Report on Airworthiness of Aging Aircraft

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Report on

Airworthiness of Aging Aircraft

Submitted to:Mr. Yeo Jian Heng

Director of Engineering Department,

Singapore Technologies Aerospace

Prepared by:

Yap Yi Shen

Deputy Head of Research Department,

Aerolion Research and Consultancy Pte Ltd

24 July 2010



Report on Airworthiness of Aging Aircraft

Acknowledgements

I would like to thank my fellow group members who contributed to this report by researching

diligently on the more technical aspects of this report. I would also like to thank Ms Priscilla

Poh on her guidance to how the report should be formatted and the layout of details and facts.

ii




Contents

PageAcknowledgements ii

List of Illustrations iv

Summary v

1. Introduction

1.1 Purpose 1

1.2 Background 1

1.3Methodology 1

1.4 Scope of Report 1

2. Factors that Affect Airworthiness and Measures against It

2.1 Corrosion on Aircraft Bodies2.1.1 Dangers Caused by Corrosion 2

2.1.2 Types of Corrosion 2

2.1.3 Ways to Detect Corrosion 3

2.2 Structural Fatigue

2.2.1 Dangers Caused by Structural Fatigue 4

2.2.2 Factors that Cause Fatigue 4

2.2.3 Ways to Detect Fatigue 5

2.3 Electrical Wiring Interconnection System

2.3.1 Dangers Caused by EWIS 5

2.3.3 Methods to Solve the Problem 5

3. Future Strategy for Aging Aircraft

3.1 Flight Safety 6

3.2 Reduction of Maintenance Cost 6

4. Conclusion 7

5. Appendices

Appendix A A-1

Appendix B A-3

Appendix C A-5

6. References R-1

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List of Illustrations and Tables

Page

Figure 1 Corrosion on Fuselage 2

Figure 2 Ultrasonic Testing 3Figure 3 Pulse-Echo Image 3

Figure 4 Stages of Structural Fatigue 4

Figure 5 Polariscope 5

Figure 6 Missing Area of Flight 243 A-1

Figure 7 Repairs Done to Right Wing of Flight 101 A-4

Figure 8 Location and Structure of Centre Wing Fuel Tank A-6

Table 1 Injury Chart for Flight 243 A-1Table 2 Injury Chart for Flight 101 A-3

Table 3 Injury Chart for Flight 800 A-5

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Summary

Even as air travel becomes safer by the day, the need to minimise any chance of accidents

happening still exist. One way is to determine the airworthiness of an airplane, to determine if

it is still safe to fly without endangering the lives of the people in it.

However, in this current financial situation where all companies are looking to cut down on

spending, there is a need to maximise each aircrafts flight time before buying new aircrafts to

replace them. Thus, there is a strong need to identify the airworthiness of aging aircrafts.

The problem of aging aircrafts can be thought of as a spiral. As an aircraft ages, the

frequency of checks and maintenance increases. This leads to a decrease in its time spent

flying. Due to increase in funds for maintenance and decrease in flights to earn income, thecompany loses money and might not be able to afford new aircrafts to replace the old ones,

which puts more strain on the old aircrafts as they have to be reused well after their average

lifespan. This in turn endangers the lives of the people on board the plane, and if any accident

happens that might lead to human casualties, the company will be forced to pay very heavy

consequences.

As such, there is a need to break this spiral before anything serious happens. The main way is

to spend more money and time into researching and developing new methods to detect and

solve the problems plaguing aging aircrafts in order to save a bigger sum of money trying toclean up the mess after any accidents happen. However, as new methods are not guaranteed

to be available anytime it is needed, more effort should be put into training engineers to

recognise the problems that might affect the airworthiness of an aging aircraft, how to detect

them and solve the problem before anything major happens.

1. Introduction

v






2.1 Corrosion on Aircraft Bodies

2.1.1 Dangers Caused by Corrosion

Corrosion is a problem that causes companies and government agencies to spend huge

amount of money to fix, and for a very good reason. Aircrafts that are attacked by corrosion

experience a drastic reduction in strength and lead to failure. Since there is no sure way to

remove corrosion, the problem requires expensive component repair or even total

replacement of the aircraft part.

2.1.2 Types of Corrosion

The 2 main types of corrosion affecting aging aircrafts are crevice corrosion and exfoliation

corrosion. Crevice corrosion is the most common type found on airplanes, occurring

whenever water is trapped between two surfaces, such as under loose paint. Airplanes that

operate near coastal areas are especially susceptible to crevice corrosion due to the high

humidity levels in the atmosphere. A famous example would be the Aloha Incident, where

Aloha Flight 243, a 19 year old Boeing 737, had a small portion of its upper fuselage blown

off while in mid-flight. Tests showed that the failure was due to crevice corrosion that

weakened the structure and due to high difference levels in pressure, the upper fuselage

cracked and broke off. For more information, please refer to Appendix A.

