Structural Integrity Handbook Issue 1.1

UNCONTROLLED COPY WHEN PRINTED

STRUCTURAL INTEGRITY HANDBOOK

ISSUE 1.1 APR 2015


Contents

Contents.................................................................................................................. 2 Figures .................................................................................................................... 3 Foreword ................................................................................................................. 4 The need for SI ....................................................................................................... 5 History of SI in the UK military ................................................................................ 6 Key definitions......................................................................................................... 7 Threats .................................................................................................................... 7 ESVRE.................................................................................................................. 10 Timing ................................................................................................................... 12

Detail and process ....................................................................................................12 SSIs and classification of structure ....................................................................... 12 Evidence record .................................................................................................... 13 Structural Examination Programme ...................................................................... 15 Statement of Operating Intent/and Usage............................................................. 16 Usage Monitoring .................................................................................................. 17 Structural configuration control ............................................................................. 19 EDPC .................................................................................................................... 20 Extensions............................................................................................................. 20 Operational Loads and Usage Validation – including OLM/ODR.......................... 21 MDRE.................................................................................................................... 23 Structural Sampling Programme ........................................................................... 23 Teardown .............................................................................................................. 25 Maintenance Schedule review .............................................................................. 25 Ageing Aircraft Programme................................................................................... 26 Damage................................................................................................................. 26 Lost usage data..................................................................................................... 27 Recovery ............................................................................................................... 27 Changes to clearances due to Validating activities............................................... 27 Pre-emptive reinforcement.................................................................................... 28 Fatigue conservation............................................................................................. 28 Cleared life ............................................................................................................ 29 Use of other evidence ........................................................................................... 29 Risk analysis ......................................................................................................... 29 Probabilistic SI management ................................................................................ 29

Practical management ..............................................................................................29 SISD .................................................................................................................... 29 SIWG .................................................................................................................... 30

Supporting information..............................................................................................31 Suggested SI training............................................................................................ 31 References and further reading ............................................................................ 31

ANNEX A SIWG AIDE-MEMOIRE Version 1.1 ....................................... 34 ANNEX B GLOSSARY AND ABBREVIATIONS ..................................... 41

2 Structural Integrity Handbook Issue 1.1


Figures Figure 1 SI Document Hierarchy .................................................................................... 4 Figure 2 Maintenance Error.......................................................................................... 10 Figure 3 ESVRE concept ............................................................................................. 11 Figure 4 ESVRE in the context of Air Safety ................................................................ 12 Figure 5 Flow chart – Understanding actual usage...................................................... 22 Figure 6 SI document hierarchy ................................................................................... 30 Cover image shows a severed front spar from a 1994 incident involving a Red Arrows Hawk

Structural Integrity Handbook Issue 1.1 3


Foreword

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RA 5720

SI Handbook

MASAAG Papers

This handbook supports RA 57201, providing additional information on the theory and practical implementation of Structural Integrity (SI) Management and its relation to the rest of the MAA Regulatory Publications (MRP). While it may be used as a repository of, and guide to, further reading on the processes mandated in RA 5720 (Figure 1), the handbook is presented in a format that also permits its use as introductory reading on SI for all staff .

The techniques and processes described in this handbook have been refined by the collected experience of many years. The contents are not themselves mandatory, but may instead be considered as a resource of proven and trustworthy methods for achieving compliance with the higher level criteria set by the Regulation and elaborated within its Acceptable Means of Compliance (AMC) and Guidance Material.

Figure 1 SI Document Hierarchy

No single approach will suit every aircraft and RA 5720 explicitly requires individuals to act on an informed assessment of the risks under consideration for their aircraft. Much of this handbook expands on the AMC in the RA but, where this is not the case or where there is a choice between different approaches, it may be found that a less complex, or even completely novel, technique can achieve compliance with less cost or disruption than initially thought. Advice is to be sought from MAA Cert S&ADS if alternative approaches are considered. It is therefore proposed that an appreciation of the contents of this handbook will provide the individuals concerned with the foundation of understanding necessary to successfully and efficiently manage SI.

This handbook is offered without prejudice to the MRP.

Suggestions for improvement may be sent by e-mail to:

[email protected]

Or by post to:

MAA Cert S&ADS #5003 MOD Abbey Wood North Bristol BS34 8QW

1 Refer to RA 5720 Structural Integrity Management



The need for SI Risk to Life.

1. The consequences of structural failure are often catastrophic. Lessons learned from decades of experience – as well as some high profile accidents – have led the MOD to prescribe a dedicated framework to manage the risks associated with the through-life management of aircraft structures. The mandated approach, ESVRE, which has developed over many years, cuts across both Type and Continuing Airworthiness, and today's RA 5720 references several essential SI-related activities now covered by other, dedicated, RAs at the same time as providing overarching regulation for SI Management.

2. The MOD, like the civil world, regulates the design and maintenance of aircraft and establishes limits within which they can safely be operated. Yet experience has shown that military aircraft are particularly likely to experience changes in usage and operating environment and to be operated for longer than was anticipated at the design stage. Numerous case studies illustrate how failure to adequately manage all of the variables involved can exacerbate risks to air safety.

Constraint on service life

3. SI often dictates useful life. The relative ease of incorporating improvements to aircraft systems means that the operational life of an aircraft is usually determined by SI considerations rather than by equipment obsolescence.

Cost

4. Structural inspections and monitoring can be the most costly element of a maintenance programme. Moreover, as aircraft age, the need for additional directed structural inspections, sampling, teardown, and life-extending modifications will increase. Through-life cost is therefore sensitive to how well SI is managed.

Integrity

5. The technical and organisational uncertainties associated with military aviation contribute to a complex range of hazards and have, over time, led the MOD to develop a dedicated framework for their management through-life. At root, this concerns nothing more than the efficient management of a variety of interdependent activities related to design, maintenance and operation. In the context of airworthiness, integrity may therefore be thought of as a measure of risk reduction specific to these activities. The overall activity called SI is regulated in the MRP (RA 5720) and is defined in MAA022:

Structural Integrity: The ability of an aircraft structure to retain its strength, function and shape within acceptable limits, without failure when subjected to the loads imposed throughout the aircraft’s service life by operation within the limitations of Release To Service (RTS) and to the usage described in the Statement of Operating Intent (SOI) or the Statement of Operating Intent and Usage (SOIU).

6. Similar frameworks have now been adopted for the management of aircraft systems and propulsion systems.

7. While support from various stakeholders is needed to manage SI, overall responsibility is assigned to the Type Airworthiness Authority (TAA) as the individual responsible for the Type Airworthiness of the air system.

2 MAA02 MAA Master Glossary



History of SI in the UK military

SIGNIFICANT TURNING POINTS IN SI

Buccaneer Incident – Ex Red Flag 1980 1954 BOAC DEHAVILLAND

COMET ► FAIL SAFE AND FATIGUE TESTING

1969 USAF F111 ► DAMAGE TOLERANCE

1977 DAN AIR BOEING 707 ► GERIATRIC (AGEING) AIRCRAFT

1979/80 RAF BUCCANEERS ► USAGE VALIDATION (MILITARY)

1988 ALOHA BOEING 737 ► WIDESPREAD FATIGUE

DAMAGE/MULTIPLE SITE DAMAGE

SIGNIFICANT SERVICE INCIDENTS

Chinook 1989 – loss of rotor sync

1974 HARRIER FLAP DRIVE SHAFT

1976 PHANTOM WING FOLD LOCK LUG

1980 HARRIER ROLL REACTION CONTROL

VALVE ROD

1989 TRISTAR WING SPAR FAILURE

1989 CHINOOK LOSS OF ROTOR

SYNCHRONISATION

1993 WESSEX LOSS OF TAIL ROTOR DRIVE

1995 TUCANO PROPELLER BLADE FAILURE

1996 HARRIER T10 WHEEL UNIT

1999 C130K FUSELAGE ROOF

2001 GAZELLE ROTOR TIE-BAR

2001 WESSEX TAIL ROTOR DRIVE

2003 HARRIER T10 WHEEL UNIT

2004 GROB TUTOR PROPELLER DETACHED

2004 HARRIER GR7 CANOPY

2004 TORNADO UPLOCK DAMPER BOLT FAILURE

2007 VC10 MULTI-ELEMENT FATIGUE DAMAGE

2010 DOMINIE WIDESPREAD EXFOLIATION CORROSION



Key definitions 8. The following key definitions, taken from MAA02, apply2:

a. Structure: Aircraft structure consists of all load-carrying members including wings, fuselage (including some transparencies), empennage, engine mountings, landing gear, flight control surfaces and related points of attachment, control rods, propellers and propeller hubs if applicable and, for helicopters: rotor blades, rotor heads and associated transmission systems. The actuating portion of items such as landing gear, flight controls and doors are subject to System Integrity Management regulation (RA 5721) as well as SI Management regulation.

b. Structural Airworthiness: This is the ability of an aircraft to maintain SI without significant hazard to the aircrew, ground crew, passengers (where relevant), or to the general public over whom the aircraft is flown.

Threats 9. Compromised SI may exacerbate risks to airworthiness. Any one, or combination of, the following threats can compromise SI:

a. Overload.

b. Fatigue, fretting and wear.

c. Accidental Damage (AD) and Environmental Damage (ED).

d. Procedural (Design, Manufacturing, Maintenance or Supply) error.

Overload

10. An aircraft encounters overload when subjected to forces that are above the Design Limit Load (DLL) for the structure. The DLL is either the maximum and most critical combination of loads and environmental conditions likely to occur during the life of the aircraft, or standard airworthiness specification cases that are unlikely ever to be exceeded in service. This may occur in extreme atmospheric conditions (gusts and air turbulence), during heavy landings, violent manoeuvring or divergent flutter. Overload may also arise because the crew or flight control system exceed RTS limitations, or if there has been an error in defining the RTS limits themselves, and more subtly, where the design and application of repairs or modifications bring about changes to load paths or where in-service loads are not fully understood. Aircraft structures are designed so that there will be no permanent deformation, loss of function or necessity for repair when loaded up to the Design Proof Load (DPL). The DPL is the product of DLL and the proof factor, which can range from 1 for a large civil type aircraft to 1.125 for a combat aircraft. In the case of an aircraft with a proof factor greater than 1, an overload event may not result in permanent deformation or structural failure; however, the overload event is to be fully investigated.

11. Aircraft are subject to fluctuating loads that can be categorised as either high or low cycle. Sources of these repeated loads include:

a. High Cycle: vibration caused by acoustic loading, non-divergent flutter, manoeuvre buffet, blade passing frequencies and rotating components in aircraft dynamic systems.

b. Low Cycle: manoeuvres, gusts, the ‘ground-air-ground’ cycle, cockpit or cabin pressurisations, landings, retractions, taxiing, rotor start/stop, changes in engine power, thermal changes, hydraulic and fuel system pressurisations and arrester hook use.



12. Fluctuating loads can pose a threat to SI if not adequately accounted for in design. The following paragraphs describe how fatigue, fretting and wear manifest themselves from this threat.

Fatigue

13. This is a process of progressive, permanent structural change occurring in a material that is subjected to fluctuating loads below the static yield strength of the material. Fatigue damage nucleates and grows on a microscopic scale until it manifests itself as cracking, the growth of which depends on material properties and geometry, the level, amplitude and frequency of fluctuating loads and the number of load cycles applied. Fatigue damage culminates in cracks that reduce the residual strength of the cracked structure and cause fracture if the residual strength falls below the applied load. Fatigue is applicable to both metallic and composite structures (for further guidance on composite materials, see AP 101A-0601-13. Fatigue damage is the predominant cause of catastrophic structural failure in metallic aircraft structures. It can also develop into Widespread Fatigue Damage (WFD), which is defined as the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet its residual strength requirements. Sources of WFD are:

a. Multiple Site damage (MSD): the simultaneous presence of multiple fatigue cracks in the same structural element.

b. Multiple Element Damage (MED): the simultaneous presence of fatigue cracks in similar adjacent structural elements.

Fretting

14. Fretting is a special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force. In the presence of an aggressive environment, the contact movement causes wear and material transfer at the surface, often followed by oxidation of the debris and the freshly exposed surface. The oxidised debris can further act as an abrasive and such degradation is termed fretting corrosion. Fretting increases the threat of initiating fatigue cracking. This can result in so-called fretting fatigue.

Wear

15. Wear is the undesired cumulative change in dimensions brought about by the gradual removal of discrete particles from contacting surfaces in relative motion, usually sliding, predominantly as a result of mechanical action. Wear is not a single process, but a number of different processes that can take place independently or in a combination, resulting in material removal from contacting surfaces through a complex combination of local shearing, ploughing, gouging, welding, tearing and other actions. The outcome of wear may result in the cumulative loss of material, and ultimately structural failure, when the residual strength of the structure falls below the applied load.

AD

16. This is the physical alteration of an item (or its surface protection where applicable) caused by contact, impact or interaction with an object which is not a part of the aircraft, or by human error during manufacture, operation or maintenance of the aircraft. Typically AD may be caused by external impact in the air (for example: mid-air collision, bird strike, wire strike, lightning strike, severe hail, weapons release self-damage or ricochet damage) or on the ground (such as Foreign Object Damage, maintenance activities, ground handling, freight loading or vehicle movements). In addition, battle damage or sabotage, although not strictly accidental, may also be

3 AP 101A-0601-1 – Employment and Repair of Aircraft Composite Materials



considered within this category, as the effects are comparable. Moreover, less obvious internal AD may arise from occupant/maintainer accidents or from overheating metallic or composite structure. AD may manifest itself as distorted, torn, punctured or otherwise distressed structure or surface protection, or as delamination or disbonding, or in less visible forms such as a change in metal heat treatment condition or ‘barely visible impact damage’ to composite materials such as fibre-reinforced plastics.

ED

17. ED is the physical degradation of structural material properties as a result of their interaction with the climate or environment. Structurally significant ED is normally caused by chemical interaction, erosion, fluid/gas absorption, thermal cycling or electro-magnetic radiation. ED may manifest itself as corrosion, stress-corrosion cracking, loss of surface finish, softening of composite material matrices (including adhesives used in laminated wood), delamination, or disbonding resulting in degradation of static, fatigue or impact strength properties.

Procedural errors can be the result of design, manufacturing, maintenance or supply errors.

18. Design error can result from failure to adhere to recognised design standards, design best practice and qualification evidence methodology. Examples of design error include:

a. Underestimating local loads or overestimating of material properties.

b. Not addressing the potential for incorrect assembly.

c. Specifying inappropriate material and manufacturing processes.

19. Manufacturing error applies to structure or an individual structural component that fails to meet the design specification. Factors leading to manufacturing error include failure to adhere to specified manufacturing drawing requirements and processes, such as:

a. Use of incorrect material.

b. Application of incorrect heat treatment.

c. Incorrect dimensioning and feature (such as holes) location.

d. Use of unauthorised or inappropriate jigs, fixtures and tooling.

20. Maintenance error (Figure 2) describes an unsound maintenance process on an aircraft structure. Factors leading to maintenance errors that threaten SI may include:

a. Inadequate training or supervision.

b. Inadequate resources.

c. Incorrect technical information.



Non-round holes

Figure 2 Maintenance Error 21. Supply error describes the supply of a component or product that does not meet the aircraft current structural design specification. Factors leading to supply errors may include non-conforming components or products, those from an unknown pedigree, those from unapproved suppliers and those that are incorrectly identified or codified.

ESVRE 22. The acronym, ESVRE, stands for Establish-Sustain-Validate-Recover-Exploit – the five activities that completely cover the process of SI Management (Figure 3) – and represents the framework identified by the MOD as best suiting the variety of aircraft and procurement models represented on the Military Aircraft Register. ESVRE is intended to be as applicable to a future fighter bought from the US as to a home-grown Remotely Piloted Air Vehicle or a historic helicopter4. It is not a purely chronological framework (like CADMID is, for example) and aspects of each ESVRE section may require action at various stages of an aircraft's life.

23. RA 5720, which is aligned with ESVRE, is the descendant of a long series of publications and guidance that developed alongside the UK military's growing knowledge and experience in dealing with structures issues – first fatigue monitoring, then corrosion control, Operational Loads and Usage Validation, structural sampling, ageing, and so on.

24. A great deal of this experience is also embodied in the set of standards for design and airworthiness known as Def Stan 00-9705, which appeared in its earliest form in 1916. Applicants for a Military Type Certificate are required6 to demonstrate compliance of Type Designs to this, or to an appropriate and equivalent, standard. Much of the initial evidence for SI is obtained during the development phase of a project, usually to support certification and qualification.

25. Fatigue testing may continue for some time after the aircraft enters service, to achieve full structural clearances. During the development phase, the nature and frequency of structural inspections and monitoring that will be necessary to maintain the fleet through life will also be determined.

26. Once the aircraft is in service, the operating conditions, usage and configuration on which the Designer based their decisions during development may change. The

4 ‘Developing Issues in Aircraft Structural Integrity’, Wg Cdr Ron Eckersley 5 Refer to Def Stan 00-970 Design and Airworthiness Requirements for Service Aircraft 6 RA 1500 Certification of UK Military Registered Air Systems



implications of any changes are assessed and SI evidence (usually kept in the static and fatigue Type Records) kept updated throughout the life of an aircraft type.

27. The work to gather initial SI evidence is classed as an Establishing SI activity. The actions to maintain SI through life and act on any changes to usage and operation are Sustaining and Validating activities, respectively.

Figure 3 ESVRE concept

28. Structural damage can be caused by any of the threats to SI or combinations thereof. More generally, failure to carry out adequate SI Management, eg Structural Configuration Control (SCC) or Usage Monitoring may undermine confidence in SI. Recovering activity, which may range from simple component exchange to a full Design Organisation (DO) repair or the retirement of the aircraft, will be required.

29. Occasionally, additional activity is needed to safely Exploit the inherent capabilities of the structure, to extend the Out-of-Service Date (OSD), for example.

30. The crucial role played by the DO at each stage of ESVRE is explicit in RA 5720. It is not, however, limited to generating, maintaining and updating the evidence record through life. It is essential that contractual arrangements allow for the DO’s participation in the exchange of information that is fundamental to in-service SI Management, such as attendance at working groups, upkeep of Usage Monitoring systems or reassessment of component lives in response to changes in aircraft usage.

