ATSB Transport Safety ReportAviation Occurrence Investigation AE-2012-013Final – 22 July 2013

Released in accordance with section 25 of the Transport Safety Investigation Act 2003

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Published by: Australian Transport Safety BureauPostal address: PO Box 967, Civic Square ACT 2608Office: 62 Northbourne Avenue Canberra, Australian Capital Territory 2601Telephone: 1800 020 616, from overseas +61 2 6257 4150 (24 hours)

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Examination of components from a GT ‘Kruza’ gyroplaneTechnical assistance provided to the Australian Sports Rotorcraft Associationnear Mangalore Aerodrome, Victoria, 14 January 2012

Contents

The occurrence.................................................................................................................1Impact with terrain 1

Context.............................................................................................................................. 3Technical assistance 3Flight control system 3Examination of the evidence 3

Rotor head components and torque tube 3Garmin GPSmap 295 4Blackberry mobile phone 4

Other gyroplanes 4

Analysis........................................................................................................................... 10Torque tube failure 10

Summary......................................................................................................................... 11Further information 11

General details................................................................................................................12Occurrence details 12Aircraft details 12

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ATSB – AE-2012-013

The occurrenceImpact with terrainOn 14 January 2012 at approximately 0930 (local time), a ‘GT Kruza’ gyroplane (registration G-762) with a pilot and student passenger on-board, departed Mangalore Aerodrome, Victoria, for a one hour training flight. At around 1130 later that morning, a member of the public identified the wreckage of the gyroplane, which had impacted terrain a short distance from the aerodrome. Both occupants had sustained fatal injuries. There were no witnesses to the accident.

The Victorian Police Service was responsible for investigating this accident; assisted by investigators from the Australian Sports Rotorcraft Association (ASRA). From an assessment of the accident site, ASRA investigators determined that the gyroplane had impacted terrain at high speed with a near-vertical nose-down attitude. The rotor system had detached from the airframe during the impact sequence and lay several metres from the majority of the wreckage. There was no evidence of fire.

A preliminary inspection of the gyroplane’s flight controls found that the rotor head torque tube had fractured through the central section where it adjoined the rotor head torque bar ( and Figure 2). Upon closer examination, ASRA investigators identified evidence of possible pre-existing cracking within the torque bar at the point of failure, and in consideration of the critical nature of this component in the flight control system, ASRA staff sought assistance from the Australian Transport Safety Bureau (ATSB) in the formal technical examination and analysis of the torque bar failure. Assistance was also sought in the examination and possible data recovery from a GPS unit and personal mobile telephone being carried on board the gyroplane.

Figure 1: ‘GT Kruza’ gyroplane, registration G-762, prior to the accident. The location of the rotor head and torque tube is arrowed.

Source: Australian Sports Rotorcraft Association

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Figure 2: The fractured torque tube as found at the accident site.

Source: Australian Sports Rotorcraft Association

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ContextTechnical assistanceA number of items were recovered from the gyroplane and submitted to the Australian Transport Safety Bureau (ATSB) with a request from ASRA to conduct additional examinations. Submitted items included components from the flight controls of the gyroplane; the fractured torque tube and the majority of the rotor head. Two electronic devices were also submitted; a Garmin ‘GPSmap 295’ and a Blackberry mobile phone.

In order to facilitate the ATSB’s technical investigation and to provide protection to the analysis findings and any electronic data recovered, an External Investigation was commenced in accordance with the provisions of the Transport Safety Investigation Act 2003.

Flight control systemPitch and bank control of the gyroplane was effected through pilot manipulation of the control column. Forward-rearward and side-to-side movement of the column produced a corresponding movement of the rotor head – tilting the plane of the rotor disk. Control rods were connected to the rotor head of the gyroplane through the 3/4-inch (19 mm) diameter torque tube, which was bolted and clamped to the torque bar ( and Source: Australian Sports Rotorcraft Association).

A 3/8-inch (10 mm) diameter pinch bolt was inserted through both the torque tube and torque bar and then tightened against a nylon-insert locking nut - the intention being to rigidly clamp both items together during operation. A hole through the torque tube accommodated the pinch bolt. The nature of the torque bar and tube assembly design represented a critical link in the flight control system of the gyroplane, with any separation of the components likely resulting in a loss of control.

Examination of the evidenceRotor head components and torque tubeAs received, the torque tube had fractured transversely into equal halves through the central bolt hole. Indications of surface fretting and wear were present on the clamping surfaces of the torque tube and bore of the torque bar - providing evidence of relative movement between the torque tube and bar during service (Figure 5).

Examination of the torque tube fracture surfaces using a binocular microscope confirmed the presence of progressive crack growth features on the entirety of the fracture surface. Repeated, concentric crack progression marks were indicative of a fatigue cracking mechanism, with origin at the external corners of the uppermost torque tube hole (through which the pinch bolt passed). The cracking had extended circumferentially around the tube diameter to the point where the cracks met the lower bolt hole and the tube fractured (Figure 7 to Figure 9).

Externally, the pinch bolt nut flanks showed paint loss consistent with the security of the nut having been checked prior to receipt by the ATSB (possibly during routine maintenance). A purple coloured ‘torque stripe’ across the nut/bolt end showed no evidence of movement or loosening between the two components (Figure 6).

Chemistry and hardnessInformation from the gyroplane manufacturer indicated that the torque tube was specified to have been produced from a thin-walled (~1.5 mm), 3/4-inch diameter 4130 medium-carbon, low alloy steel tube. To verify, a section of the tube was removed and examined under the scanning electron microscope, which, under semi-quantitative analysis, showed the steel to be alloyed predominantly with chromium and manganese with minor additions of molybdenum – consistent

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with the 4130 specification. The measured hardness of the tube (266 HV0.5) was consistent with the 4130 alloy steel in the quenched and tempered condition.