Figure 1: Corrosion on Fuselage

Source: Google ImagesExfoliation corrosion is caused when corrosion products building up along grain boundaries

exert pressure between the grains, with the end result being a leafing effect. Aircraft parts

made of aluminium alloy are especially dangerous, as aluminium alloys that have been

extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains,

are particularly prone to this damage. This will cause cracks in the structure, and might lead

to further widening of the cracks and potential disaster, as seen in Figure 1.

2.1.3 Ways to Detect Corrosion

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2

http://corrosion-doctors.org/Forms-intergranular/anisotropic.htm

http://corrosion-doctors.org/Forms-intergranular/anisotropic.htm




Most corrosion happens where it is hard to detect visually, like under loose paints and

unsealed joints. Non-destructive testing (NDT) plays an important role in this effort by

enabling the detection of early signs of corrosion so that corrective action can be taken before

the damage becomes severe. This form of detection is able to detect for corrosion in hidden

areas, yet without causing any form of damage to the aircraft. Common NDT methodsinclude ultrasonic, eddy-current testing and thermography.

The ultrasonic is most widely used for detecting corrosion on aircrafts, as it is non-hazardous

to the material or the personnel using it as well as having high penetrative power. Ultrasonic

testing works by launching very short ultrasonic pulse-waves with centre frequencies ranging

from 0.1-15 MHz and occasionally up to 50 MHz into materials to detect for flaws. When

there is any form of corrosion detected, the ultrasound waves that bounce back will not be of

the same form as other normal waves, thus showing a difference in results, as can be seen in

Figure 2. For more powerful equipments, it is also possible to show the size and exactlocation of the corroded area. These images are called pulse-echo image, as seen in Figure 3.

Figure 2: Ultrasonic Testing

Source: Google Images

Figure 3: Pulse-Echo Image

Source: http://www.machinerylubrication.com/

2.2 Structural Fatigue

viii3

http://en.wikipedia.org/wiki/Ultrasonic_testing

http://en.wikipedia.org/wiki/Eddy-current_testing


http://en.wikipedia.org/wiki/Ultrasound


http://en.wikipedia.org/wiki/Ultrasonic_testing






2.2.1 Dangers Caused by Structural Fatigue

Structural fatigue refers to progressive and localised structural damage that occurs when a

material is subjected to cyclic loading. This causes the material to crack, as seen in Figure 4,

or even break apart if the problem is not fixed. The damage is cumulative, and can also becaused by other forms of damage like corrosion.

Figure 4: Stages of Structural Fatigue


Structural fatigue is one of the worst problems a pilot can ever face while in mid-flight, since

it may lead to a part of an aircraft breaking off, causing the aircraft to go out of control. A

more recent example would be the Chalk’s Ocean Airways Flight 101 Incident in Miami,where a 58 year old G73T seaplane’s right wing broke off in mid-flight and the whole aircraft

crashed into the sea, killing everyone on board. Investigations found out that the right wing

suffered from fatigue and cracks had started to appear, but was not discovered and repaired

by the engineers of the company owning the aircraft. For more information, please refer to

Appendix B.

2.2.2 Factors that Affect Fatigue

Over time, fatigue is sure is appear if the material is subjected to cyclic stress. However, there

are certain factors that can cause fatigue to happen more quickly and shorten the material’s

life. Some factors include the surface quality, the material type, direction of loading,

environment and residual stresses. Also, fatigue is accumulative and would not heal if left

alone for even a long period of time. For aircrafts, environment plays an important role as

environmental conditions can cause erosion, corrosion, or gas-phase embrittlement, which all

affect fatigue life. In the case of Aloha Flight 243, the environment played a crucial role in

the disaster. The aircraft operates on the Hawaii islands, near coastal regions that cause

corrosion which in turn leads to structural fatigue.

2.2.3 Ways to Detect Fatigue

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1 2

3 4

4




NDT is also used to detect for fatigue. However, there is a slight difference in that optical

NDT are more favoured to detect fatigue. This is because optical NDT allow for a full-field

analysis of the inspected area without any need for physical contact with the surface. They

can also sometimes provide more or different information where the other techniques fail or

cannot be applied. Using an instrument called polariscope, like the one in Figure 5, polarisedlight is shown through onto the component and by using a simple formula, the stress levels

could be calculated.