31. It was stated earlier that the TAA is ultimately responsible for ensuring that all of these activities are carried out – this is reflected in the wording of RA 5720 – but Figure 4 illustrates the importance of contributions from other stakeholders to achieving SI.



Figure 4 ESVRE in the context of Air Safety

Timing 32. Adequate funding is key to supporting the aircraft’s SI Strategy throughout the anticipated service life of the aircraft.

33. Measures to assure SI are to be planned in advance and included in the aircraft Through Life Management Plan (TLMP), and also in any project whole life cost forecasts.

Detail and process

SSIs and classification of structure 34. Aircraft structure comprises assemblies, components and features (elements and details) that have traditionally been classified according to their relative contribution to the function or residual strength of their higher assembly. Various terms such as Primary Structure, Class 1 Structure, Vital Parts or ‘Grade A’ Parts, Principal Structural Element (PSE), Airworthiness Limitation Items, Structural Significant Items (SSIs) and Safety of Flight Structure have been used. There is no universally accepted method for selecting or identifying important structure, or commonly accepted interpretation of ‘critical’ or ‘significant’ in this context. Nevertheless, SI can be established only when all items whose failure would have an unacceptable impact on the structural airworthiness of the airframe as a whole are identified, assessed and recorded in documents that are maintained and accessible to SI stakeholders. Such items of structure are defined in the following paragraphs.

35. An SSI is defined in MAA022 as:

a. SSI: Any detail, element or assembly, which contributes significantly to carrying flight, ground, pressure or control loads and whose failure could affect the SI necessary for the continued safe and controlled flight of the aircraft.

36. Selection of SSIs is pivotal to the derivation of the Preventive Maintenance programme by such methods as the Operator/Manufacturer Maintenance Development Processes, as developed by the Maintenance Steering Group Task Force (currently known as MSG-3) and published by the Air Transport Association of America and



Reliability Centred Maintenance (RCM) published within JAP (D) 100C-227. The characteristics of SSIs have to be assessed in terms of their sensitivity and vulnerability to the threats to SI. The characterisation of Safe Life or Damage Tolerant SSIs is dependent upon a fatigue damage assessment.

SSI classification by design philosophy

37. Safe Life (specifically, either Stress Life or Strain Life) is a design philosophy that establishes a finite service life within which the probability of fatigue cracks developing and compromising residual strength is acceptably low. Safe Life SSIs are those items of structure designed to have a finite fatigue life, or those where application of a Damage Tolerant approach is not possible. The Safe Life is qualified by calculation, often supported by test as in the case of the full scale fatigue test required by Def Stan 00-970. Safe Life SSIs are subject to an analysis of their vulnerability to AD and ED during development of a Preventive Maintenance programme. This analysis is essential because the life of an SSI can be adversely affected by both AD and ED and thus occurrence of these threats is detected by an appropriate inspection regime.

38. Damage Tolerance is a design philosophy which leads to a structure that can retain the required residual strength for a period of use after the structure has sustained specific levels of detectable fatigue damage, AD or ED. Airworthiness of Damage Tolerant structure is therefore assured by a specified inspection regime. Damage-tolerant SSIs are either structure that was designed and demonstrated to be Damage Tolerant or fail safe, or structure which was designed to be Safe Life but which has subsequently been re-categorised as Damage Tolerant because of failure on test or in service, or life extension beyond the designed Safe Life limit where SI can be assured using an inspection-based regime. Damage Tolerant SSIs are subject to a fatigue assessment as well as AD and ED analyses.

Evidence record 39. Aircraft accepted into UK military service may be designed to satisfy one of a number of different design requirements or standards, such as Def Stan 00-970, or international military or civil standards. Notwithstanding the wealth of evidence required for certification and qualification of the aircraft structure, the minimum evidence required to sustain the management of the aircraft throughout its service life is usually summarised in the static and fatigue evidence document set.

40. Static qualification evidence is usually found in the form of a Static Type Record (STR), or equivalent document. Whether an aircraft type is procured in accordance with the 5000 series RAs or some alternative acquisition management standard or procurement model, a Type Record is the ideal vehicle for the collation and summary of static strength evidence.

41. An STR or equivalent document comprises a general arrangement and description of the aircraft, a summary of static design assumptions and criteria, a summary of critical loading, shear force, bending moment, torque and mass distributions, and a summary of static reserve factors. The static evidence contains all the relevant supporting stress and test reports and calculation files. For modern designs, evidence may have been obtained from aerodynamic and loads models (such as Finite Element Models) and so it is necessary that these models should be maintained and updated through life, in the same manner as the static evidence document itself. The scope of the static evidence document, in terms of the structural components involved, includes all SSIs.

7 Refer to JAP (D) 100C-22 Guide to Developing and Sustaining Preventive Maintenance Programmes, available on the MAA website



42. Fatigue qualification evidence, usually in the form of a Fatigue Type Record (FTR) or equivalent document, is mandated for aircraft designed to Def Stan 00-970 and is recommended by RA 53098. Notwithstanding how an aircraft type has been procured, an FTR or equivalent document is the ideal vehicle for the collation and summary of fatigue qualification evidence to support the DH-facing Safety Assessment for RTS and sustain SI.

43. For multi-national or off-the-shelf designs, for both the static evidence and fatigue evidence, it may be possible to develop a multi-national fatigue evidence document or to adapt existing evidence into an acceptable format. However, in due course it will be necessary to develop a national fatigue evidence document to reflect the UK military as-flown usage and configuration as it develops and diverges from that of other operators.

44. The initial issue of an FTR or equivalent document generally consists of a single Part 1 (see RA 5309(1)8) that comprises a statement of the fatigue principles used and a summary of fatigue and/or Damage Tolerance analyses for each item of structure. As with the static evidence document, the FTR also lists all relevant reports and may be supported by computer models.

45. Furthermore, because both usage and fatigue evidence evolve significantly over the life of a type, the fatigue evidence is to incorporate additional sections, in accordance with RA 5309:

a. A historical record of the fatigue substantiation for the aircraft.

b. A reassessment in light of service usage and fatigue test results.

c. A reassessment of inspection requirements shown to be necessary by (b).

d. A life extension document (as required).

46. SI requirements must be considered during certification activities so that applicable evidence can be captured within the Structural Integrity Strategy Document (SISD) (see Page 29).

47. Loads models, including computer modelling, are to be substantiated by test evidence and verified by an Independent Technical Evaluator (ITE) as part of certification and qualification activities.

48. Resourcing and tasking of DO support is needed to support certification (in accordance with the Military Air Systems Certification Process (MACP)) and through-life SI management. RA 15006 contains regulation concerning the MACP.

49. Adequate arrangements are to be made with appropriate organisations to support computer models used to generate qualification evidence. These are then available and maintained in a usable state for the life of the platform.

50. Review of structural qualification evidence: Both the static and fatigue qualification evidence will initially have been compiled during the design and qualification of an aircraft type, as SI Establishing activities. However, as the as-flown configuration and usage diverge over time from that assumed during design, and as information becomes available from any ongoing testing and SOIU review/Operational Loads and Usage Validation, it will become necessary, progressively, to update the qualification evidence documents.

51. Repairs, modifications or changes in usage or configuration may invalidate the original qualification. In order to assure the continued airworthiness of the fleet, there is an ongoing requirement to validate and update the structural qualification evidence against measured loads data and other relevant information. The Project Team (PT)

8 Refer to RA 5309 Fatigue Type Record for Aircraft



ensures that the DO is tasked to review and update the qualification evidence whenever it may have been invalidated.

Structural Examination Programme 52. Once an aircraft enters service, its structure is subjected to the four main groups of threat to SI: overload, fatigue, AD/ED and procedural error (design, manufacturing, maintenance and supply). For all SSIs, with the current state of technology, the only counter to AD/ED is to examine for and repair any damage. Furthermore, safe lives and Damage Tolerance examination thresholds and intervals are based on analysis and/or testing of structure that has not been exposed to AD/ED. Therefore, there is a need to examine for AD/ED to ensure that the occurrence of AD/ED does not invalidate fatigue and Damage Tolerance clearances. In addition to the examinations for AD/ED that may be necessary for both Safe Life and Damage Tolerant structures for these two reasons, Damage Tolerant structures rely on examinations for cracks to counter the threat posed by fatigue (in contrast to Safe Life structures, which are not examined for fatigue cracking per se). For all structures, therefore, a Structural Examination Programme (SEP) is required.

53. The SEP is accomplished through the structures-related parts of flight servicing and scheduled maintenance procedures, and through structural sampling and teardown. In the context of the ESVRE framework, the structural elements of flight servicing and scheduled maintenance are Sustaining activities. Structural sampling and teardown are Validating activities; see RA 5720(4). Furthermore, as aircraft age, the likelihood of interaction between the different threats to SI increases. PTs ensure that procedures are in place to identify the interaction of these threats in order to enable appropriate action to be taken – see RA 57239. An effective SEP, supported by structural sampling, is a key element that enables the early identification of likely threats.

54. Each SSI can be characterised in terms of its fatigue or Damage Tolerance clearance, which will depend on the original design philosophy. It will either be Safe Life or Damage Tolerant. For Safe Life components the life is determined so that the component is retired before fatigue cracks are likely to have compromised residual strength. For Damage Tolerant components, testing will have shown when a fatigue crack is likely to develop and how long the crack will take to reach a critical length. The likely time of crack initiation will determine when in the aircraft’s life the Damage Tolerance inspections start (the examination threshold), while the rate of crack propagation determines the inspection periods (examination interval).

55. Maintenance Schedules: In parallel with the above characterisation, each SSI is also assessed for its vulnerability and susceptibility to AD/ED. This is normally carried out as part of the Maintenance Schedule development process. However, typical Maintenance Schedule development processes, such as RCM or MSG-3, commonly only deal with AD or ED mechanisms acting independently. It is therefore necessary to consider the vulnerability and susceptibility of each SSI to the interaction of AD and ED (for example, the onset of corrosion following impact damage causing the breach of a corrosion protection coating) and to the interaction of AD and ED with fatigue or static loading (for example, stress-corrosion cracking or corrosion-accelerated fatigue), even though this may not be accounted for by the RCM process. The result of the assessment of vulnerability and susceptibility to AD/ED is to designate each item as either At Risk (AR) or Not At Risk (NAR) of AD/ED.

56. The Maintenance Schedule review process identifies which SSIs are AR. These items are then included in the Master Maintenance Schedule (MMS) and marked as SSIs to ensure they are examined at a suitable frequency to detect AD or ED before it becomes a hazard to SI.

9 Refer to RA 5723 Ageing Aircraft Audit



57. For Safe Life items that are NAR, and Damage Tolerant items with a long threshold before inspections begin, it is possible that they may not have been included in the MMS. Therefore, to maintain SI for those items, it is necessary to validate the assumptions that led to determination of the item being NAR and determination of the Safe Life or the Damage Tolerance examination threshold. This is achieved by carrying out sample checks of the items before they reach the end of their Safe Life or first Damage Tolerance inspection. Consequently, PTs ensure that the SSI list is cross-referred to the MMS and that SSIs that are not included in the MMS are subject to structural sampling to confirm that they do not suffer from AD, ED or fatigue damage earlier than expected.

58. Structural Examination – Reporting: The results of the Maintenance Schedule review process are promulgated in a Topic 5V (Sampling Requirements and Procedures) or equivalent, which lists all SSIs (both AR and NAR) and states the means by which they are to be examined. The list of SSIs is supported by the use of diagrams, drawings or photographs as appropriate. The Topic 5V also specifies the method for recording the result of each examination and feeding back this information to the PT; positive and negative reporting is always required to enable the PT to ensure SI is maintained. The use of omnibus reporting for inspected SSIs where no structural problems are found may be considered, with detailed reporting only when a structural arising is found. The PT ensures that the inspection results are analysed to determine whether SI or availability is being compromised and whether Recovering action is required. Furthermore, the results are utilised in Maintenance Schedule reviews and Continued Airworthiness activities. The Topic 5V examinations are called up in the Topic 5A1 or equivalent Maintenance Schedule as SSI-coded activities and cross-referred or hyperlinked to the Topic 5V entry.

59. For Safe Life structures, the threat posed to SI by fatigue is addressed by retiring individual airframes or major components before their probability of failure reaches an unacceptable level (typically higher than 1 in 1000). The Safe Life is derived by applying a statistical scatter factor to the predicted life (from testing or analysis), hence Safe Life methods result in the majority of structures being retired before any fatigue cracks are likely to have reached the point where the residual strength is compromised.

60. For Damage Tolerant (examination-dependent) structures, the threat posed to SI by fatigue is addressed by examining the structure for cracks. These examinations commence at the point in the aircraft’s life (the examination threshold) at which the crack is predicted to be detectable by the inspection technique and are repeated at every inspection interval.

Statement of Operating Intent/and Usage 61. The Statement of Operating Intent (SOI) is the means by which the UK Services’ future operating intent for a particular aircraft type and major mark are conveyed formally to the DO. The SOIU replaces the SOI when the aircraft has accumulated sufficient representative in-service usage data. The SOI is part of the Aircraft Document Set (ADS) and its production is mandated for all UK military aircraft. The SOI is published as the Topic 15S within the ADS. The DO may use SOI information as the basis for deriving a Design Usage Spectrum for underpinning fatigue and Damage Tolerance clearances and associated inspections.

62. SI can be highly sensitive to changes in aircraft usage. Continued Airworthiness assurance therefore requires regular reviews of aircraft usage.

63. The SOIU describes and quantifies actual usage over a period of time and describes intended future aircraft use. The SOIU formally conveys this data to the DO so that it can be analysed in comparison with original or extant usage assumptions and the implications fed into revised fatigue and damage-tolerance inspection thresholds.



64. The SOIU must be reviewed annually to check its continued validity and triennially to carry out a quantitative update, including fleet fatigue and usage data derived from IAT (including MDRE, HUMS, FDR, ADR, etc).

65. The Aircraft Usage Validation Process document10 is the primary source of further detailed guidance on SOI and SOIU creation and the SOIU review cycle.

Usage Monitoring 66. For both Safe Life and Damage Tolerant structures, Usage Monitoring through Individual Aircraft Tracking (IAT) is necessary to manage the timing of either the retirement or examination of individual airframes, as appropriate. Usage Monitoring systems have therefore been developed to identify when SSIs are to be retired or examined, as appropriate. These systems vary in the metrics used and their complexity and may include:

a. Structural Health Monitoring.

b. Health and Usage Monitoring Systems.

c. Fatigue meter and fatigue meter formula (FMF).

67. In this context, it is important to note that Usage Monitoring for IAT is primarily used only to address the threat to SI posed by fatigue. Some usage monitoring systems do have the additional ability to monitor flight parameters for potential overload conditions.

68. IAT can comprise direct or parametric measurement techniques at both fleet and individual aircraft level. Depending on the type of structure and the predominant fatigue loading action, metrics may be any one of (or a combination of) Fatigue Index (FI), flying hours (which may be factored to account for differences in role severity), landings or other relevant data.

69. FI is defined as a measure of the fatigue usage of a structure in comparison with the qualified usage, at which examination, modification or withdrawal from service is necessary. The design Safe Life of a structure will nominally be 100 FI, though this figure may be increased or decreased for in-service release as a result of fatigue testing, loads monitoring or in-service arisings. For many legacy fixed-wing aircraft, FI is calculated using an FMF, incorporating normal acceleration data (captured by a ‘fatigue meter’ counting accelerometer) plus mass, configuration and other relevant usage parameters. Normal acceleration is defined as normal (at 90°) to the plan of the aircraft in the direction conventionally annotated ‘z’. This is usually caused by a turning manoeuvre, pitching manoeuvre or gust and often measured in multiples of acceleration due to gravity, ‘g’.

70. Fatigue and usage data will normally be captured post-sortie by type-specific means such as MOD Form 724 (Flying Log and Equipment Running Log), MOD Form 725 (Flying Log and Fatigue Data Sheet), Technical Logs, electronic equivalents such as LITS/Gold post-sortie data feedback, or by automatic data download from an on-board system.

71. The fatigue damage accrual for some structural features on both helicopters (eg, transmission components) and fixed-wing aircraft (eg, fin and tailplane for agile combat and trainer aircraft) can be dominated by high-frequency, cyclic stresses in structure. It increases with the magnitude of the stress cycle and accumulates with each application, eventually leading to initiation and growth of cracks. The severity of individual manoeuvres directly affects the fatigue consumption of components. Gentle handling should be encouraged, unless operational circumstances dictate otherwise. Excessive vibration can cause an increase in fatigue damage rates and necessitate

10 see the MAA website under Certification



structural repairs. Fatigue damage to helicopter rotors can be reduced by avoiding a complete shutdown of the rotor during sorties.

72. Certain operational environments and roles can cause high rates of fatigue damage in excess of those suffered by the majority of a fleet. Air Staff and PTs would normally consider the regular transfer of aircraft between various roles and environments to avoid high rates of fatigue damage being concentrated upon a limited number of aircraft.

73. Good practice requires systems are to be in place for usage data capture and computation (either by internal agencies or external contractors), for managing overall fleet usage, for monitoring individual aircraft usage against safe lives or examination thresholds and intervals, and for sponsoring changes to Usage Monitoring, data capture and computation systems as and when required. Data is to be entered promptly onto electronic systems or forwarded to the appropriate agency, as directed by the PT.

74. The output of IAT systems is flight by-flight and component-by-component cumulative lifing data, which is used to manage retirement, replacement, modification or examination as appropriate to the individual aircraft/component and fatigue design philosophy. This data is therefore essential to fleet management and to allow interchangeability of lifed components. For a Safe Life airframe, the rate of consumption of fatigue life will determine the ability of the fleet to reach its OSD and the need for a life extension programme (LEP). For a Damage Tolerant design, the rate of consumption of fatigue will determine the examination burden; this may necessitate supplemental structural examinations for very high-life aircraft and will determine the economic life of the fleet.

75. It is important for the PT to manage the consumption of fatigue to achieve the defined OSD. Occasionally, therefore, it may be appropriate to determine and promulgate a usage budget to individual aircraft in conjunction with the RTSA and the Duty Holder (DH), where differences in operation from the rest of the fleet are significant, eg dual control or specially instrumented aircraft used for trials or training exercises.

76. Modification or refurbishment programmes to mitigate fatigue can be initiated where necessary to maintain capability, achieve target OSD, enhance capability or extend life (see RA 572411 and RA 572512).

