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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
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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.
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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
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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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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
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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
Source: Google Images
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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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
Source: Google Images
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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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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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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Figure 7: Repairs done to Right Wing of Flight 101
Source: National Transportation Safety Board 2007, p.26
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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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.
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Figure 8: Location and Structure of Centre Wing Fuel Tank
Source: National Transportation Safety Board 2000, p.31
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.
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