Figure 5: Polariscope


2.3 Electrical Wiring Interconnection System

2.3.1 Dangers Caused by EWIS

Deterioration in an aircraft electrical wiring interconnection system (EWIS) is often difficult

to identify and repair. This is due to the wires being designed in a ‘fit and forget’ principle in

older aircrafts. The wires are also bundled together in huge looms, which means that only the

outer wires could be checked. If the wires malfunction, it might cause the loss of critical

functions or information of equipments. In the case of a short circuit, a fire might even break

out, like the incident of TWA Flight 800. The centre wing fuel tank exploded due to ignition

of the flammable fuel in the tank. The fire was started by a spark from exposed wires near the

fuel tank. For more information, please refer to Appendix C.

2.3.2 Methods to Solve the Problem

Since wires are still very important as they are needed to transmit important information from

critical instruments like the pitot tube to measure the speed of the aircraft, it is not possible to

remove them. However, some companies are replacing the wires with fibre optics to reduce

the need for bulky looms. These will save more space and yet increase the speed of

transmission of data, with the only hindsight being the high cost needed. As for the problem

of sparks causing ignition, the idea of nitrogen inerting is being examined for future

implementation, which extinguishes any sparks that appear.

3. Future Strategies for Aging Aircrafts

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Engineers have long identified the need to do more researching and developing more efficient

or more cost-effective solutions to the technical problems like those mentioned above.

However, research progress have somewhat come to a stall lately, and there is a need to

reiterate this problem quickly.

3.1 Flight Safety

First and foremost, flight safety should be the priority of all research since the value of

human life cannot be evaluated. From the 1950s to the 2000s, mechanical failure accounted

for 22 percent of all fatal aircraft accidents. Although this is not as much as pilot error, at 50

percent, the 22 percent should be brought as low as possible. A potential solution for

corrosion can be the use of nanotechnology. Nanotechnology employs the method of coating

the aircraft in a layer of corrosion control coating at the nanoscale. This layer will prevent

and combat corrosion degradation by directly targeting the thermodynamic enablers to

corrosion. Specifically, it will detect and repair small coating damage, detect and signalmaintainers of moisture intrusion, detect corrosion and signal aircraft maintenance engineers

of its presence, release inhibitors to combat corrosion, replenish its corrosion inhibitors from

the environment, and integrate needed repairs to the coating.

3.2 Reduction of Maintenance Cost

Secondly, focus should also be placed on reducing the maintenance cost. Cheaper solutions

need to be found to solve the problems of an aging aircraft, yet without compromising the

effectiveness of the repair methods used, like finding how to use the nanotechnology stated

above in a more cost-effective way. This will encourage airlines to maintain their aircrafts

more often to prevent any accidents from happening. The 6 main steps to go about reducing

maintenance cost, as John W. Lincoln (2001) stated, are:

• Conduct surveys to determine problems

• Identify and prioritise solutions requiring research and development

• Establish research and development roadmaps

• Obtain management and customer approval

• Execute research and development efforts

• Transition technology to the operator

The reason behind the reluctance of companies not putting in more money into developing

ways to cut maintenance costs is due to their worries that the money might not generate

satisfying results. With this in mind, government agencies should do more financially by

helping to fund researches for the strategies mentioned above.

4. Conclusion

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The problem of the airworthiness of each aging aircraft should be determined on a case-by-

case basis. The airworthiness of each aircraft is as different as every human being even if

they are of the same model built on the same day, due to the difference in working

conditions, maintenance frequency and procedures.

Also, technology is only as useful as the person utilising it. Besides searching for cost-

effective maintenance methods, more effort should also be put into training maintenance

engineers of a higher sense of responsibility and skill. Most aircraft accidents that occur due

to mechanical problems are because of the engineer’s failure to spot or repair the problem

well enough. Thus, engineers need to constantly upgrade their knowledge and skills to meet

ever-increasing problems, as well as cultivate the sense of responsibility towards their job and

the next batch of passengers boarding the plane they are maintaining.

Appendix A

Flight Number: Aloha Airlines Flight 243

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7




Date and Time: April 28 1988, 1346 HST

Aircraft: Boeing 737-200, N73711

Age: 19-years old, 89,090 flight cycles, 35,496 flight hours

Location: Kahului, HawaiiInjury Report:

Injuries Crew Passengers Total

Fatal 1 0 1

Serious 1 7 8

Minor 0 57 57

None 3 25 28

Total 5 89 94Table 1: Injury Chart for Flight 243

(National Transportation Safety Board 1989, p.10)

Description: The 19-year-old Boeing 737-200 was making a flight from Hilo to Honolulu,

Hawaii. During mid-flight, at 24,000 feet above sea level, approximately 18 feet of the cabin

skin and structure aft of the cabin entrance door and above the passenger floor line separated

from the airplane, as seen in Figure 6.