77. The DO responsibilities are:

a. Specify for each SSI the usage parameter to be used and either the Safe Life or the Damage Tolerance examination threshold and interval, in terms of the appropriate usage metric.

b. Provide the Usage Monitoring system and implement changes to it as required.

78. Most Helicopters are fitted with Health and Usage Monitoring Systems (HUMS) in accordance with RA 450013 and there are opportunities for exploiting data from these systems in the IAT role. Helicopters which spend more time in unusually biased or severe flying conditions (eg heavy lift operations or mountain flying) may also load parts of the structure or rotating components considerably more severely than in the Designer’s assumptions. Unmonitored, such unusual usage could lead to structural failure of a rotating component before the predicted Safe Life has been reached. For this reason, fleet managers at all levels are to ensure that wherever possible, individual aircraft experience as wide a range of roles as possible. Should it be necessary,

11 Refer to RA 5724 Life Extension Programme 12 Refer to RA 5725 Out of Service Date Extension Programme 13 Refer to RA 4500 Health and Usage Monitoring



through modification or location, to limit an airframe to one type of flying only, details are to be brought to the attention of the Designer in order that the fatigue implications can be assessed.

79. Any period of unmonitored operation or change in usage of the aircraft will threaten SI, as either can lead to aircraft being operated beyond the assumptions underpinning SI. Specific threats include:

a. Inadequate validation of sortie profile codes (SPCs) leading to incorrect recording of aircraft usage and incorrect critical part lifing assumptions.

b. Lack of monitoring or analysis of aircraft usage on an individual aircraft basis leading to inappropriate sortie profile mix and incorrect critical part lifing assumptions.

c. Change of sortie profiles.

d. Differing environments.

e. New working practices or changes to maintenance processes.

f. New operating practices.

80. Where fatigue and usage data is lost or compromised, either due to unserviceability of aircraft systems or failure of data capture or computation, fill-in data will generally be derived from mean fatigue usage data. However, this attracts a penalty factor to account for uncertainty. Since this represents a loss of available Safe Life or time to next examination, it is important that such occurrences are minimised.

Structural configuration control 81. Throughout the life of an aircraft there are risks to SI from changes in structural configuration. The structural configuration of aircraft at the point of delivery can be expected to vary from the baseline design due to differing build standards and build concessions. Furthermore, the structural configuration of in-service aircraft is likely to diverge from the as-built configuration as individual aircraft are subject to damage, repairs, modifications and Special Instruction (Technical) (SI(T)). These deviations or changes may affect aircraft mass, centre of gravity (C of G), internal structural layout or local structural stiffness, and may therefore alter internal load distributions. Such deviations or changes may compromise static strength, fatigue and Damage Tolerance clearances, and may jeopardise future repair, modification and life extension. Static and fatigue strength may be threatened if repairs are not adequately assessed or qualified (including static and fatigue qualification), and if the potential interaction of adjacent structural repairs is not addressed. The migration of lifed structure between aircraft further complicates the problem. Furthermore, if the DO is unaware of the structural configuration of an aircraft, then recommendations for modification, repairs and life extension will be based upon a presumed configuration usually based on the information held in the aircraft master drawing set. To mitigate these risks, expensive and time-consuming whole-fleet aircraft surveys may be required if SCC has not been maintained throughout the life of the platform. SCC is a process that assists the PT to identify the cumulative risk to SI arising from deviations or changes from the build standard, and to avoid additional costs and logistics risks in the future.

82. A PT is required to take appropriate measures to ensure that a suitable system is put in place to maintain overall SCC of each aircraft within the fleet. Accurately recording structural arisings will assist decision-making and trend analysis for targeting appropriate husbandry or maintenance, and will be further aided if the SCC system can handle graphical material. Ideally a single database should be used to maintain SCC, although it may utilise data from a range of different sources eg as a minimum, structural concessions, repairs, modifications, accidental and environmental damage for all aircraft. For legacy aircraft it may be necessary to carry out a structural survey of



the whole fleet to establish a baseline. The database includes all relevant arisings to individual aircraft and is maintained for the life of the aircraft. The data should permit a fleet-wide assessment of structural health.

83. Key elements of the database are:

a. Build concessions.

b. The extent of damage (such as AD and ED) present before repair.

c. The condition of the material after the repair has been carried out (such as the thickness remaining after blending).

d. Repairs, which may originate from the Topic 6/Structural Repair Manual, from an approved DO or from an approved Aircraft Repair Organisation.

e. Un-repaired damage (damage within authorised limits), so as to provide a complete picture alongside embodied repairs and any further damage that may occur.

f. Modifications.

(1) DO-produced.

(2) Service-produced.

84. The regulation, including responsibilities, for technical data exploitation can be found in RA 114014.

85. For safe and effective operation of aircraft, the weight and C of G must remain at all times within the limits specified in the RTS. If these conditions are not satisfied, the consequences may range from an increased consumption of fatigue life that will threaten SI, to failure to maintain adequate control and stability, to loss of the aircraft. If weight and C of G are deliberately changed through modification or change of role, then it may be necessary for the PT to initiate a review of static and fatigue qualification in the light of the cumulative effects of the changes. Regulation and guidance on aircraft weight and C of G can be found in RA 425615.

EDPC 86. As aircraft age, ED becomes more widespread and is more likely to occur concurrently with other forms of damage such as fatigue cracking. Similarly, aircraft structures will suffer from ED mechanisms such as erosion and degradation of composite material properties and surface finish. ED degrades SI and, if uncontrolled, will reduce the inherent ability of the structure to sustain loads in the presence of other forms of damage. Establishment and promotion of an EDPC Programme to minimise environmental damage to the aircraft structure should be in place. The EDPC Programme defines the minimum requirements for preventing and controlling safety-related ED problems within the fleet (see RA 450716).

Extensions 87. Fatigue safe lives and Damage Tolerance examination thresholds and intervals will normally be determined by the DO, on the basis of analysis and testing in accordance with a suitable design and qualification standard, and promulgated in the FTR. Where there is an anticipated need to extend lives, thresholds or intervals, this can be achieved through the DO via a life extension programme. However, in exceptional cases there may be an urgent need for a temporary extension of lives, or examination thresholds and intervals, of SSIs for a limited number of aircraft. In such cases, the PT may consider an extension.

14 Refer to RA 1140 Military Air System Technical Data Exploitation 15 Refer to RA 4256 Aircraft Weighing 16 Refer to RA 4507 Aircraft Environmental Damage Prevention and Control



88. Any temporary extension beyond the cleared fatigue Safe Life or Damage Tolerance examination threshold or interval, for a single airframe or component, required in exceptional circumstances, is authorised by an appropriate PT engineer with the necessary delegated airworthiness authority and with the agreement of the relevant 2-Star Cluster Leader (see RA 100317 and RA 100618).

Operational Loads and Usage Validation – including OLM/ODR 89. The structural design, test and qualification and validation methodology for an aircraft requires the DO to make assumptions about the aircraft’s intended usage and the associated loads acting upon the structure during its service life, as well as their frequency and magnitude of application. As an aircraft’s role, operating patterns (including operational theatre and individual manoeuvres or SPCs) or configuration change, the relationships between flight parameters and local structural stresses may also change. Therefore, the actual in-service aircraft’s usage and the loads associated with this usage can be determined at regular periods throughout the aircraft’s life, for comparison with the aircraft’s design usage assumptions and the fatigue spectra used in test and qualification. Usually, the SOIU does not provide the level of detail required to validate all design usage and loads assumptions. Similarly, the use of simple parametric usage monitoring systems, such as those that rely simply on flying hours or ‘g’ counts coupled with FMF, may not provide the required level of detail. Furthermore, FMF accuracy relies upon a known mix of sortie profiles and confirmed relationships between flight parameters and structural loads.

90. Operational Loads and Usage Validation is the preferred term for the direct or indirect measurement of representative in-service data for comparison with the assumptions supporting aircraft and component clearances and structural lives (see Figure 5). This activity, which builds upon the loads validation or flight loads survey activity undertaken during the design process, is commonly achieved using OLM, for fixed-wing aircraft, and Operational Data Recording (ODR), for rotary-wing aircraft, but the complexity of the solution chosen will be determined by the aims of the programme and existing confidence not only in actual usage, but also in the relationship between usage and the loads experienced by the airframe. The TAA, in consultation with the DO, may make use of routine Usage Monitoring systems as well as parametric data, and instrumentation systems developed especially for the task. Data from less complex systems may confirm a need to progress to a more complex one.

91. The main aim of Operational Loads and Usage Validation is the substantiation of assumptions made during design, qualification and test regarding usage and associated loads. This process may comprise:

a. The validation of the fatigue usage spectrum and the assumptions used during the design, structural qualification and test of the aircraft, including the review of fatigue clearances as well as maintenance and inspection periodicities.

b. The validation of the aircraft’s Usage Monitoring system, including any lifing, damage or FI algorithms.

17 Refer to RA 1003 Delegation of Airworthiness Authority and Notification of Air Safety Responsibility (DE&S) 18 Refer to RA1006 Delegation of Engineering Authorizations


http://defenceintranet.diif.r.mil.uk/libraries/corporate/MAA/Regulatory%20Publications/1000%20Series/RA1003_Initial_Issue.pdf

http://defenceintranet.diif.r.mil.uk/libraries/corporate/MAA/Regulatory%20Publications/1000%20Series/RA1006.pdf


SOIU

Operational Loads & Usage Validation

SOI (Initially)

Lifing clearances

Evidence record

Actual usage

Usage Monitoring

Figure 5 Flow chart – Understanding actual usage

92. The benefit of considering the Operational Loads and Usage Validation requirement during platform development is that onboard systems that meet the requirement can be included in the design (see MASAAG Paper 10919 Chapter 7). This reduces the need to install the necessary instrumentation through expensive retrofits once in service, although this may be required for legacy fleets whose IAT systems fall short of the full requirement or where component/assembly-specific validation programmes (eg undercarriage or tail rotor) are required. The precise instrumentation solution, such as strain gauges or parametric systems, would then be agreed with the DO. Once installed, a permanent installation can be activated either continuously or as and when required.

93. The PT will normally task the DO to define the usage validation requirements and proposed methodolgy for meeting these requirements. Depending on when this task is initiated, it may be possible to obtain benefit from the knowledge and evidence gathered by the DO or Original Equipment Manufacturer during the structural design and qualification phase.

94. The complexity of Operational Loads and Usage Validation programmes and their management is such that PTs may appoint a project manager and form a dedicated working group including appropriate DO representation. Similarly, the Front Line Command may appoint the technical and operating unit project officers responsible for facilitating the programme on unit on a day-to-day basis.

95. Limited guidance on the specification of an Operational Loads and Usage Validation system can be found within Defence Standard 00-970, Part 1, Section 3, 19 Refer to MASAAG Paper 109 Guidance for Aircraft Operational Loads Measurement Programmes



Leaflet 38. Comprehensive guidance for planning and undertaking an OLM/ODR programme can be found within MASAAG20 Papers 10919 and 12021.

MDRE 96. Where the method of IAT cannot adequately characterise aircraft usage, a Manual Data Recording Exercise (MDRE) can be employed to record additional flight condition parameters during a representative sample of sorties. An MDRE involves an additional crewmember recording the occurrence and duration of all relevant flight conditions throughout each sortie.

97. The MDRE includes a sufficient number of sorties of each kind flown by the fleet to obtain statistically significant results. In lieu of an MDRE a suitable, automatic, means of recording usage data may be employed.

98. It is important that the F724 already includes an SPC column, so that the record of usage in-service can be validated by the results of MDRE and Operational Loads and Usage Validation.

Structural Sampling Programme 99. The Structural Sampling Programme (SSP) is part of the SEP (the derivation of the SEP is described on Page 15). A properly implemented Structural Sampling Programme (SSP) helps to mitigate the unquantified risk of critical structural failure from unanticipated causes. Sampling is applicable to all SSIs that are not otherwise examined during flight servicing or scheduled maintenance. Predominantly these will be items that have been sub-classified as ‘Not At Risk’ from AD/ED and demonstrated, by test or analysis, to be free from fatigue cracking during the expected service life of the aircraft. However, where there are also ‘At Risk’ SSIs and parts of AR structure that are not normally accessible for inspection, such as the inside surfaces of hollow flying control rods or torsion boxes, it is most important that these are be sampled regularly through the life of the fleet. Structural sampling, combined with the feedback from scheduled examinations, validates the scheduled examination element of the SEP. Furthermore, by examining the actual condition of all critical structure, the SEP validates the assumptions used to forecast, for the lifetime of the aircraft, the effects of usage and exposure to AD/ED, including the interaction of the different threats to SI. If structural sampling uncovers unexpected damage, Recovering, and/or Exploiting activities are employed. Unexpected damage may also indicate a lack of understanding of local in-service loading that could be addressed by OLM/ODR including targeted instrumentation to improve understanding and inform the development of effective repair or modification action. Non-SSI components are considered for sampling where failure would result in unacceptable economic or operational impact.

100. To achieve the necessary SI Validating objectives, the PT actively manages structural sampling so that the necessary sampling information is gathered and interpreted and any necessary follow-up action completed. Policy for sampling programmes is detailed in the Support Policy Statement of the aircraft Topic 2(N/A/R)1 and sampling activities, plus the resulting corporate knowledge gained from sampling, is to be captured in the SISD and the aircraft’s Topic 5V or equivalent. Where the SSP highlights significant deviation between expected and actual in-service damage, this information is to be fed back to the DO.

101. Structural sampling is planned so that by the time the fleet leader reaches 80% of its original design life, or revised life if less, examples of all parts of all SSIs have been examined either through scheduled maintenance or sampling. Where it is not possible to examine all parts of all SSIs, alternative approaches can be explored in conjunction with the DO to assess the condition of the SSIs. For example, if it is

20 Military Aircraft Structural Airworthiness Advisory Group, a subgroup of the Joint Air Safety Committee hierarchy 21 Guidance for Helicopter Operational Data Recording Programmes



physically impossible to access and inspect an SSI, the DO may be tasked to assess the risk associated with not being able to carry out inspection and propose alternative analyses or mitigations, including teardown. Some of the threats to SI, if not correctly anticipated by the RCM analysis, could result in indications early in the aircraft’s life (such as AD and ED), whereas others would not be apparent until late in the aircraft’s life. Therefore, it may be appropriate to sample at both points. The decision on how much further sampling is required will need to be determined through a regular review of the sampling results and the level of risk. Such analysis and decisions are examined periodically by the SI Working Group (SIWG) (see Page 30). It is advisable to continue the sampling process right up to the OSD, as the cumulative exposure to damage from AD, ED and the probability of unexpected fatigue cracking increases towards the end of the aircraft’s life. Creeping deferment of fleet OSDs has, in the past, also exposed shortfalls in the lifing of, for example, older modifications, which could have been picked up by sampling.

102. Candidate aircraft for structural sampling inspections are selected carefully to maximise the probability of finding any unanticipated damage that could affect the fleet. The probability of damage increases with exposure to the threats to SI. Therefore, aircraft that lead the fleet in terms of flying hours, FI, age, maintenance time or exposure time in corrosive or dusty environments, or environments significantly different from that assumed during development (eg, embarked ops), or that have experienced the worst combination of these factors, are selected for sampling. For example, corrosion arising rates by tail number may indicate the environmental exposure level. A statement describing how representative the chosen aircraft are of the fleet as a whole may provide useful evidence that thought has been given to where this relatively small sample size lies in the overall aircraft population. It is also useful to have a number of alternative candidate aircraft on standby to take full advantage of the available sampling opportunities. For removable components, the history and usage of the component is considered, since it may differ from the usage of the aircraft to which the component is currently fitted.

103. Where possible, sampling should be done within a scheduled maintenance activity, and should include feedback directly into the RCM analysis. The following opportunities are worth considering for implementing the structural sampling plan:

a. Planned sampling may usefully be carried out concurrently with scheduled maintenance on the selected aircraft, when access to the structure to be sampled is improved by the scheduled maintenance activity.

b. Opportunities for sampling inaccessible structure may arise from modification programmes and extensive repairs. PTs can take full advantage of those occasions when aircraft are sufficiently stripped to enable examination of normally inaccessible structure, or when individual aircraft, assemblies or major components are scrapped following major damage or life expiry. Although the aircraft concerned may not be the highest lifed within the fleet, value may still be obtained from opportunity sampling of these airframes. This may be in addition to, or instead of, sampling on the originally selected aircraft. A simple process to consider structural sampling requirements should be considered for inclusion in the instructions for repair scheme applications. Structural sampling opportunities should also be considered as part of the modification authorisation process (MOD Form 714 / 715 or equivalent). Unless absolutely necessary, SSP should not be undertaken in a first-line environment.

c. If there is no suitable maintenance opportunity for the selected candidate aircraft within the required timeframe and no emergent opportunity afforded by modification, incident or accident, it may be necessary to undertake sampling by directed inspections using an SI(T). Due to the large amount of preparatory work likely to be required to access the structure to be sampled, this option is likely to have the greatest adverse impact on aircraft availability.



104. Finally, there is little fundamental difference between sampling of critical structure and critical systems, and consideration can be given to harmonising these policies where possible, to avoid duplication of effort.

Teardown 105. A Teardown (also known as sampling and forensic examination) is defined as a progressive, detailed, controlled and destructive examination of aircraft structure. The teardown of a full-scale fatigue test specimen airframe at the conclusion of testing is normally required for completion of fatigue and Damage Tolerance qualification, and is therefore an SI Establishing activity. Teardown as part of the structural sampling programme involves teardown of the whole or selected parts of an ex-service aircraft or teardown of removable components or sub-assemblies. This may be the only way to achieve some of the structural sampling requirements and may be done on an opportunity basis. However, The same level of consideration as to where to focus the teardown is required as it is for structural sampling. Teardown may also be used to examine in detail particular areas of SI concern on an airframe and to develop and validate examination techniques. Teardown can also be used to provide on going confidence in repairs by removing them and inspecting the underlying damage site for evidence of any damage growth.

106. Teardown of a high-life aircraft is a useful tool to support life extension activities (see RA 5724(1) & (2)11).

107. Detailed guidance on the conduct of structural sampling and teardown can be found in MASAAG Paper 10522 and Teardown Handbook produced by the Technical Co-Operation Program (TTCP), and details of potential structural sampling areas, based upon the SSI list, are detailed in the Topic 5V or equivalent.