Garmin GPSmap 295The Garmin GPSMAP 295 was a portable GPS navigation device capable of storing flight path information. As-received, the outer casing of the device was severely deformed (Figure 10). The device was decontaminated and then disassembled to view the condition of the internal circuitry.

The circuit board was deformed and cracked in several locations. The EMI shield was removed from the circuit board and multiple electronic chips were found to have detached from the circuit board under the influence of the accident impact loads (Figure 11).

While the discrete memory chip which contained the most recent flight path information was identified, it had sustained damage to several of its ball-grid array connection pads. A local solder repair of the electronic connection between the memory chip and the circuit board pad was attempted; however no valid data was able to be recovered.

Blackberry mobile phoneSmart-phone devices, such as the BlackBerry unit recovered from the accident site, contain componentry and functionality to enable display and recording of positional and track information when running dedicated applications for this purpose. Due to the level of in-built encryption in this class of device however, and in the absence of a specific password for the unit, data recovery was not possible.

Other gyroplanesThe manufacturer of the gyroplane reported to ASRA investigators that only one other of their aircraft had been manufactured with the same rotor head and torque tube design as G-762. Immediately after this accident, the control system for that gyroplane was removed from service, disassembled and the torque tube examined. No evidence of cracking was found.

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Figure 3: Close view of the rotor head taken during construction of the Kruza G-762 prior to final painting. Several key components are labelled.

Source: Australian Sports Rotorcraft Association

Figure 4: Rotor head and fractured torque tube from G-762, as received by the ATSB.

Source: ATSB

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Figure 5: Fractured torque tube showing the point of failure intersecting a central bolt hole. The darkened areas on the tube surface are fretting damage from relative movement between the torque tube and torque bar.

Source: ATSB

Figure 6: Torque bar clamping arrangement. The purple ‘torque stripe’ was intact on the pinch bolt which indicated that the bolt had not been adjusted since the initial installation. Paint loss on the nut flanks was indicative that the pinch bolt may have been checked for security during routine maintenance.

Source: ATSB

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Figure 7: Fracture surface of the failed torque tube. Flashing from drilling the central hole remains in place.

Source: ATSB

Figure 8: Evidence of crack progression and ‘beach marks’ on the fracture surface confirms that the torque tube failed by fatigue.

Source: ATSB

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Figure 9: Evidence of crack progression and ‘beach marks’ on the fracture surface confirms that the torque tube failed by fatigue.

Source: ATSB

Figure 10: A Garmin GPSmap 295 and a Blackberry mobile phone device were recovered from the accident and submitted to the ATSB with a request from ASRA to recover the data. The units are shown in the as-received condition.

Source: ATSB

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Figure 11: Internal circuit board of the GPSmap 295 showing the displaced memory chips.

Source: ATSB

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AnalysisTorque tube failureFailure of the rotor head torque tube occurred as a result of a progressive fatigue cracking mechanism, initiated at the internal corners of a through-drilled hole for the assembly clamping bolt. The manifestation of fatigue cracking in this region was a direct result of the development of dynamic (operational) bending stresses within the component, to levels that exceeded the fatigue endurance of the tube material.

The design of the torque bar and tube unit provided for the translation of control system loads into cantilever bending loads at the junction between the bar and tube. By virtue of the rigidity differences between the hollow-section tube and solid bar, bending load concentration within the tube would be expected at the tube entry into the torque bar, with further amplification arising should movement within the assembly occur as a function of inadequate clamping. Local stress-raising features in loaded areas can also significantly increase the predisposition to fatigue cracking – in this instance, the placement and production of the pinch bolt hole can be shown to have significantly concentrated the stresses at the hole corners – making those regions ideal fatigue crack initiation sites.

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SummaryThe Australian Sports Rotorcraft Association (ASRA) requested the Australian Transport Safety Bureau (ATSB) to provide technical assistance in the examination of items recovered from the wreckage of a GT ‘Kruza’ gyroplane that collided with terrain near Mangalore Aerodrome, Victoria on 14 January 2012. The following conclusions were drawn from the examinations performed:

Fracture of the rotor head torque tube was directly associated with the development of fa-tigue cracking that had initiated from stress concentration effects around a clamping bolt hole that passed, by design, through the centre of the tube.

The level of surface fretting and evidence of movement between the tube and torque bar suggested a level of inadequate clamping force between the components.

There was no evidence of loosening of the clamping bolt locking nut.

There was some evidence to suggest that the tightness/security of the clamping bolt nut had been checked at some time before the accident.

No data was able to be recovered from either the Blackberry mobile phone or the Garmin GPSmap 295 device.

Further informationThe investigation into the circumstances of this accident was conducted by the Victorian Police Service, supported by the Australian Sports Rotorcraft Association. The involvement of the Aus-tralian Transport Safety Bureau was limited to the technical examinations summarised within this report.

Requests for further information regarding the occurrence should be directed to the Victorian Po-lice Service or ASRA.

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General detailsOccurrence details

Date and time: 14 January 2012

Occurrence category: Accident

Primary occurrence type: Loss of control leading to impact with terrain

Location: Near Mangalore Aerodrome, Victoria

Aircraft details Manufacturer and model: GT Gyroplanes ‘Kruza’

Registration: G-762

Total time in service 310.9 hours

Engine model Subaru EJ25

Type of operation: Flying training

Persons on board: Crew – 1 Passengers – 1

Injuries (fatal): Crew – 1 Passengers – 1

Damage: Destroyed