Figure 6: Missing Area of Flight 243

Source: National Transportation Safety Board 1989, p.11

Aftermath: The captain managed to pull off an emergency landing on Kahului Airport on

Maui safely, miraculously with only one casualty.

Cause: The NTSB concluded that accident was caused by metal fatigue exacerbated by

crevice corrosion. The crevice corrosion was in turn due to the plane operating in coastalregions, where water was able to enter a gap that was not fully closed by an epoxy adhesive

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A-1

http://en.wikipedia.org/wiki/Metal_fatigue


http://en.wikipedia.org/wiki/Corrosion


http://en.wikipedia.org/wiki/Corrosion






Location: Miami, Florida

Injury Report:

Injuries Crew Passengers Total

Fatal 2 18 20Serious 0 0 0

Minor 0 0 0

None 0 0 0

Total 2 18 20

Table 2: Injury Chart for Flight 101

(National Transportation Safety Board 2007, p.12)

Description: Flight 101 was a regularly scheduled passenger flight to Bimini, Bahamas,with 2 flight crew members and 18 passengers on board. Shortly after takeoff from the Miami

Seaplane Base, it crashed into a shipping channel adjacent to the Port of Miami. All 20

people aboard the airplane were killed, and the airplane was destroyed. Of about 15 witnesses

interviewed, most reported that the airplane’s right wing separated from the rest of the

airplane in flight, that smoke or fire came from the wing or a fireball in the sky, and that the

airplane subsequently descended into the water. About one-half of these witnesses reported

that they heard an explosion associated with the wing separation.

Aftermath: Lifeguards who patrolled Miami Beach on foot and on jet skis were the first to

respond to the accident scene. Miami emergency dispatch notified the Miami Coast Guard

and the Miami Beach Police Department by telephone about the accident. The Miami Coast

Guard also launched an HH-65 helicopter to the accident scene about 7 minutes after

receiving notification of the accident and began recovery efforts about 6 minutes afterward.

Cause: The NTSB concluded that accident was due to the in-flight failure and separation of

the right wing during normal flight. This was in turn due to pre-existing fatigue fractures and

cracks in the rear Z-stringer, lower skin, and rear spar lower spar cap, and this multiple-

element fatigue damage reduced the residual strength capability of the wing structure and

caused the fatigue failure of the wing during normal flight operations.

The right wing was found to have repairs done to it on May 6 1992. A crack was observed on

the wing and two doublers were attached, as seen in Figure 7. However, the doubler repair to

the accident airplane’s lower wing skin was ineffective because the doublers did not restore

the load-carrying capability of the skin in the area of the fuel sump drain and the repair did

not properly address the underlying cause of the skin cracking, which was the cracked or

fractured rear Z-stringer.

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A-3




Figure 7: Repairs done to Right Wing of Flight 101


Other Contributions: The failure of the Chalk’s Ocean Airways maintenance program

to identify and properly repair fatigue cracks in the right wing and the failure of the FAA to

detect and correct deficiencies in the company’s maintenance program contributed to the

accident, along with the strain endured throughout 39,743 landings.

Deaths: Everyone on board the plane died, including three infants under the age of 2.

Results: The NTSB advised the FAA after the incident to identify the systemic deficiencies

in the maintenance program oversight procedures that led to this incident and modify those

procedures to ensure that the maintenance program plans for commercial operators are

adequate to ensure the continued airworthiness, both structural and otherwise, of the

operator’s fleet.

Appendix C

Flight Number: Trans World Airlines Flight 800

Date and Time: July 17 1996, 2031 EDT

Aircraft: Boeing 747-131, N93119

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A-4




Age: 25-years-old, 16,869 flight cycles, 93,303 flight hours

Location: East Moriches, New York

Injury Report:

Injuries Crew Passengers TotalFatal 18 212 230

Serious 0 0 0

Minor 0 0 0

None 0 0 0

Total 18 212 230

Table 3: Injury Chart for Flight 800

(National Transportation Safety Board 2000, p.22)Description: Flight 800 was making its way from JFK International Airport in New York

to CDG International Airport in Paris. While flying over the Atlantic Ocean, the plane’s

centre wing fuel tank exploded, plunging the aircraft into the Ocean and killing everyone on

board. Many witnesses in the vicinity of the accident at the time that it occurred stated that

they saw and/or heard explosions, accompanied by a large fireball over the ocean, and

observed debris, some of which was burning, falling to the water.