Maintenance Schedule review 108. The structural elements of the Maintenance Schedule for an aircraft type will normally be reviewed in the course of the Maintenance Schedule review; see RA 435123 for regulation on the conduct and timing of these reviews. The structural element of this review is an SI Validating activity. In-service SI arisings and changes of operating intent and usage are to be taken into account when reviewing the SEP elements of the Maintenance Schedule.

109. In reviewing the structural elements of the Maintenance Schedule, in-service SI arisings (including those from Topic 5V or equivalent reporting) and failures, results of sampling and teardown, plus the implications of major changes in fleet disposition, operating environment, roles and usage, are be taken into account. It is essential that schedule changes generated by the MOD or contracted schedule review body do not extend examination intervals derived from fatigue and Damage Tolerance analyses, unless supported by the DO. Moreover, it is also recognised that, for any scheduled structural examinations driven by fatigue considerations, historical examination results will be of very limited predictive value and therefore do not provide a sound basis for increased examination intervals; the absence of cracking to date may not indicate that it would be safe to adjust examination intervals. A further consideration is that schedule review activity is to be managed to ensure continuity in trend monitoring of SI arisings both before and after a schedule is revised. For example, Schedule Identification Numbers (SINs) for directed examinations of individual SSIs are usually left unchanged; however, if the SINs are changed, pre-and post-review SINs are to be tracked.

22 MASAAG Paper 105 Teardown Inspections – Guidance and Best Practice 23 Refer to RA 4351 Production and Maintenance of Maintenance Schedules



Ageing Aircraft Programme 110. Ageing Aircraft aspects should be considered as a through life programme with consideration given to ageing within the SISD. See RA 572324 for regulation on the Ageing Aircraft Audit. Relevant reading on the subject can be found on the MAA website.

Damage 111. Although damage may be detected on individual aircraft, it will usually be necessary to assess whether such occurrences have fleet-wide implications. An evaluation of the extent to which damage to a single aircraft may indicate the potential for similar damage to be present on, or likely to be sustained by, other aircraft in the fleet may need to be undertaken. For example, new arisings of AD/ED may be indicative of an emerging fleet-wide SI problem that can be addressed. For fatigue, the occurrence of unexpected cracking in an in-service aircraft may indicate that Safe Life, fatigue or Damage Tolerance clearances are grossly in error and a significant portion of the fleet may be at risk. Therefore, all instances of in-service fatigue cracking and damage to SSIs as identified in the Topic 5V or equivalent, plus previously unreported cracking and damage to other structure, may be brought promptly to the attention of the PT and subsequently to the DO. The DO alerts the PT to any potential read-across damage information from other operators of the same aircraft type.

112. Suspected structural damage that is not readily apparent may have occurred following a specific incident to an individual aircraft or operation of aircraft in an adverse environment. Recovery actions are prompted by, and dependent upon, the following triggering events:

a. Reported overloads (including exceeding RTS operating limits).

b. Reported lightning strikes.

c. Reported bird strikes.

d. Impacts to composite structure.

e. Exposure of structure (metal or composite) to any elevated temperatures that could cause degradation in material properties.

f. Exposure to corrosive substances:

(1) Caustic soda, acids, mercury, vehicle decontamination compounds, etc (see RA 450716).

(2) Bodily fluids (see RA 410325).

(3) Exposure to a maritime environment.

g. Operating in an environment that may invalidate the Maintenance Schedule, such as embarked operations.

113. As far as practicable, type-specific procedures are published within the ADS, detailing the action, including the appropriate inspections, to be taken following each of the events listed above. However, the impact of unusual or unexpected events will depend on the specific structure affected and specialist advice is to be sought from the DO. If the inspections confirm the presence of damage, the principles of recovering detected damage apply.

114. Arisings are reported to the PT and monitored by the SIWG.

24 Refer to RA 5723 Ageing Aircraft Audit 25 Refer to RA 4103 Decontamination of Aircraft after Spillage of Body Fluids



Lost usage data 115. If usage data is lost or not recorded at whole aircraft or lifed-item level, the trigger points for retirement of Safe Life structure or examination of Damage Tolerant structure become uncertain. Conservative assumptions are therefore to be made about the missing usage data. IAT programmes can be compromised by failures of monitoring equipment (such as an unserviceable fatigue meter, strain gauge, other sensor or data logger), or loss of sortie data (such as errors in post-sortie feedback recording, data corruption or transfer failure). Sorties where this occurs are classified as unmetered sorties.

116. Unmetered sorties are to be positively identified within the IAT record so that fill-in data can be applied and re-evaluated as necessary. This fill-in data is normally based on the mean usage rate for flying of a similar nature to that for which data is missing, multiplied by a standard scatter factor of 1.5. For example, for a fixed-wing aircraft structure lifed in FI, the fill-in FI for a missing sortie record could be based on the mean FI per flying hour rate for the same SPC, multiplied by 1.5. In most cases this factor will be conservative, resulting in a significant life penalty, but where usage is known to be highly variable, it may not be adequate. However, where more statistical evidence is available and more accurate fill-in data is required, the fatigue consumption rate equivalent to the 90th percentile of usage severity for similar sorties is to be used. Since this represents a loss of usable Safe Life, or a reduction in the time to the next Damage Tolerance examination, such loss of data is to be minimised.

117. The PT monitors the percentage of unmetered sorties and determines the extent and impact of the lost sortie information and the reason for the loss. Recovery action to prevent recurrence is to be carried out. Provided the data loss is not from the same aircraft or unit, or within a single or the most fatigue damaging SPC, the following percentages can be used as guidelines for the percentage of unmetered sorties on which the PT is to act: <2% acceptable, 2-5% further investigation required, >5% the structural airworthiness of the fleet is to be assessed and urgent recovery action undertaken if it is found that structural airworthiness has been compromised. The rate of, and recovery of, unmetered sorties is monitored by the SIWG. If the penalty fill-in life exceeds the Safe Life of the aircraft structure or lifed item, Exploiting measures can be employed.

Recovery 118. The need for repairs to meet extant design standards is especially relevant for ageing aircraft and for aircraft with multiple adjacent repairs.

119. The repair of detected damage to individual aircraft will be effected by undertaking repairs detailed within the ADS or schemes provided by either the Aircraft Repair Organisation (see RA 4815(2)26) or the DO; these processes are outside the scope of SI management and this RA. However, in order that structural configuration control is maintained, the extent of the damage and its repair is to be recorded on a suitable database. Refer to RA 440327 and RA 4815(2).

Changes to clearances due to Validating activities 120. Validation activities such as SOIU reviews and Operational Loads and Usage Validation may introduce changes to fatigue formulae or fatigue and Damage Tolerance clearances that may reduce available Safe Life or render Damage Tolerance examinations overdue. In the short term, such reductions may indicate that the risk of fatigue failure is higher than originally thought and, in extreme cases, that safe lives have been over-flown. Action is then required to recover SI to restore risks to As Low as Reasonably Practicable (ALARP) levels.

26 Refer to RA 4815(2) Procedures for Good Maintenance Practices 27 Refer to RA 4403 Expedient Repair



121. Once adverse lifing revisions have been verified, it is essential that the PT re-evaluate the position of the fleet in relation to Safe Life and/or Damage Tolerance clearances. Subject to the results of this re-evaluation, it will be necessary to quantify risk and generate recovery options. Options for Recovering of SI may include early retirement of the fleet or individual airframes, transfer to a Damage Tolerance (examination dependent) regime, additional testing, modification, repair or replacement, or the imposition of operating restrictions. These options are, as far as possible, to be based on quantitative analyses by the DO. In all cases, it is important that the risk presented is thoroughly assessed and the acceptance/mitigation endorsed by the SIWG and then accepted by the appropriate Letter of Airworthiness Authority holder. Furthermore, it will be important to address the understandable, intuitive reaction to under-estimate risk when there is a lack of physical damage on in-service aircraft.

Pre-emptive reinforcement 122. This section is being rewritten and has been temporarily removed. Interim guidance on this subject may be obtained by contacting [email protected].

Fatigue conservation 123. The rate of fatigue usage of an airframe will determine when the Safe Life is reached (for retirement or embodiment of a structural modification) or, for Damage Tolerant designs, the point at which the airframe becomes uneconomic to examine and repair. These safe or economic lives are frequently the constraint on the useful calendar life of a type and are therefore managed to achieve the required airworthiness, availability and capability until the required OSD. Whereas fatigue budgeting is an SI Sustaining measure, fatigue conservation is an SI exploitation measure that may be required if the fatigue budget would be insufficient to achieve the necessary OSD.

124. Fatigue-damaging loads on the life-limiting structure of fixed-wing aircraft are driven by key parameters such as normal acceleration, ground-air-ground cycle, pitch and roll accelerations, mass properties and time spent in buffet conditions. Life may be prolonged by identifying and avoiding, or only undertaking when absolutely necessary, particularly damaging manoeuvres or configurations. For a helicopter, the loading situation is more complex, involving high-cycle loading (from the rotating assemblies) and a low-cycle element (similar to fixed-wing manoeuvring and exposure to gusts), so the accuracy of the SPCs is particularly important. This makes it harder to identify the most damaging manoeuvres or types and phases of flight or configurations for a helicopter, but it may be possible to use knowledge of design spectra or MDRE, ODR or HUMS data to derive operating advice for fatigue conservation purposes.

125. The following procedure is to be used to implement fatigue conservation measures on a fleet or individual aircraft:

a. Identify shortfall in required fatigue budget.

b. Identify the most damaging manoeuvres/operations/role fits using, for example, the following information and techniques:

(1) Analyse Operational Loads and Usage Validation data, HUMS data and results from MDRE, as available, to identify the most significant contributors to structural life consumption.

c. In consultation with the DH, identify means of reducing or eliminating fatigue-damaging operating methods.

d. DH: promulgate and enforce the revised operating methods.



e. Review the effectiveness of the revised operating methods.

Cleared life 126. The potential requirement for a structures LEP should be identified in the SI Strategy and Plan from their inception. A decision point is to be marked and reviewed periodically so that any LEP work can be started and completed before the current planned OSD. Practically, it can take ten years between starting a major LEP and getting the evidence to give the required life clearances. The FTR and supporting technical reports are used as evidence for this activity. RA 572411 and Def Stan 00-970 Part 1 Section 3 (Structure) Leaflet 39 (Fatigue Life Extension) provides a good template for structures LEP activity, regardless of the design standard originally used for the aircraft

Use of other evidence

Risk analysis

Probabilistic SI management 127. These sections are being rewritten and have been temporarily removed. Interim guidance on these subjects may be obtained by contacting [email protected].

Practical management

SISD 128. Establishing the Strategy allows all SI stakeholders to underwrite the intended approach while maintaining visibility of the actions required. Moreover, the SI strategy will form part of an aircraft’s Safety Assessment evidence and TLMP. It is essential that development of a SI strategy to meet this Regulation is considered early in the acquisition cycle and well before the In-Service Date.

129. The SISD covers the following aspects:

a. Details of overarching strategy: what measures are considered necessary to support the fleet throughout its life and to address ESVRE activities, and details of any necessary deviations from MOD regulation.

b. Details of strategy implementation: an introduction, implementation of the strategy to ESVRE principles, the SI meeting strategy including frequency and membership, the SI Plan and a record of historic SI activities and decisions.

130. Compromised SI can affect an aircraft’s airworthiness, capability, availability and associated whole-life costs at any point in its service life. The aim in sustaining an SI Strategy is to maintain and promulgate the PT’s intended approach to implementing the required acquisition cycle and through-life SI management activities for an aircraft in terms of methodology, timescale and financial commitment. To achieve this, a PT develops a SI Strategy to ensure that there is a clear understanding across all SI Stakeholders of what is to be achieved and what the short and long-term objectives are in meeting the vision. Sustaining the strategy allows all SI Stakeholders to continue to update and underwrite the intended approach while maintaining visibility of the actions vital to maintaining structural airworthiness. The strategy is promulgated in an SISD, which includes an SI Plan. This communication is particularly important as it will form part of the platform’s Through Life Management activities and ultimately the associated Safety Assessment, and will also act as a record of the evidence and rationale behind the SI decisions, such as termination of full-scale fatigue testing, taken throughout the



life of the aircraft. Therefore, this document outlines the strategy and details the activities required to implement and maintain UK MOD SI regulation.

131. The SISD will include relevant items such as design philosophy, verification and validation approach, major modification and capability upgrade programmes, integration of new stores, change in fleet disposition, fleet draw-down and OSD plans.

132. The SI Plan needs to be integrated within the SI Strategy Document and the TLMP (see Figure 6). The SI Plan may incorporate all elements of the ESVRE framework or equivalent for SI management, identifying all of the SI activities planned to achieve the necessary Structural Airworthiness, capability, availability and cost, until the funded OSD. It may be useful to mirror the RA headings of RA 5720 plus selected relevant additional line items such as major modification and capability upgrade programmes, integration of new stores, changes in fleet disposition, fleet draw-down and OSD plans. Accordingly, the SI Plan is owned by the PT and made available to all SI Stakeholders.

Airworthiness Strategy

System Integrity Strategy Propulsion Integrity Strategy

Structural Integrity Strategy

Safety Case Through Life Management Plan

Structural Integrity Plan

Figure 6 SI document hierarchy 133. It is essential that development of an SI Strategy is considered early in the acquisition cycle and well before the aircraft is in service. The SISD and SI Plan are living documents and should be reviewed periodically by structure specialists in the SIWG. They should incorporate all elements of the ESVRE framework for SI Management, broken down into individual programme, recurring and one-off activities, key milestones and decision points, as appropriate to the position of the aircraft type in the project life cycle. Detailed guidance on the content and approach required of the SISD may be found in the MAA SISD Template28.

134. SI planning is a continuous activity that ensures that all SI-related activities are captured, coordinated and adequately resourced.

SIWG 135. The SIWG forms a fundamental part of the governance of the through-life safety management of the Air System. At the top level, the SIWG should keep in mind questions that define its aim:

a. Is there sufficient evidence to prove that, at current and forecast usage rates, the platform will safely achieve its OSD? If not, what do we need to do to address this?

28 Structural Integrity Strategy Document Template



b. How do we ensure that risks to air safety arising from the four main threats to SI are managed, tolerable and ALARP?

136. The PT is to have processes that enable it to monitor, measure and sustain SI. The effectiveness of these processes is ratified at the SIWG. Although executive airworthiness responsibility rests with the TAA, there are a number of stakeholders in SI Management who will contribute to the SI decision-making process. A SIWG is a suitable forum for discussing a SI Strategy, structural issues and risks.

137. The SIWG is to be sufficiently ahead of higher level meetings (Project Safety Panel, etc) to allow SI risks to be raised at these forums.

138. SIWGs should be initiated by the PT at an appropriate time prior to the In-Service Date and in consultation with MAA Cert S&ADS. In multi-national projects, although multi-national forums may be established to progress SI issues common to the partner nations, it is likely that a UK-only SI meeting structure will also be necessary to progress national issues associated with UK configuration, usage and operating practices and SI standards.

139. Current risks to Structural Airworthiness are to be discussed by all SI stakeholders in sufficient detail to allow the LoAA holder to understand the current level of risk. Risks are to be formally recorded in order to provide an audit trail for SI decisions. Any Structural Airworthiness risk identified that is potentially generic in nature or broad-based is to be raised by the PT at the appropriate Airworthiness Management Group.

140. A SIWG 'Aide-Memoire' is available at Annex A.

Supporting information

Suggested SI training 141. The following training courses may be suitable for staff involved in SI Management. Applications can be made via the MAA website.

a. Aircraft Structural Integrity Course (ASIC) (MAA).

b. Airworthiness of Military Aircraft Course (DA-CMT).

References and further reading MAA regulation and further reading on SI can be found on the MAA website at www.gov.uk/maa

AAPWG Paper 10 – A Framework for Ageing Aircraft Audits

Aircraft Topic 15S – Statement of Operating Intent and Usage.

Aircraft Topic 5V – Sampling Requirements and Procedures.

Aircraft Usage and Validation Programme

AOF – Aquistion Operating Framework for further information on Risks and Hazards Via the Defence Intranet

AP 101A-0601-1 – Employment and Repair of Aircraft Composite

AP 101A-1500-0 – Joint Service Aircraft Battle Damage Repair Manual.

Def Stan 00-970 Pt 1 Sect 3 Lft 36 (Structural Inspection Programme)

Def Stan 00-970 Pt 1 Sect 3 Lft 37 (Ensure Fatigue Qualification Testing covers fleet usage to OSD)

Def Stan 00-970 Pt 1 Sect 3 Lft 38 (Conduct a continuous or periodic OLM/ODR programme throughout aircraft life)


http://www.gov.uk/maa


Def Stan 00-970 Pt 1 Sect 3 Lft 38 (Fatigue and Usage Monitoring and HUMS. Assess Fleet Leader status and need for budgeting)

Def Stan 00-970 Pt 1 Sect 3 Lft 38 (SOIU Review)

Def Stan 00-970 Pt 1 Sect 3 Lft 39 (Life extension measures)

Defence Standard 00-970 – Design and Airworthiness Requirements for Service Aircraft.

Guidance for Helicopter Operational Data Recording Programmes, Terry, G. K., Reed, S. C., and Perrett, B. H. E.

Handbook of Best Practice in Teardown of Aircraft Structures, Second Edition, TTCP, Sep 2011

JAPD-100C-20 – Preparation and Amendment of Maintenance Schedules.

JAPD-100C-22 – Procedures for Developing Preventive Maintenance.

JSP 886 – The Defence Logistics Support Chain Manual

MAA Structural Integrity Strategy Document Template

MAA02 MAA Master Glossary

MASAAG Paper 104 – Recommendations for the future shape of the Ageing Aircraft Structural Audit

MASAAG Paper 105 – Teardown Inspections - Guidance and best

MASAAG Paper 106 – Repair Assessment Programme for Military Transport Aircraft

MASAAG Paper 109 – Guidance for Aircraft Operational Loads Measurement Programmes.

MASAAG Paper 116 – Widespread fatigue damage in military aircraft

MASAAG Paper 118 – Interaction of Corrosion and Fatigue

MASAAG Paper 123 – Development, Validation, Verification and Certification of Structural Health Monitoring Systems for Military Aircraft

Materials.

practice.

RA 1003 – Managing the Delegation of Safety and Airworthiness Responsibilities.

RA 1210 Ownership and Management of Operating Risk (Risk to Life)

RA 1500 Certification of UK Military Registered Air Systems

RA 4103 Decontamination of Aircraft after Spillage of Body Fluids

RA 4256 Aircraft Weighing

RA 4350 Through Life Management of Technical Information

RA 4351 Production and Maintenance of Maintenance Schedules

RA 4403 Expedient Repair

RA 4500 Health and Usage Monitoring

RA 4507 Aircraft Environmental Damage Prevention and Control

RA 4815(2) Procedures for Good Maintenance Practices

RA 5309 Fatigue Type Record for Aircraft

RA 5720 Structural Integrity Management

RA 5723 Ageing Aircraft Audit

RA 5724 Life Extension Programme

RA 5725 Out of Service Date Extension Programme




Classic articles (available to ASIC delegates):

Managing an Ageing Aircraft Fleet, Wg Cdr Andy March

Developing Issues in Aircraft Structural Integrity Wg Cdr Ron Eckersley, (Ex WC SSG)

Helicopter Fatigue and Qualification Case History, M Overd, Westland Helicopters Ltd

Helicopter Structural Integrity, Sqn Ldr D Thomas

Fibre-reinforced Polymer Composites, Dr D Bray, DSGT, RAFC Cranwell

The Hawk Life Extension Programme, Wg Cdr M J Kilshaw (adapted from original articles)

Future Fatigue Monitoring Systems, Sqn Ldr S Armitage, (ex SM17b (RAF)) and D M Holford, DERA

Fatigue Management – Current RAF Practice, the Future and some Case Studies, Sqn Ldr S Armitage

Tornado Structural Integrity, Sqn Ldr M A Sibley (adapted from original paper)

Eurofighter Typhoon – Production Major Airframe Fatigue Test (PMAFT), M.D. Greenhalgh, BAE Systems

Structural Airworthiness Requirements for Aircraft Fatigue Design, Alison Mew, QinetiQ(F)

Ageing Aircraft in Military Service, Sqn Ldr N Lea

Economic Benefits of Usage monitoring in UK Military Rotary Wing Aircraft Sqn Ldr L Sumner

Aviation Occurrence Reporting CAL Boeing 747 Accident Report (ASC-AOR-05-02-001) – Executive Summary, ASC

Repairs to Damage Tolerant Aircraft, T Swift (FAA)

Safe Life Scatter Factors – Their Origins, Applicability and Opportunities for Exploitation, Sqn Ldr T Bleakley

The Structural Audit – A Positive Contribution To The Sustainment Of Airworthiness And Safety, M J Duffield – Systems & Structures Airworthiness – Structural Integrity (Large Aircraft) QinetiQ(F)

The Nimrod Review https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/229037/1025.pdf

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/229037/1025.pdf

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/229037/1025.pdf


ANNEX A SIWG AIDE-MEMOIRE Version 1.1

1 Introduction a. Introduction.

b. Actions arising from previous minutes

and not covered under agenda items.

The SIWG forms a fundamental part of the governance of the through-life safety management of the Air System. At the top level, the SIWG should keep in mind 2 questions that define its aim:

a. Is there sufficient evidence to prove that, at current and forecast useage rates, the platform will safely achieve its OSD? If not, what do we need to do to address this?

b. How do we ensure that risks to air safety arising from the 4 threats to structural integrity are managed, tolerable and ALARP?

2 Fleet Planning

a. Fleet Statistics.

b. Fleet Size.

c. Fleet Disposition.

Check on consumption statistics (e.g. FI/1000hr, landings/hr – whatever is important for that type of platform) looking for significant changes. Stats need to include historic or design assumptions for comparison. Average data can be misleading if fleets-within-fleets issues exist.

Review of how many aircraft there are in the fleet and how they are operated – does this reflect design assumptions or has Designer been made aware of variations.

Identification of fleets-within-fleets issues. Identification of changes in bases or operations (e.g. rough strip).

3 Establishing SI

a. SI Strategy Document and SI Plan.

b. SOI.

Do SI Plan and Strategy exist? Make sure you have read the Plan and Strategy. Are they just repeats of the template or have they been thought about? Has the Designer been consulted / included in the Strategy / Plan? Post-RTS and after the SI Strategy and SI Plan have both been established this agenda item may be moved to Sustaining SI, where the discussion should focus on when the Strategy and the Plan were last updated and whether they still deliver a regulatory compliant and suitably safe outcome (eg in light of updated or new regulation, or in light of in-service findings necessitating a change to the strategy).

Has the SOI been produced? Alternatively, when is it planned to be published? Has Requirements Manager been heavily involved in SOI development. Has

34


the Designer been tasked to review and identify implications for lives. This agenda item can be marked as N/A if the SOI has been updated to become an SOIU.

c. Release to Service – SI aspects.

d. Structural Design Certification:

(1) Classification of Structure & review of SSI list.

(2) Static Type Record.

(3) Fatigue Type Record.

e. Fatigue Test Clearances.

f. SARC Assessment.

This should cover both the initial RTS, with specialist advice from the SI Advisor, and ongoing RTS amendments (with implication for SOI/U). Looking for significant issues or shortfalls (e.g. limited clearances or envelope restrictions – looking for coherent plan to get to full RTS). Also are there any RTS issues that will require validation in service? For example, probabilistic gust or birdstrike analysis will require validation from in-service data. What is the structural design certification basis for the aircraft (e.g. stress-life, strain-life, damage tolerance)? Where is this defined and recorded (e.g. designed / certified to Def Stan/EASA/FAA/BCAR/JSGS etc)? Does the platform have a clearly identified SSI list (or eqivalent) that has credibility and has been reviewed, involving Designer? If not, what plans are in place to develop list? SSIs are required for developing RCM-based maintenance schedules. Does the platform have an up-to-date and credible STR? If not, what documents constitute the equivalent and are they identified and controlled in the same way. Are processes in place to ensure that modifications likely to affect the STR or equivalent are identified and STR amendments actioned? Does the platform have an up-to-date and credible FTR? If not, what documents constitute the equivalent and are they identified and controlled in the same way. Are processes in place to ensure that modifications likely to affect the FTR or equivalent are identified and FTR amendments actioned? What plans are in place for incorporating OLM/ODR/IAT results into the FTR (or equivalent) to move from design assumptions to actual usage for lifing. What are the fatigue clearances for the fleet? Where are they promulgated and how are they managed in service. What plans are in place to take interim fatigue clearances to meet the life-time requirements. Are adequate plans in place for ongoing fatigue tests or tests likely to be needed to generate lifing evidence? Red/Amber/Yellow/Green. Review previous meeting’s score (and the reasons for that score if not green) to ensure consistency and to monitor progress of long-running issues.

35 Structural Integrity Working Group – Aide-Memoire v1.1


4 Sustaining SI a. Usage Monitoring – method of Individual

Aircraft Tracking (IAT); fatigue monitoring/budgeting/formulae; structural and transmission Health and Usage Monitoring Systems.

b. Structural Examination Programme (SEP) (to include discussion on structural examination to counter threat posed by hazardous incidents, corrosion, erosion and other environmental degradation).

(1) SEP Reporting Process

(Topic 5V).

(2) Scheduled maintenance plan (Topic 5A1).

c. Structural Configuration Control.

d. OSD.

e. Obsolescence Management Plan

Does platform have a clear plan for usage monitoring / IAT/ HUMS? This has been an area of weakness on many platforms. Often too much focus on ‘fitting systems’ and not enough on what is going to be done with the data and through life management of the system, data and results and what are the aims of IAT/HUMS ? Where FMF are used how are they validated and reviewed? This should include measures to identify when the IAT has been compromised (e.g. excessive un-metered rates or system failures) and plans to rectify the situation if this occurs. Is there sufficient fatigue life (however it is calculated) remaining to see the fleet through to its OSD?

How has the SEP been developed? Was RCM used in conjunction with an up- to-date SSI list? How is the PT going to review the SEP and ensure that it is fit for purpose? – reliance on F765s alone is not sufficient.

Does a Topic 5V exist? If not are there plans in place to develop one, in conjunction with Designer? How will it be done and how does the PT intend to exploit the 5V to support SI (e.g. sampling opportunities)? Also did the 5V development identify any shortfall in existing SEP (e.g. hard to inspect or un- inspectable at risk SSIs)?

How has the maintenance schedule been developed? Has the Designer been fully involved in its development? Also ensure that mechanism is in place to ensure that structural inspections are undertaken by authorised tradesmen (MAP-B473).

Historically this has been a challenging issue. What measures are in place to retain config control and how are these measures going to be validated to ensure they are working (e.g. periodic mod and repair checks at deep maintenance)?

What is the declared OSD for the platform, how does this relate to the Design Service Goal (DSG) and when will any potential LEP requirements be identified? Creeping life extension should be avoided.

Are there any structural materials or coatings used on the ac type that are obsolete/obsolescent? If so, how are repairs to structural items made from obsolete/obsolescent materials going to be performed and what plans are in

36


f. SIWG Frequency and Composition

g. SARC Assessment.

place for identifying and approving new coatings.

Does the SIWG take place twice yearly? Are the key stakeholders noted in RA5720(3) present? Does the Chairperson’s LOAA allow him/her to chair SIWGs?

Red/Amber/Yellow/Green. Also review previous meeting’s score (and the reasons for that score if not green) to ensure consistency in the scoring and to monitor progress of long-running issues.

5 Validating SI a. SOIU Review.

b. Operational Loads and Usage Validation.

c. Structural Sampling.

d. Teardown.

e. Maintenance schedule review (Including Topic 5V returns).

When were the last annual and triennial SOIU reviews started and, therefore, when are the next reviews due to begin? What plans are in place for the next reviews? Are adequate data collection and analysis tools in place to collate the data for 3-yearly review? What calendar period does the useage data come from? Does the review also include updating the ‘Intent’ element? What mechanisms are in place for Designer to analyse the revised SOIU and report back on lifing implications in context of the DSG?

What OLM/ODR/MDRE is planned or underway? This section should include reports from OLM/ODR/MDRE Working Groups. Encourage identification of likely OLM/ODR/MDRE validation requirements early in the programme. Additional OLM/ODR/MDRE requirements may still be required for ac with strain gauge-based monitoring systems due to incomplete coverage (e.g. landing gear). May also require validation of RTS assumptions if not possible by other means (e.g. time at flight-level for birdstrike or gust risks).

What sampling has been done and what is planned? Opportunity sampling can prove extremely valuable and does not have to be restricted to Cat5 aircraft. What can be done alongside mod programmes? Use Topic 5A1 to identify candidates. Particular focus should be on any ‘at risk’ SSIs that are not inspectable (e.g. inside flying control rods).

As per structural sampling. Invaluable particularly for validation of corrosion protection / prevention. Successful programme requires careful thought as to targeted areas. Requires Designer involvement and also benefitted by involvement from other users (particularly if older fleets exist elsewhere).

What plans are in place for review of the schedule? How will the schedule review be done? What data are available and is the data considered for purpose? Is the Designer heavily involved in this process and are fatigue-life related schedule elements clearly identified.



f. Review of Type Records.

g. NDT Technique Review

h. Ageing Aircraft Measures.

i. SARC Assessment.

What plans are in place for the review of type records (or documents identified as meeting these requirements)? Should be linked in with fatigue test completion, OLM/ODR programmes, mod programmes etc. STR sometimes forgotten as in-service focus is on fatigue – care needs to be taken over mass growth (often incremental).

NDT review should be about both whether the techniques are still required, and whether they are still appropriate. Techniques are often introduced rapidly in response to an in-service or fatigue test arising and should be reviewed if they are going to be part of a longer-term SI measure.

When was previous AAA started and when is next planned? How are issues identified by AAA managed within the platform and what measures are in place to ensure the completion of remedial measures is endorsed by the SIWG? Again important that the Designer is involved in this process (see RA5723 for background information).


6 Recovering SI a. Structural Problems.

(1) Review Contents of Structural Arising Database.

(2) Review EDPC WG/Database.

b. Modifications with SI implications.

c. Special Instructions Technical SI(T) with SI implications.

d. Repair Assessment Programme

Identification of structural issues and recovery plans.

Report from EDPC WG. Should include issues taken on by SIWG and directives back to EDPC WG. Confirm that there is an ED Prevention and Control Programme in place and that this has been reviewed by SMEs at SIWG (including Designer) and that issues and progress are tracked by SIWG.

Review of recent modifications to confirm that SI implications have been identified and addressed (e.g. mass growth (global or local), changes in flight control system, reduced access for maintenance etc).

Review of tech instructions issued to ensure SI implications identified and addressed (e.g. removal of surface finish, making access for inspections).

Where applicable, what RAP surveys have been undertaken and what is

38


(MASAAG 106)(mandatory for large ac only).

e. Compromised IAT.

f. SARC Assessment.

planned? How is the information being preserved and updated? Has the Designer reviewed the RAP reports and have remedial actions been implemented (e.g. fatigue analysis of repairs undertaken and inadequate repairs replaced with adequate repairs)? What measures have been implemented to prevent reoccurrences in the future (e.g. revised procedures, cancellation of inadequate repair instructions or removal of inadequate repairs from the Topic 6).

It is likely that an IAT system will be compomised at some time during the life of the fleet, particularly (but not exclusively) with modern electronic structural health monitoring systems. What is the percentage of compromised IAT data? Are there any trends that could identify a significant issue for collecting IAT data (eg large percentage of compromised IAT data from a single airframe)? What is the gap-filling method and does it comply with the regulation?


7 Exploiting SI a. Review of Structural Hazards or Risks.

b. Fatigue Conservation Measures.

c. Exploiting In-service experience.

d. Life/OSD Extension Programme.

Identify if any of the discussions during the meeting invalidate the structural hazards/risks that already comprise the loss model, meaning that those hazards/risks need to be reconsidered in light of new evidence, or whether new hazards need to be considered.

If it has been identified that there is insufficient fatigue life (however it is calculated) remaining until the OSD, what measures could be introduced to ameliorate the situation? For example, VC10 adjusted the aileron upset angle to reduce wing bending. RW platforms could extend cleared safe lives of some rotables by reducing the time spent in hover. Historically measures have been introduced too late and they have been less successful than anticipated. Therefore this should include methods of monitoring fatigue conservation measures.

Should include any relavant experience from other users or similar types. Reports from User Groups on SI issues can be highly relevant.

Identification of plans for LEP. Historically often identifed too late. Significant LEP should be idenfied 10 years before OSD. Deadlines for decisions should be identified in Strategy/Plan. Often an issue for elevation above SIWG.



40

e. Exploitation of OLM/ODR/HUMS/SHM

/MDRE data.

Data can be essential in meeting OSD or in LEP. Plans to exploit data to convert from design assumed usage to actual usage should be identified.

8 Ac Disposal Plan

a. Aircraft Disposal Plan Potential source of structural sampling / teardown to support remaining fleet.

9 AOB a. AOB 10 SARC Score a. Overall SARC Assessment. Red/Amber/Yellow/Green. Also review previous meeting’s score (and the

reasons for that score if not green) to ensure consistency in the scoring and to monitor progress of long-running issues.

11 New SI Risks a. Consideration of new SI Risks that should be registered within Risk Management system

Formal pull through of SI issues identified as structural issues in the SIWG into the risk register (where not already identifed as entries)

12 Upward Reporting

a. Actions from and reports to higher fora.

Should include significant risks (technical and programme (e.g. LEP)) and significant non compliances with RA

13 Next Meeting a. Arrangements for next meeting


SI Glossary Version 1.1 41

ANNEX B GLOSSARY AND ABBREVIATIONS

Readers are advised to consult MAA02 for the most current definitions relating to Air Safety. The following list is offered as an additional resource.

Term Abbreviation Definition

Naval Service Modification (N)SM (MAA02 Issue 5: The obsolescent RN terminology for a Service Modification. Regulations for NSMs are given in AP100N-0140 Ch 18.)

First Line 1st Line Obsolete. Refer to MAA02.

Second Line 2nd Line Obsolete. Refer to MAA02.

Third Line 3rd Line Obsolete. Refer to MAA02.

Fourth Line 4th Line Obsolete. Refer to MAA02.

724/725 (MOD Forms 724/725) Hard copy data collection forms used to record sortie details including mass, fuel, stores, duration, landings and counts from fatigue meter (qv) if fitted. The data recorded on the F725 was the original basis for fatigue calculation in an MOD central computer.

Armament and Explosive Regulations

A & ERs

Ageing Aircraft Audit AAA provides assurance that the Structural Integrity, System Integrity and Propulsion Integrity, and hence the airworthiness risks, of a fleet’s aircraft are being managed appropriately from the perspective of ageing.

Aircraft Armament Electrical Installation

AAEI Refer to MAA02.

Aircraft Assisted Escape System AAES Refer to MAA02.

Aircraft Armament System AAS Refer to MAA02.

Ageing Aircraft Structural Audit AASA Subdivision of an AAA, considering the failure modes and airworthiness consequences related to aircraft structure.

Aircraft Armament Suspension Equipment

AASE Refer to MAA02.

Aircraft Charge Certificate ACC Refer to MAA02.

Accident In specific circumstances, an occurence that is not repairable by the aircraft maintenance staff at the aircraft operating location. (MAA02 Issue 5: An occurrence which results in: a person being killed or suffering a major injury or an aircraft sustaining Category 4 or 5 damage. With respect to Aviation Risk Management an accident is the realization of the Hazard becoming a harmful outcome.)

Aircraft Control System ACS Refer to MAA02.

Aircraft Cross-Servicing ACS Refer to MAA02.

Active Control of Structural Response

ACSR

Accidental Damage AD The physical alteration of an item (or its surface protection where applicable) caused by contact, impact or interaction with an object, which is not a part of the aircraft, or by the result of human error during manufacture, operation or maintenance of the aircraft. Often used with ED (qv) when making assessment to determine suitable design approach or maintenance policy for specific component.

Assistant Directorate Engineering Policy

AD Eng Pol Obsolete. Refer to MAA02.

Assistant Directorate Forward Support

AD FS

Assistant Director/Airworthiness, Design Requirements and Procedures

AD/ADRP

Accidental Damage/Environmental Damage

AD/ED See Accidental Damage; Environmental Damage.

Automatic Direct Exchange Scheme

ADES Refer to MAA02.

Anti-deterioration maintenance ADM Refer to MAA02.