Aftermath: Remote-operated vehicles (ROVs), side-scan sonar , and laser line-scanning

equipment were used to search for and investigate underwater debris fields. Victims and

wreckage were recovered by Scuba divers and ROVs; later scallop trawlers were used torecover wreckage embedded in the ocean floor. In one of the largest diver-assisted salvage

operations ever conducted, over 95% of the airplane wreckage was eventually recovered.

Cause: The source of the ignition that caused the explosion was found to be due to sparks

produced during a short circuit outside the tank. Figure 8 shows the location of the centre

wing fuel tank. The electrical wires in charge of transmitting information from the fuel

quantity indicative system to the cockpit produced excess voltage that ignited the flammable

fuel and air mixture in the tank. Due to the difficulties in checking the state of the wires,

insufficient attention has been paid to the condition of aircraft electrical wiring, resulting in

potential safety hazards.

xvii

A-5

http://www.answers.com/topic/remotely-operated-vehicle

http://www.answers.com/topic/side-scan-sonar

http://www.answers.com/topic/scuba-diving


http://www.answers.com/topic/commercial-trawler

http://www.answers.com/topic/remotely-operated-vehicle

http://www.answers.com/topic/side-scan-sonar


http://www.answers.com/topic/commercial-trawler




Figure 8: Location and Structure of Centre Wing Fuel Tank


Other Contributions: Contributing factors to the accident were the design and

certification concept that fuel tank explosions could be prevented solely by precluding allignition sources and the design and certification of the Boeing 747 with heat sources located

beneath the centre wing fuel tank with no means to reduce the heat transferred into the tank

or to render the fuel vapour in the tank non-flammable.

Deaths: Some of the notable deaths among the 230 dead included: Pam Lychner, an

American crime victims' rights advocate; Michael Briestroff, a French ice hockey player; and Jed

Johnson, an American interior designer.

Results: This accident changed the long-standing view that electrical wires would not causeany major accidents. Rules for more stringent installations and inspections of electrical wires

were implemented and designs for the centre wing fuel tank were reviewed.

Controversy: Although this explanation was the most widely accepted, some people

believed that the explosion was instead caused by a missile. Some witness allegedly saw a

flaming object flying across the sky and hitting the plane. Also, some had claimed that the

U.S. Navy was holding a military exercise near the area and the missile could have been

launched by a Navy vessel, and that the whole investigation was a cover-up. However,

investigations by the CIA and experiments by the NTSB have concluded that the flaming

missile witnessed was 0% accurate.

xviiiA-6




References

Aging Aircraft-Electrical Wiring [online]. Available from:

http://www.skybrary.aero/index.php/Ageing_Aircraft_-_Electrical_Wiring [Accessed

18 July 2010]

Aging Aircraft-Structural Failure [online]. Available from:

http://www.skybrary.aero/index.php/Ageing_Aircraft_-_Structural_Failure [Accessed

18 July 2010]

Aircraft Corrosion [online]. Available from:

http://corrosion-doctors.org/Aircraft/Aircraft.htm [Accessed 18 July 2010]

Airworthiness [online]. Available from:

http://www.skybrary.aero/index.php/Airworthiness [Accessed 18 July 2010]

Avram, J., 2009. THE ELIMINATION OF CORROSION . . . IS NANOTECHNOLOGY THE ANSWER TO THE USAF’s #1 AGING AIRCRAFT DILEMMA?, Alabama: Air University

Lincoln, W., 2001. Managing the Aging Aircraft Problem, RTO-MP-079(II).

Manchester.

National Transportation Safety Board, 1989. Aircraft Accident Report on Aloha

Airlines Flight 243. Washington D.C.: National Transportation Safety Board.

National Transportation Safety Board, 2007. Aircraft Accident Report on In-Flight

Separation of Right Wing. Chalk’s Ocean Airways Flight 101. Washington D.C.:

National Transportation Safety Board.

National Transportation Safety Board, 2000. Aircraft Accident Report on In-Flight Breakup Over The Atlantic Ocean. Trans World Airlines Flight 800. Washington

D.C.: National Transportation Safety Board.

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http://ieo.dit.ie/technical_ndt.html#optndt [Accessed 20 July 2010]

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Report on Airworthiness of Aging Aircraft