Aircraft Document Set ADS The documents that have a prime airworthiness function for each aircraft type. They include the Release To Service (RTS), Air System Safety Case, Aircraft Maintenance Manual (AMM), Operating Data Manual(ODM), Flight Reference Cards (FRCs), Support Policy Statement, Engineering Air Publications (including the Flight Test Schedule (FTS)) and the Statement of Operating




Intent and Usage(SOIU). The documents comprising the ADS may be held electronically. (MAA02 Issue 5)

Air Engineer Officer AEO Refer to MAA02.

Aeroelastic (effect) Process where aerodynamic forces on a structure cause it to twist or bend thereby modifying the forces and, in turn, the twisting or bending. Resultant effects may be aeroelastic divergence (qv), steady aeroelastic distortion of the structure, or oscillation. The oscillations may be damped and stable, neutrally stabile (Limit Cycle Oscillation, LCO qv) or divergent (Flutter qv.).

Aeroelastic Divergence Specific case of aeroelastic (qv) deflection of the structure when the deflection caused by aerodynamic forces acts to increase those forces, leading to increasing deflection of the structure until it fails.

Automatic Flight Control System AFCS

Aerospace Ground Equipment AGE

Aircraft Ground Engineer AGE

Above Ground Level AGL

Aircraft Integrity Monitoring Equipment Committee

AIMEC (MAA02 Issue 5: The committee responsible for provisioning AIME.)

Airframe Structure qv that defines and supports the aerodynamic profile of an aircraft.

Airworthiness (MAA02 Issue 5: The ability of an aircraft or other airborne equipment or system to be operated in flight and on the ground without significant hazard to aircrew, ground crew, passengers or to third parties; it is a technical attribute of materiel throughout its lifecycle.)

As Low As Reasonably Practicable

ALARP As Low as Reasonably Practicable (Risk). The principle for Safety management in the MoD. Refer to MAA02 for a detailed definition.

Airworthiness Management Group AMG 6-monthly meetings, structured around Combat Air, Air Support and the Helicopter Clusters, attended by PTLs and specialists to discuss platform-specific and cross platform airworthiness issues.

Aircraft Maintenance Manual AMM

Artificial Neural Network ANN An Artificial Neural Network (ANN) is an information processing paradigm that is inspired by the way biological nervous systems, such as the brain, process information. The key element of this paradigm is the novel structure of the information processing system. It is composed of a number of highly interconnected processing elements (neurones) working in unison to solve specific problems. ANNs, like people, learn by example. An ANN is configured for a specific application, such as pattern recognition or data classification, through a learning process.

Air Navigation Order ANO Statuary Rules for air navigation of civil aircraft in UK.

Aircraft Operating Authority AOA Refer to MAA02.

Aircraft Post Crash Management APCM Refer to MAA02.

Auxiliary Power Unit APU Refer to MAA02.

At-Risk AR A term used in the assessment of structure in its vulnerability and susceptibility to AD/ED.

Armament Role Change Harness ARCH Refer to MAA02.

Airborne Radio Installation ARI Refer to MAA02.

Aircraft Repair Organization ARO Refer to MAA02.

Aviation Safety Implementation Group

ASIG Consists of representatives from all aspects of military aviation across the 3 services, DLO and DPA who have authority to implement policy.

Aircraft Structural Integrity Program

ASIP As used by the US Department of Defense, an organized and disciplined approach to ensure the achievement of the desired level of structural safety, performance, durability, and supportability with the least possible burden throughout the aircraft’s design service life (refer to MIL-STD-1530C(USAF)).

Asymmetric Usage Aircraft are generally symmetric (left and right sides) and the structural usage on each side is normally balanced. However, some manoeuvres are asymmetric (rolling, turning, yawing) and loading depends on the direction of motion. Bias towards one loading direction may arise.

Audit Team Leader ATL

Airworthiness Flying Limitation AWFL Document that contains flight limitations, to which the aircraft is flown. It is normally issued by the Designer and may give the full intended clearance rather than something less. It is used to give clearance for flight trials so that an RTS can be issued.

Bending Moment The reaction induced in a piece of structure subject to bending,




measured as a force multiplied by a distance. For example, the lift forces on a wing create a bending moment that is greatest at the wing root.

Bending Moment Relief Alleviation of aerodynamic or inertia-induced tensile stresses in a structure by opposing inertia or aerodynamic forces. Sometimes employed as a fatigue reduction measure when outboard wing stores or aerodynamic control devices are fitted/used to reduce fatigue stresses in wing root/centre fuselage structure.

Built-In Test Equipment BITE

Basic Level Budget BLB

Brinelling Engineering term for the indentation of raceways by ball/roller bearings that are overloaded causing plastic deformation. Note “False Brinelling” which has the appearance of true brinelling but is caused by wear.

Brittle Failure Fracture of a material with little or no prior deformation.

Buffet When the airflow over a structure is no longer smooth but becomes unstable, often characterised by random vortices, the forces on the structure start to fluctuate. Often occurs at high angles of attack or when airbrake, spoilers etc are deployed.

Buzz A high frequency alternating loading, including oscillating shock waves, on a strut, etc that can cause it to resonate at its natural frequency and may cause fatigue failure.

Barely Visible Impact Damage BVID Term used in relation to composite structure where little or no damage is visible to the eye, but significant sub-surface damage exists.

Centre of Gravity C of G or CG The point of equilibrium. Normally, only the fore and aft (longitudinal) location of the centre of gravity is important for symmetrically loaded aircraft. If the lateral, or vertical position of the centre of gravity is likely to have any material effect, reference to it will be made in the Weight and Balance Data of the Air Publication concerned and, when applicable, on the Trim Sheet. (MAA02 Issue 5)

Civil Aviation Authority CAA Refer to MAA02.

Controller Aircraft Release CAR Obsolete term used to indicate that an aircraft had been cleared for service use within specified limits, comparable with the civil Type Certificate (qv). Replaced in 1996 by term MCofA but quickly changed to Military Aircraft Release (MA Release) to prevent confusion with civil terms.

Condition Based Monitoring CBM

Configuration Control CC Refer to MAA02.

Centre of Pressure A theoretical point where the aerodynamic force is considered to act on an aircraft, wing, etc at a specific speed and configuration. Point will move backward or forwards in different situations.

Crew Escape System CES Refer to MAA02.

Carbon Fibre Reinforced Plastic CFRP

Communications and Information Systems

CIS Refer to MAA02.

Clearance The limitation in terms of percentage maximum load for static loading or life expressed in either flying hours, landings or FI for fatigue loading, imposed on a structure pending further analysis/testing to prove full load or life. Full clearances with no limitations are issue don successful completion of all testing/analysis.

Cleared Life Interpretation of qualified life for a particular aircraft or group of aircraft, taking into account actual service usage and variability effects.

Contractor Logistic Support CLS Refer to MAA02.

Configuration Management CM Refer to MAA02.

Configuration Management Plan CMP Refer to MAA02.

Contractor-Owned Contractor-Operated

COCO Refer to MAA02.

Cold Working of Holes Creation of residual compressive stresses in surrounding parent material rendering externally applied tensile load cycles less damaging in fatigue. May be used to enhance structural features subject to fatigue.

Civil-Owned Military Aircraft COMA Obsolete. Refer to MAA02.

Component Life Register The CLR is part of Topic 5A1, the Master Maintenance Schedule. The CLR is a list of components that are replaced out-of-phase with scheduled maintenance.

Civil-Owned Military-Registered COMR Obsolete. Refer to MAA02.




Communications Security COMSEC Refer to MAA02.

Concession (Sometimes called a Build Concession or, in the US, an Engineering Disposition). A departure in production, modification or repair from that laid down in the design standard which is subsequently underwritten by the Design Organization's quality organization and the customer. Categorized as Major or Minor or Cat 1, 2 or 3. Major Concessions are granted on MOD Form 77 (UK only) and could affect safety, fatigue, etc. A Minor Concession may be recorded by the manufacturer, providing the affected part complies with static strength, fatigue life, fit, form and function, and interchangeability requirements. Concessions may have to be reconsidered if fatigue lives are extended.

Configuration (MAA02 Issue 5: The functional and physical characteristics of materiel as described in technical documents.)

Corrosion The deterioration of a material or its properties due to the reaction of that material with its chemical environment. Usually associated with the chemical degradation of metals as they invariably attempt to revert to an ore-like, oxide state at a lower energy level.

Corrosion Control Register Method of tracking corrosion history of individual aircraft and entire fleets by recording of the pre- and post-corrosion-removal states in a book, file or database. The contents of the register are then subject to periodic review by the PT/Service Provider.

Corrosion Fatigue Failure of a metallic component caused by the simultaneous effect of fatigue and corrosion. Not to be confused with Stress Corrosion Cracking (qv).

Control of Substances Hazardous to Health

COSHH Refer to MAA02.

Crack Stopper Hole A hole drilled in the skin of a structure at the tip of a crack, intended to blunt the crack, reduce the stress concentration and thereby reduce the tendency of the crack to grow.

Creep Failure Excessive deformation or distortion of a material at high temperatures, without fracture occurring.

Critical crack size The crack size associated with the onset of rapid, unstable crack growth under application of load. This is further defined within Def Stan 00-970, Pt1, Sect 3, Lft 36 – ‘Fatigue Inspection Based Substantiation’ as: The crack size associated with the onset of rapid, unstable crack growth under application of load corresponding to the residual strength requirement. (MAA02 Issue 5: The length of a crack for a given stress at which unstable crack growth will occur.)

Customer Supplier Agreement CSA Refer to MAA02.

Configuration Status Record CSR Refer to MAA02.

Contingency Maintenance CTM Refer to MAA02.

Corporate Technical Services CTS

Clearly Visible Impact Damage CVID Term used to describe damage to composite structure that can be identified with the naked eye.

Cockpit Voice Recorder CVR

Convergence Working Group CWG

Contractor's Working Party CWP

Design Authority DA Obsolete. Refer to MAA02.

Design Approved Organisation Scheme

DAOS A system whereby organizations that have been assessed as competent to design and supply equipment to a satisfactory standard are approved by the MAA. (MAA02 Issue 5)

Defence Aviation Repair Agency DARA Obsolete. Refer to MAA02.

DASB Obsolete. Refer to MAA02.

Defence Aviation Safety Centre DASC Tri-Service MOD Organisation that co-ordinates UK Military Flight Safety policy, awareness and records, for all UK registered military aircraft.

Defence Aviation Safety Management System

DASMS – Has a policy that risks must be identified and their likelihood and severity evaluated before their tolerability can be judged.

Design Change Proposal DCP Usually follows a Production Permit (qv).

Declaration of Design and Performance

DDP This document is the design certificate for the Tornado. Can be for the Equipment, Major Assembly or the Aircraft.

Debonding Failure of a glue bond between structures that are made up separately and subsequently joined.

Def Stan 00-970 Design and Airworthiness Requirements for Service Aircraft, which determines the UK build, test and qualification requirements for military aircraft.

Delamination Failure of the matrix between layers of composite materials resulting in layers separating.




Dangerous Engineering Substance

DES Refer to MAA02.

Defence Environment and Safety Board

DESB Responsible to SofS for Defence. for all matters of this nature.

Designer Refer to MAA02.

Detail Design office term describing part of a structure, normally where there is a joint or change in geometry and where formerly, in hard copy drawing days, a specific drawing of that area would be called for and local stresses may have been assessed.

Deterministic An approach in which parameters are given a single value to simplify mathematical treatment and allowance for uncertainty is made by applying (safety) factors to results. (cf Probabilistic).

Disbonding See Debonding.

Divergent Oscillation Describes motion of whole aircraft or part of structure when the forces causing oscillation are undamped. Oscillation increases in amplitude usually, leading to catastrophic failure.

Design Limit Load DLL See Limit Load.

Designer Modification DM

Design Organization DO The organization appointed by the PTL to be responsible for the design or design change of an airborne system or its associated equipment, and for certifying the airworthiness of the design by issue of a Certificate of Design. (MAA02 Issue 5)

Date of Segregation DoS Refer to MAA02.

Defence Ordnance Safety Group DOSG Refer to MAA02.

Design Proof Load DPL

Data Reporting, Analysis and Corrective Action System. Common System

DRACAS Used in industry, it encompasses all aspects of tests and trials and includes performance data. DRACAS is the preferred system for MOD contracts (DEF STAN 00-40 refers). (MAA02 Issue 5: A data reporting and analysis system to record safety problems and use this data to improve safety management.)

Defence Systems Approach to Training

DSAT Refer to MAA02.

Defence Storage and Distribution Centre

DSDC Refer to MAA02.

Design Service Goal DSG

Defence Science and Technology Laboratory

Dstl Refer to MAA02.

Damage Tolerance DT Approach developed by USAF which differs from the Safe Life philosophy in that it assumes cracks to exist in the structure already at the very first load cycle. Damage tolerance is an attribute of a structure that permits it to retain a level of residual strength for a period of unrepaired usage after the structure has sustained specific levels of fatigue, corrosion, accidental or environmental damage and/or discrete source damage. A distinction is made between inspectable and non-inspectable structures, and areas classified either as Slow Crack Growth, Fail Safe Multiple Load Path, or Fail Safe Crack Arrest structures. This concept has been extended to include ”Inspection Dependent” structure where cracks are assumed to exist upon expiry of the Safe Life and is an approach that may be used during life extension activities.

Ductile Failure Fracture of a material with significant permanent (plastic) deformation.

Durability May be considered as a quantitative measure of the structure's resistance to the threats to SI (qv), especially fatigue cracking under specified service conditions. This means that the economic lifetime, including all inspections, replacements or repairs, should exceed or at least equal the design life based on damage tolerance. The traditional means to achieve durability has been through conventional fatigue testing and analysis. However, recently, a fracture mechanics philosophy, combining a probabilistic format with a deterministic crack growth approach has been advised.

Dynamic Fatigue Test Test in which high frequency loading is simulated such as that experienced in buffet, whereas the conventional fatigue test applies lower frequency loads.

Engineering Authority EA Refer to MAA02.

European Aviation Safety Agency EASA Refer to MAA02.

Electronic Counter Measure ECM Refer to MAA02.

Environmental Damage ED The physical degradation of material properties as a direct result of interaction with the climate or the environment.




Engineering Development and Investigation Team

EDIT Refer to MAA02.

Environmental Damage Prevention and Control

EDPC An EDPC Programme is a comprehensive and systematic approach to managing the risk to airworthiness, capability, availability and costs arising from ED throughout the life of an aircraft.

Electro-Explosive Device EED Refer to MAA02.

Electrical Firing Circuit EFC Refer to MAA02.

Early Failure Detection Cell EFDC Refer to MAA02.

Engine Health and Usage Monitoring Systems

EHUMS Refer to MAA02.

Electro-Magnetic Compatibility EMC Refer to MAA02.

Endurance Limit The alternating stress level in a material below which it will not fail in fatigue no matter how many times the stress is applied. The item effectively has an infinite fatigue life below this level of stress.

Engineering Disposition see Concession.

External Quality Audit EQA Refer to MAA02.

Expedient Repair ER Refer to MAA02.

Engineering Record Card ERC Refer to MAA02.

Engineering Sciences Data Unit ESDU Now known simply as ESDU International plc. Providers of engineering data such as properties of alloys, etc, based on empirical and theoretical research, for use by industry and research establishments.

ESVRE ESVRE UK MOD approach to managing SI for in-service aircraft in which the 5 areas of Establishing, Sustaining, Validating, Recovering and Exploiting SI comprehensively cover all activities needed to economically manage structural airworthiness risks.

Electronic Technical Information ETI Refer to MAA02.

Engine Usage Condition Monitoring and Management System

EUCAMS Refer to MAA02.

Federal Aviation Administration FAA Refer to MAA02.

Fleet Air Arm FAA

Factor The term factor is used in several applications on aircraft structures, ie usage factor, scatter factor, safety factor, reserve factor and stress factors, given as a number.

Fail Safe The Fail Safe design philosophy was a forerunner to the Damage Tolerance philosophy and relies on continued Structural Integrity being predicated on the presence of alternate loads paths or crack arrest features and the ability to detect the failure. These can however only withstand the redirected loads for a limited time before further failure. Therefore, SI is maintained through suitably timed inspections aimed at determining whether failure of the primary load path has occurred.

Federal Aviation Requirement FAR Certification standards published by the FAA.

Fastener Generic name for single rivet, nut/bolt, screw or latch used to join or attach aircraft structure. A latch (such as a Quick Release Fastener) may itself be attached by fasteners (rivets).

Fatigue Allowable The maximum stress a detail could experience when the Limit Load was applied so that the detail’s fatigue life when subjected to the expected loading spectrum would coincide with specified aircraft life.

Fatigue Failure. Fracture of a material due to cyclical application of a load. Fracture usually occurs by a three-step proce: 1. Nucleation (qv) of the crack. 2. Slow, cyclic propagation of the crack. 3. Catastrophic failure of the metal.

Fatigue Index The non-dimensional measure of fatigue usage of an aircraft or specific items of its structure such as fuselage frames or empennage. For an aircraft/component in which there are no premature failures during Full Scale Fatigue Testing (qv), the Safe Life (qv) will usually be set at 100 Fatigue Index (FI).

Fatigue Life Normally used to mean the point at which the risk of failure from fatigue of an item subjected to repeated stresses is predicted to reach an unacceptably high level, at which point the item is withdrawn from service. Can also be used to mean the proportion of this life consumed to date.

Fatigue Meter A basic aircraft instrument, which records exceedances of pre-set levels of acceleration normal to the aircraft longitudinal axis (in the aircraft z direction) during a sortie. Separate counters indicate the




number of exceedances above a specified level but additional counts will only register if, in the intervening interval, the acceleration falls below the priming (threshold) value for that level. Used only on some fixed wing platforms. Now being superseded by load monitoring/measuring systems to give a more accurate picture of aircraft usage. See also FMCS.

Fatigue Strength The maximum cyclic stress a material can withstand for a specified number of cycles before failure occurs.

Flight Data Recorder FDR Refer to MAA02.

Finite Element FE

Flying Hours fg hr Refer to MAA02.

Front Line Command FLC Refer to MAA02.

Flutter Refer to MAA02.

Fatigue Monitoring and Computing System

FMCS Fatigue monitoring system previously used on limited number of Harrier GR9 and all T12 ac. Relies on 16 strain-gauged monitoring points, on-board rainflow cycle counting and direct computation of fatigue damage for downloading via hand-held solid-state unit to PC-based ground station.

Failure Modes Effects and Criticality Analysis

FMECA Refer to MAA02.

Fatigue Meter Formula FMF A formula which predicts the proportion of total fatigue life consumed by an aircraft or component during a sortie. The formula may include mass, configuration, landing or pressurisation data in addition to normal acceleration exceedances obtained from the Fatigue Meter (qv).

Foreign Object Damage FOD Refer to MAA02.

Fatigue Quality Factor FQF The ratio of the Fatigue Allowable (stress) to the actual stress in a component at Limit Load. A sort of “Fatigue Reserve Factor”.

Failure Reporting, Analysis and Corrective Action System. Common System

FRACAS Used in industry, is essentially similar to DRACAS but limited in scope. It is only concerned with failure data, unlike DRACAS, which encompasses all aspects of tests and trials and includes performance data. DRACAS is the preferred system for MOD contracts. (MAA02 Issue 5)

Fatigue Research Advisory Group FRAG A meeting group of fatigue specialists from Industry and MOD which discusses and exchanges information on fatigue research as well as directing modest joint projects.

Fatigue Research Database FREDA A 'M-Vision' database of fatigue information established and maintained by the organisations involved in its development.

Flight Servicing Competency Check

FSCC Refer to MAA02.

Full Scale Fatigue Test FSFT (Often known as Main Airframe Fatigue Test or MAFT qv). A programme lasting several years in which a representative development, production, or preproduction, airframe is subjected to repetitive, simulated flight and landing loads to establish the crack free period of Safe Life designs or the inspection free period for Damage Tolerant (qv) aircraft. For UK and UK-collaborative designs, the loading spectrum is that anticipated in-Service and the test is run for 3-5 time the Design Service Goal of the aircraft. Other nations may use more severe loads than those expected in service to accelerate testing so that only twice the equivalent flying hours are required. Damage tolerant designs are usually tested to twice the equivalent inspection free period, following which the test is normally used to validate the damage tolerance analysis by monitoring the development of both natural and artificially induced cracks.

Functionally Significant Item FSI Refer to MAA02.

Fatigue Type Record FTR A document used to record the method by which the fatigue life of all Structural Significant Items were substantiated.

Flight Test Schedule FTS Refer to MAA02.

Fixed Wing FW

Ground Air Ground GAG Refer to MAA02.

Generic Aircraft Release Process GARP Refer to MAA02.

Government Furnished Equipment GFE Refer to MAA02.

Generic Helicopter Health and Usage Monitoring Systems

GHHUMS

Generic Health and Usage Monitoring System

GHUMS UK programme of individual helicopter monitoring systems for Lynx, Sea King and Chinook (See HUMS qv).

Ground Maintenance System GMS A GMS is the equipment and software used to transform and manage GSS-processed HUMS data and manually input maintenance data into information pertinent to the management of




aircraft maintenance at 1st through to 4th Line. A GMS may be aircraft-specific, such as Typhoon’s Engineering Support Subsystem (ESS), or corporate, such as Logistics Information Technology Strategy (LITS) or Work Recording and Asset Management (WRAM). (JAP 100A-01 Ch 7.2.5)

Gaseous Oxygen GOX Refer to MAA02.

Ground Resonance Phenomenon in helicopters when rotors turning on the ground excites a natural frequency associated with roll about the undercarriage legs and can lead to rapid, violent catastrophic rolling onto side.

Ground Support Equipment GSE Refer to MAA02.

Ground Support System GSS A GSS is the equipment and software used to download, further process, analyze, display and store health and usage monitoring data from aircraft HUMS. The GSS may be used to reconfigure airborne HUMS data collection parameters. (MAP-01 Ch 11.2) (MAA02 Issue 5)

Gust An atmospheric phenomenon in which a localised air mass has vertical, or lateral velocity appreciably different to that of the surrounding conditions thereby momentarily changing the aerodynamic loads on the structure. The magnitude and intensity of the gust increases as altitude decreases towards sea level, therefore gusts may be particularly significant during the take-off or landing phases.

Hardened Aircraft Shelter HAS Refer to MAA02.

Hazard Refer to MAA02.

High Cycle Fatigue HCF When the number of repeated loads applied to an item is more than about 107 then the item is said to be subjected to high cycle fatigue loading.

Hazard Division HD Refer to MAA02.

Hot-Spot Gauges Description applied to strain gauge applied to a specific problem area in an aircraft structure to measure strain directly. The stress is inferred by direct relationship although the external loading on the item of structure is not necessarily known accurately as the installation is not calibrated in a load frame. (The strain gauge itself will be calibrated though).

Headquarters Strike Command HQSTC

Health and Usage Monitoring Systems Support Cell

HSC Refer to MAA02.

Health and Usage Centre HUC Refer to MAA02.

Health and Usage Monitoring System

HUMS The term Health and Usage Monitoring (HUM) encompasses a variety of techniques including operational load monitoring, vibration analysis, visual inspections, oil and wear debris analysis. The data obtained through such monitoring is used to preserve and enhance the airworthiness of the platform. Health and Usage Monitoring Systems (HUMS) have been developed to support condition-based maintenance by the acquisition of health and usage data from on–board sensors.

Helicopter Under Slung Load Equipment

HUSLE Refer to MAA02.

Identification and Authentication cycle

I&A cycle Refer to MAA02.

Issue Authority IA Refer to MAA02.

Individual Aircraft Tracking IAT A term applied to a process that leads to the output from a system that collects aircraft-by-aircraft cumulative lifing data, which is used to manage retirement, modification or examination as appropriate to the aircraft type and fatigue design philosophy.

International Committee on Aeronautical Fatigue

ICAF Bi-annual conference and symposium covering all aspects of aeronautical fatigue. Member Nations present a review of aeronautical fatigue work in each country during the first 2 days, following which there is a 3 day Symposium.

Inspector of Explosives IE Refer to MAA02.

Interactive Electronic Technical Publication

IETP Refer to MAA02.

Integrated Ground Support Facility IGSF Tyhpoon term concerned with GSS (qv).

Integrated Logistic Support ILS Refer to MAA02.

Incident Refer to MAA02.

Inspection Threshold The point at which inspection should be carried out expressed in terms of units of usage (Flying Hours, FI, years etc.) The initial threshold is based on the smallest detectable defect in the structure concerned. (MAA02 Issue 5)




Integrated Operational Support IOS

Internal Quality Audit IQA Refer to MAA02.

Inspect and Repair As Necessary IRAN Refer to MAA02.

Information System IS

Independent Safety Advisor ISA A person or team independent of the design activity who carries out process audits of the Designer’s work for adherence to the Safety Plan, reviews the safety documentation including the safety assessment , and carries out independent safety audit where commissioned by the PTL. (MAA02 Issue 5)

Independent Structural Airworthiness Advisor

ISAA A competent individual, independent of the Designer, who carries out an independent technical evaluation of aircraft structures. They must have a first degree in an engineering discipline and prior experience of 5 years minimum in aircraft structures design, structures safety assessment or structures maintenance, relevant to aircraft types for which advice will be given.

Independent Technical Evaluator ITE A person or team independent of the Design organization (DO) who carries out an independent technical evaluation and analysis of the data evidence supporting the contractor’s Safety Assessment including a qualitative assessment of aircraft handling. (MAA02 Issue 5)

In-Use equipment IU Refer to MAA02.

Joint Aviation Authority JAA Refer to MAA02.

Joint Airworthiness Committee JAC A sub committee of the DASB (qv). Refer to MAA02.

Joint Air Publication JAP Refer to MAA02.

Joint Aviation Requirement JAR Refer to MAA02.

Joint Force Harrier JFH

Joint Helicopter Command JHC Refer to MAA02.

Joint Services Specification Guide JSSG US forces combined guide for weapons systems issued following acquisition reform initiatives to emphasise performance requirement and to replace single service documents. For aircraft structures the guide is 2006.

K Factors Factors applied to the Endurance Limit obtained in a laboratory on the rotating beam specimen to generate the Endurance Limit for a mechanical element. These account for Surface finish, size effects, reliability, temperature, stress concentrations and other miscellaneous effects. K – Stress Intensity Factor: This is a measure of the severity of the stress at a fatigue crack tip. As the crack grows, K increases until it reaches a critical value (Kc or Klc) at which fast fracture will occur. Kt -Stress Concentration Factor: The factor is defined as the ratio of stress at a fatigue critical feature to nominal stress based on minimum. This term is used to simplify the selection of appropriate S-n curves in classic Safe Life analysis. The factor is defined as the ratio of stress at a fatigue critical feature to nominal stress based on minimum area. factors for most geometries of aircraft joints were catalogued by RAE in the 1960s and are now available as ESDU (qv) data.

Light Aid Detachment LAD Refer to MAA02.

Land (Also known as Edge Distance) Distance between a row of fasteners and the free edge of the material.

Landing, Full Stop, Roller Important loading actions on most aircraft types differentiated between whether the aerodynamic loading on the lifting surfaces is fully dissipated (Full stop) or not (Roller or touch-and-go).

Life Cycle Costing LCC Refer to MAA02.

Life Extension Programme LEP Measures taken to extend the period of operation of a platform, as expressed in flying hours, calendar time or more directly in fatigue usage beyond the original cleared life.

Limit Cycle Oscillation Specific case of aeroelastic (qv) oscillation in which a steady state is reached neither divergent nor damped.

Logistic Information System LIS Refer to MAA02.

Logistic Information Technology Strategy

LITS Refer to MAA02.

Limit Load LL The maximum load that the aircraft should experience in service.

Load Often used indiscriminately by engineers to mean: the acceleration of an aircraft; the external force exerted on an aircraft structure; or the resultant stresses induced in a loaded structure.

Load Path Method of visualising the build up of stresses throughout a structure by considering the notional “flow” of stress from the point(s) of application of the load into more remote supporting structure.




Load Spectrum/G-Spectrum, Stress Spectrum, Usage Spectrum

A statistical expression of the number of times the load (normally expressed in acceleration multiples of g) on an aircraft structure exceeds specified levels. Data is often presented with a logarithmic x-axis indicating exceedances per 10 to the power x flying hours.

Loading Action The source and nature of the forces introduced into the structure at a particular point in the aircraft’s operation. eg during landing.

Local Strain A method developed in the US for use in predicting the life to crack initiation by assessing the strains around a stress concentration using techniques such as Neuber's rule as an alternative to considering stresses.

Low-Cycle Fatigue Low cycle Fatigue, stress reversals up to about 103 cycles, needs to be considered for the design of other components when the possibility of some very high loads during the lifetime of the component is high.

Liquid Oxygen LOX Refer to MAA02.

Low-Profile Platforms LPP Refer to MAA02.

Line Replaceable Unit LRU Refer to MAA02.

Logistic Support Analysis LSA Refer to MAA02.

Logistic Support Analysis Record LSAR Refer to MAA02.

License to Act LTA

Materials & Structures Group M&SG Obsolete: See MIG

Military Aircraft Release MA Release Obsolete: Refer to MAA02.

Military Aviation Authority MAA Refer to MAA02.

MAA Regulatory Policy MAA01

MAA Master Glossary MAA02

Military Air Systems Certification Process

MACP

Military Air Environment MAE Refer to MAA02.

Major Airframe Fatigue Test MAFT See full Scale Fatigue Test.

Manual of Maintenance and Airworthiness Processes

MAP

Military Aircraft Structures Airworthiness Advisory group.

MASAAG

Master Armament Safety Switch MASS Refer to MAA02.

Main Airframe Static Test MAST A programme conducted in the pre-production phase of a new design in which simulated flight and ground loads are applied to confirm that the aircraft will not collapse before the Ultimate Load (qv) is reached. The overall clearance may be achieved in stages, with each stage clearing specific packages of flight trials. Usually, the test will be taken above ultimate loads in a selected critical load case to explore the potential strength.

Mobile Aircraft Support Unit MASU

Materials Assurance and Technology Section

MATS

Military Certificate of Airworthiness MC of A Obsolete Short-lived (1996) term intended to replace CAR (qv) but quickly changed to MA Release to prevent confusion with civilian terminology.

Miniature Detonating Cord MDC Refer to MAA02.

Manual Data Recording Exercise MDRE Simple method of collecting flight condition data by installing a trained observer in a helicopter who, for a sample of sorties, categorises type and records the duration of flight condition.

Multi-Element Damage MED MED is the simultaneous presence of fatigue cracks in similar adjacent structural elements, such as frames and stringers.

Mid Sortie Mass Term used in a fatigue meter formula to identify the assumed mass of the aircraft for all damaging manoeuvres. Strictly not the mass at mid-sortie by time but a percentage of the mass including stores, fuel etc, at the take off and landing. The percentage used may vary depending on the configuration the aircraft is flown in.

Materials Integrity Group MIG An organisation within Navy Command that is responsible for scientific and technical advice on materials and monitoring of equipment in the air environment.

Military Aircraft An aircraft in respect of which there is in force a certificate issued by the SofS (for Defence) that the aircraft is to be treated as a military aircraft for the purpose of the Air Navigation Order (qv).

Miner's Rule/Law Fundamental axiom of fatigue damage that an item cycled through a variable amplitude stress programme will fail when the




sum of the ratios of number of load cycles at a given stress amplitude to the number of cycle required to fail the item at that constant amplitude is equal to unity. The Miner's Rule can be said to be "Calibrated" to an actual failure at a known number of load cycles of variable amplitude on a fatigue test.

Master Maintenance Schedule MMS

Main Operating Base MOB

Modification Defined as a change, authorised by the customer, to the Baseline Build Standard (BBS) of an item of the weapon system, including hardware, software and support material. It is considered a modification if it affects the fit, form or function of the weapon system.

Monitored Structure Considered monitored if there is a fleet wide load monitoring procedure in place which records the fatigue consumption on an individual A/C or component basis. Traditionally, fatigue consumption for monitored structure is determined using Fatigue Meter Formulae (FMFs), which relate A/C normal acceleration in service to the results of MAFT.

Military Registered Civil Owned Aircraft

MRCOA Term for aircraft previously known as: COM - Military type aircraft not in employment of the crown, often used for flight testing of military aircraft prior to expor COM- Aircraft usually of civil design and qualification, employed on contractorised tasks (eg flying training). Airworthiness of the aircraft is maintained to MOD regulations by a MOD appointed In-Service PT with CAA oversight. (RA1121.) (MAA02 Issue 5)

Machine-readable device MRD Refer to MAA02.

Main Rotor Head MRH

Master Record Index MRI Obsolete. Refer to MAA02.

MAA Regulatory Publications MRP

Multi-Site Damage MSD The simultaneous development of fatigue damage in large numbers of adjacent features qv such as fastener qv holes that would result in the coalescence of fatigue cracking and major structural failure if not checked. Cause of the Aloha B-737 cabin rupture in 1980´s. Also see WFD.

Maintenance Steering Group MSG

Not-At-Risk NAR A term used in the assessment of structure in its vulnerability and susceptibility to AD/ED.

Non-Destructive Examination NDE

Non-Destructive Testing NDT

Neuber's Rule A method of predicting fatigue life for notched members if the notch root strain history and the smooth specimen strain-life data or fatigue properties are known. One of the advantages of this method is that it accounts for changes in local mean residual stresses.

Neural Network See Artificial Neural Network.

Not-In-Use equipment NIU

Nucleation The beginning of the physical changes in the material properties to allow a crack to develop when subjected to a load.

No Volts Safety Test NVST

Nz Vertical acceleration.

Officer Commanding Engineering and Supply Wing

OC ESW

Officer Commanding Handling Squadron

OC HS

Operational Data Recording ODR UK validation programme for monitoring in service strains on helicopter components with strain gauges, together with detailed flight parameters such as accelerations, roll rates, turn rates, side slip.

On-the-Job-Training OJT Refer to MAA02.

Operational Loads Measurement OLM (Sometimes referred to, incorrectly, as Operational Loads Monitoring). An OLM programme is designed to capture the actual in service fatigue usage of an aircraft for comparison with the assumptions made in design and qualification, particularly the major fatigue tests of components and the airframe. It normally covers both monitored structure, where the aim is to validate the normal fleetwide fatigue monitoring process, and unmonitored structure which is normally cleared simply in terms of FH, landings etc. Strain gauges bonded to the surfaces of metallic components give electrical outputs, which are recorded and




converted into strain. In addition other parameters are recorded. On some programmes, the airframe/component may be calibrated in a loading rig in order to determine actual loads,(including bending moments and torque).

Out-of-Service Date OSD The point at which an aircraft or system is planned to leave service.

Overload Situation in which a structure is subjected to a single high load that exceeds its limit strength. If proof strength is exceeded the structure may be permanently deformed and if the higher ultimate strength is exceeded there is a high risk of structural failure. The actual strength (proof, ultimate) is not often known precisely, it is the CLEARED STRENGTH that should be known. The airframe is cleared to its req’d strength, not its actual or potential strength. So, it will not necessarily permanently deform above proof load or fail above ultimate load.

Publication Authority PA Refer to MAA02.

Preliminary Design Review PDR A review held with the customer during development. Any permitted exceptions shall be detailed as design deviations.

Post Design Services PDS Covers mods, development, repair, fault assessments and investigations. (MAA02 Issue 5)

Project Engineer PE Refer to MAA02.

Protected Earth Monitoring System

PEMS Refer to MAA02.

Potential Explosive Site PES

Penetrant Flaw Detection PFD Refer to MAA02.

Production Flight Test Schedule PFTS Refer to MAA02.

Proof Installation PI Refer to MAA02.

Personal Identification Number PIN

Point In The Sky PITS A representative flight condition, expressed in terms of configuration (stores etc), mass, fuel load, mach number, altitude which design engineers use to define a particular load (flight manoeuvres, gusts or other loading actions qv) for either static strength or fatigue analysis.

Permanent Joint Headquarters PJHQ Refer to MAA02.

Portable Maintenance Data System

PMDS Typhoon term for the data transfer memory module used to get data from aircraft to the GSS (qv).

Plastic Media Stripping PMS Method of removing surface finish by directing a controlled stream of plastic beads at the surface to wear finish away.

(NDE) Probability of Detection POD Faults present in materials cannot always be detected using Non Destructive Testing. A statistical approach can be used to describe the likelihood, with a specified confidence level of finding a specified flaw within a specified structural configuration.

Point On the Ground POG A representative condition, expressed in terms of configuration (stores etc), mass, fuel load, prior to take-off and post landing which design engineers use to define a particular load (ground manoeuvring or other loading actions qv) for either static strength or fatigue analysis.

Pressurisations The number of loading cycles caused by pressurising the cabin or cockpit above the external pressure either when flying at altitude or during maintenance on the ground. A combat aircraft operating a HI-LO-HI sortie may experience more than one pressurisation per sortie.

Probabilistic SI approach An approach which aspires to consider the full range of probable strengths of a structure with the full range probable loads applied to it to determine the probability of the structure surviving (or failing). This is emerging technology. Cf Deterministic.

Production Permit Requested when it is obvious before manufacture that the product will not be to the actual spec. For example, when producing old products an alternative component may have to be used if it has become obsolescent. On occasions an alternative raw material may be used.

Proof Load Test A Proof Load test may be carried out on the MAST to demonstrate the Proof Strength req’t, ie no permanent deformation).

Principal Structural Element PSE A basic, functional feature within an airframe structure: for example, wing carry-through structure or undercarriage back-up structure. Each PSE contains a number of sub-elements or SSIs that are located in the same region and subjected to similar stress fields and loading spectra.

Project Team PT Replacement term for Integrated Project Team (IPT) which, between 2000 & 2008, was the body responsible for managing a project from Concept to Disposal. (MAA02 Issue 5)




Project Team Leader PTL Refer to MAA02.

Quality Assurance QA Refer to MAA02.

Quality Audit QA Refer to MAA02.

Quality Assurance Representative QAR Refer to MAA02.

Qualified Aircrew Instructor QFI Refer to MAA02.

Qualified Helicopter Instructor QHI Refer to MAA02.

Quality Management System QMS Refer to MAA02.

Quality Occurrence Report QOR Refer to MAA02.

Qualification Programme Plan QPP BAE SYSTEMS term related particularly to the fatigue life qualification work on Tornado (PDT qv 1283 etc).

Quality System Coordinator QSC Refer to MAA02.

Quality System Owner QSO Refer to MAA02.

Qualification Verification that a system meets its design specification.

Qualification Life Amount of safe life demonstrated by test, calculation or other means against a specified design loading requirement.

Regulatory Article RA

Radiation Hazard RADHAZ Refer to MAA02.

Rainflow Stress Cycle Counting Rainflow stress cycle counting is the most common and practical form of stress cycle counting. Rainflow counting is used to measure the likely impact of the most damaging stress cycles in a sequence that combines load reversals in a manner that defines cycles by closed hysteresis loops. The rainflow counting technique uses a plot with strain time history plotted vertically downward on a time axis. The history is plotted so that the largest strain magnitude occurs as the first and last peaks or valleys, with the lines connecting strain peaks ending up looking like a series of ‘pagoda roofs’. The fatigue damage is then counted using a series of conventions, by counting the ‘rainflow’ of drops falling from the roof of the pagoda.

Repair assessment Programme RAP A programme of identifying and then assessing the repairs carried out on an aircraft structure, from both a static strength and damage tolerance perspective, to ensure continued structural airworthiness.

Residual Current Device RCD Refer to MAA02.

Reliability Centred Maintenance RCM The systematic approach for identifying preventive maintenance tasks for an equipment or item in accordance with a specified set of procedures and for establishing intervals between maintenance tasks [with minimum expenditure of resources] (DEF STAN 00-40). Note: Outside the MAE, RCM is commonly known as Maintenance Steering Group (MSG) logic. (MAA02 Issue 5)

Repair and Design Liaison Office RDLO Refer to MAA02.

Request for Engineering Information/Instruction

REI Now Repair Instructions and Aerospace Equipment Design Requests (MAP-01 Ch 9.13.2)

Residual (Static) Strength Test At the cessation of a successful Full Scale Fatigue Test (qv), an aircraft may be subjected to a design static loading case, usually wing-up, nose down bending, to confirm that the structure will still sustain a percentage (for example 80%) of the ultimate load (qv).

Residual Strength The static strength of a structure (or component) after a specified amount of usage or degradation.

Reserve Factor RF The ratio of the failure stress of a component to the stress at ultimate load (Normally 1.5 x Limit Load). Ideally, this factor is 1.0 for minimum weight but other factors such as AD/ED (qv) may be more important.

Royal Fleet Auxiliary RFA Refer to MAA02.

Regulatory Instruction RI

Regulatory Notice RN

Regional Non-Destructive Testing Team

RNDTT Refer to MAA02.

Resident Project Officer RPO Obsolete. Refer to MAA02.

Repair and Salvage Unit RSU

Routine Technical Instruction RTI Refer to MAA02.

Release to Service RTS The release document that authorizes Service flying on behalf of the Service Chief of Staff. The RTS refers to the Safety Assessment documentation for the aircraft or equipment, including the limitations and aircraft description, and defines the as-flown standard of the aircraft. (MAA02 Issue 5)




Remote Viewing Aid Equipment RVA Refer to MAA02.

Rotary Wing RW

Support Authority SA Obsolete RAF term. Refer to MAA02.

Senior Aircraft Engineer Officer SAEO

Safe Life The period during which the risk of unexpected structural collapse of an aircraft's structure, determined by statistical prediction, is deemed to be acceptably small. The life depends on both the flying hour rate of the aircraft and the severity of its usage. In the UK, the 2 parameters are combined, for convenience, into the single non-dimensional parameter Fatigue Index (qv). Thus, the Safe Life is expressed as a specific number of Fatigue Index units or 'FI'. Other operators, notably the USN, periodically update the expected Flying Hours to life expiry using the most recent structural usage data. Tornado is designed to safe life principles, that is, fatigue tests using a specified design loading spectrum, and appropriate scatter factors to produce sufficient confidence not to require dedicated structural inspections to maintain safety. Safe life limitations can also be expressed in terms Flying Hours and/or Landings.

Service Amendment Leaflet SAL Refer to MAA02.

Station Administrative Management Aid

SAMA

Sampling Normally taken to mean structural sampling, a programme to validate that design assumptions and sustaining measures applied in service use are adequate by examining parts of structure that were assumed not to need routine examination, on a sample of fleet.

Structural Airworthiness SAw Refer to MAA02.

Scatter Statistical term describing the spread of data points. Often used in context of fatigue or strength testing or fatigue lives.

Structural Configuration Control SCC The maintenance of effective control of the approved configuration of materiel (applying to aircraft structure).

Stress Corrosion Cracking SCC This is cracking in a metallic material originating at a site subjected to continual tensile stress, which is susceptible to corrosion and which is subjected to the agents of corrosion. The tensile stress may result from externally applied loads or may be residual, arising from the metal working processes during manufacture. Not to be confused with corrosion fatigue (qv).

Supply Control Centre System SCCS

Senior Civilian Engineer SCE Refer to MAA02.

Structural Control Point SCP The particular SSI or feature within a PSE that represents the lead fatigue feature and for which the structural examination requirement would be most frequent. By examining the SCP only, the condition of all sub-elements within the PSE can be inferred. (MAA02 Issue 5)

Service Deviation SD Obsolete. Refer to MAA02.

Strategic Defence and Security Review

SDSR

Station Engineering Management Aid

SEMA Obsolete. SEMA was an early information technology system designed to manage selected engineering functions for Phantom and Tornado aircraft.

Structural Examination Programme

SEP Set of measures including directed inspections (sustaining) and opportunity inspections and Teardowns (validating) that underwrite continued SI for in service aircraft.

Service Provider (or Support Provider)

The company/group delivering a contracted service or capability to the MOD.

Simulated Flight Hours SFH

Special Flying Instruction SFI Refer to MAA02.

Serious Fault Signal SFS

Safety, Health, Environmental and Fire

SHEF

Structural Health Monitoring SHM

Shot Peening Process during manufacture of metallic parts in which residual tensile stresses at the surface are relieved by blasting the item with lead shot or other media thereby reducing the susceptibility to surface fatigue cracking.

Servicing Instruction SI Refer to MAA02.

Significant Item SI Refer to MAA02.

Structural Integrity SI The ability of an aircraft structure to retain its strength, function and shape within acceptable limits, without failure when subjected




to the loads imposed throughout the aircraft’s service life by operation within the limitations of Release To Service (RTS) and to the usage described in the Statement of Operating Intent (SOI) or the Statement of Operating Intent and Usage (SOIU). Refer to MAA02 Issue 5.

SI Plan Plan forming part of the TLMP in which the key structural management events are marked to allow for coordination and budgeting.

Special Instruction (Technical) SI(T) An instruction, issued by the PT, that gives instructions to undertake a work package to repair or prevent a potential fault. There are 4 types of SI(T): STI, SI, UTI and RTI. (MAA02 Issue 5)

Strategic/Source Intelligence Fusion Testbed

SIFT

Structural Integrity Strategy Document

SISD The document that communicates the Structural Integrity strategy and methodology and records historic structural integrity actions/events employed. The document covers the strategic SI actions and contains the SI plan and the explanation behind the actions contained within it.

Structural Integrity Working Group SIWG Meeting between all SI stakeholders including PT, Designer, Air staff and Specialist Structural adviser to address SI issues and obtain TAA endorsement that the approach to SI assurance on a specific aircraft type is being satisfactorily undertaken. Usually held twice a year and chaired by an LoAA holder at OF4/B2 level.

SLAP/SLEP Service Life Assessment Programme/Structural Life Extension Programs (US) by Boeing and McDonald Douglas.

SLE Structural Life Extension programme for the Tornado.

Service Modification SM Refer to MAA02.

Schedule Maintenance Agency SMA

Subject Matter Expert SME Refer to MAA02.

Safety Management System SMS Refer to MAA02.

S-N Diagram (or Curve) To determine the fatigue strength of materials under the action of fatigue loads, specimens are subjected to repeated or varying forces of specified magnitudes while the cycles or stress reversals are counted to destruction. Due to the statistical nature of fatigue, a number of tests are completed. The results are plotted on an S-N Diagram. The ordinate of the S-N diagram is called the Fatigue strength; a statement of this strength must always be accompanied by a statement of the number of cycles N to which it corresponds.

Safety of Flight Structure SofFS That structure whose failure would cause direct loss of the aircraft or whose failure, if undetected, would result in loss of the aircraft. (US Joint Services Specification Guide 2006, though with “air platform” substituted by “aircraft” in UK)

Statement of Operating Intent SOI Used at the project inception stage to formally describe and communicate the aircraft’s expected usage to the Designer.

Statement of Operating Intent and Usage

SOIU The aircraft Topic 15S, the SOIU is a narrative and graphical description of the expected and/or actual usage of an aircraft type. Specific types of sorties are summarised in the SPCs (qv) and the purpose of the SOIU is to provide Designers, aircrew and engineers with information on which to base fatigue management decisions. Part of the aircraft document set used to underpin airworthiness as set out in the platform safety case.

Special Order Only SOO Refers to modifications applicable to a limited range of ac tail numbers. Not unique to SI work.

Statement of Work SOW UK industry term used to define the specific actions expected of the company in order to fulfill a task. Normally drafted by the company who will undertake the work for approval by the tasker. Not specific to SI work.

Sortie Profile Code SPC A code number used within the MOD to identify a specific type of sortie. Among many uses, SPCs have been used to develop the fatigue formulae for aircraft with fatigue meters, the SPC continues to provide a useful summary of generic sortie types and are defined in the SOIU (qv).

Superplastic Formed/Diffusion Bonded

SPFDB A fabrication technique used in the manufacture of the Eurofighter Typhoon Titanium foreplane.

Support Policy Statement SPS

Service Engineered Aircraft Radio Installation Modification

SRIM Obsolete. Refer to MAA02.

Structural Significant Item SSI Any detail, element or assembly, which contributes significantly to carrying flight, ground, pressure or control loads and whose failure could affect the structural integrity necessary for the continued safe and controlled flight of the aircraft. (MAA02 Issue 5, see also AP 100C-22, MSG-3 and MASAAG 103B)




Structural Sampling Programme SSP See Sampling.

System Security Procedures SSP Refer to MAA02.

Standard Serviceability Test SST Refer to MAA02.

Static Overload See Overload.

Static Strength The static load at which a structure fails.

Static Test See MAST.

Strike Command STC

Special Trial Fit STF

Special Technical Instruction STI

Stochastic A time dependent variable often associated with statistics.

Static Type Record STR Properly known as the Type Record (qv) but sometimes “Static” used to differentiate from Fatigue Type Record (qv).

Strain Engineering term describing the ratio of the change in length of a loaded component to it’s original, unloaded length.

Strain Gauge A device based a wheatstone bridge that has an electrical current running through it, the device is bonded to the structure and as the structure deforms due to the application of load the electrical resistance of the gauge changes. The changes in resistance are then recorded, from which the deformation and strain can determined and subsequently the stress in the structure.

Strength Envelope Description of the limitations on an aircraft’s static strength and usually portrayed graphically by normal acceleration, altitude and velocity boundaries.

Stress Rupture Fracture. Fracture of a material due to prolonged exposure at a high temperature.

Structural Configuration Control Database

A database on which the structural configuration, in terms of repairs, corrosion, damage, modifications etc of individual aircraft can be recorded.

Structural Sampling (Scheduled, Opportunity)

See Sampling.

Structure Aircraft structure consists of all load-carrying members including wings, fuselage (including some transparencies), empennage, engine mountings, landing gear, flight control surfaces and related points of attachment, control rods, propellers and propeller hubs if applicable and, for helicopters: rotor blades, rotor heads and associated transmission systems. The actuating portion of items such as landing gear, flight controls and doors must be subject to System Integrity Management regulation (RA 5721) as well as Structural Integrity Management regulation. (MAA02 Issue 5)

Special-to-Type Test Equipment STTE Refer to MAA02.

Support Authorities Obsolete. The original multi-disciplinary group (MDG), incorporating air, engineering, supply, finance, contracts and Post Design Services staffs which provided all aspects of support management for a particular ac, weapons system or range of equipment. Nominated engineer staff, the Engineering Authority (EA), was responsible for exercising engineering judgement wrt airworthiness. (MAA02 Issue 5)

Support Provider See Service Provider.

Severe Wind And Moisture Prone. SWAMP Refer to MAA02.

Service Working Party SWP Refer to MAA02.

Security Operating Procedures SyOPs Refer to MAA02.

System A combination of physical components, procedures and human resources organised to achieve a function. The system organised to achieve the function of SI is the aircraft Structure. (MAA02 Issue 5)

Type Airworthiness Authority TAA The Type Airworthiness Authority is the individual, often an aircraft PTL, who on behalf of the Secretary of State for Defence, oversees the airworthiness of specified air system types. As the TAA the PTL responsibilities are as laid down and agreed in their Letter of Airworthiness Authority from their respective Director. (MAA02 Issue 5)

Typed Air Station TAS Refer to MAA02.

Teardown Usually termed a Structural Teardown, this is a destructive dismemberment of a Structure during which key joints are carefully disassembled and key features are inspected for previously unknown degradation from fatigue, corrosion, delamination, disbonding.

Technical Log Civilian equivalent of Form 700 incorporating F724/725 data and fault rectification information similar to MOD F707A.




Temporary Earth Electrode TEE Refer to MAA02.

Technical Enabling Services TES Obsolete. Refer to MAA02.

Test Factor Sometimes called the Life Factor or Safety Factor. A factor applied to the results of a test to allow for uncertainty and to introduce an acceptable level of confidence when the factored life is imposed on in-service structures. Various figures are used in practice depending on national practice, the uncertainty of loads applied in service and/or whether the structure’s usage is monitored or unmonitored in service.

Test Life The life (to failure or end of test) of a component or whole aircraft as measured in a representative fatigue test where no Safety Factors are considered.

Threats to SI The 4 threat groups to SI are: Overload; Accidental and Environmental Damage; Fatigue, fretting and wear; and Maintenance/Supply Error.

Technical Information TI

Trial Installation TI

Technical Information sponsor TI sponsor Refer to MAA02.

Top Level Budget TLB Refer to MAA02.

Test and Measuring Equipment TME

Trade Qualification Annotation TQA

Turn Round TR Refer to MAA02.

Tail Rotor Blade TRB

Type Record UK mandatory summary of the structural design of an aircraft with the loading assumptions and Reserve Factors for all Structure. Definition of which structure to include has been interpreted differently by different manufacturers over the years but intention is that SSIs qv are included as a minimum.

Unmanned Air Vehicle Air Vehicle UAV Obsolete. Refer to MAA02.

Uninstalled Engine Test Facility UETF Refer to MAA02.

Unsatisfactory Feature Report UFR Refer to MAA02.

Ultimate Factor Safety factor between the Limit Load and the Ultimate Load. Conventionally, Ultimate was 1.5 time Limit Load but this factor has been lowered to 1.4 for some parts of the Eurofighter Typhoon structure because of the constraints to pilot input imposed by flight control software. It can be and is used wherever it can be shown that a system will control a load eg A pressure relief valve maximum value could be used with 1.4.

Ultimate Load Load on the structure at Limit Load x Ultimate Factor and at which structural failure would normally occur with loss of SI.

Ultimate Strength A material property giving the stress at which the material will fail. Usually qualified by terms “tensile”, “compressive” or ”buckling” to indicate mode of failure.

Unmonitored Structure Structure for which there is no fleet wide load monitoring procedure in place for UK military aircraft. Therefore, generally, structure which is affected by loading actions other than normal acceleration is considered as unmonitored structure. Unmonitored structure is qualified by a mixture of MAFT, separate fatigue tests and also by calculation.

Usage Factor Comparative measure of the fatigue life consumed by component in service with that for a component on a fatigue test. Needed when flight data indicates that the stresses measured in the test specimen are unrepresentative of those in Service for the equivalent flight condition.

Urgent Technical Instruction UTI Refer to MAA02.

Very Light Aircraft VLA

Vne Never exceed velocity – specified because at greater speeds the aerodynamic loading would exceed limit loads in parts of the structure, unstable aeroelastic phenomena would develop or flight control would become ineffective.

Weibull Distribution Statistical distribution defined by 4 parameters, sometimes used instead of Log Normal distribution to describe fatigue life against probability of failure for a large population of test samples. Weibull distribution is asymmetric and for fatigue lives will assume zero probability value at time zero whereas Log Normal distribution has a non zero value at time zero.

Widespread Fatigue Damage WFD WFD in a structure is characterized by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement.




Work Recording and Asset Management

WRAM

Documents

Structural Integrity Handbook Issue 1.1