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Both military and civilian aircrafts fleets are operated
throughout the world to increase the time period of the serviceability
of aircrafts and thus there is a great need to address the challenges
of aging aircrafts. The detection of corrosion is of greatest concern
when structural problems of aging aircrafts are discussed. Many
non-destructive testing techniques are used for the detection of
deformities occurred at the surface and sub-surface. Corrosion is an
example of such deformities. There is a greater need for the
implementation of some advanced techniques for the inspection of aging
aircrafts. The advanced techniques should be used especially for the
detection of corrosion in the components that are constructed with
composite materials. In this paper, laser-ultrasonic detection method
is described that is used for the detection of hidden corrosion in
aircraft lap joints. The detection of hidden corrosion has been
recognized as a serious problem in the maintenance of aging aircraft
structural elements such as lap joints. In the presence of corrosion,
the thickness of the metal skin may be significantly reduced and reach
a level (generally above 10% of metal loss) that requires repair or
replacement. The Industrial Material Institute (IMI) has developed a
novel method that uses the spectral analysis of laser-ultrasonic
waveforms to determine the residual metal skin thickness of the top
skin of a lap joint. Previous work has shown that a characteristic
equation can be derived that predicts the resonance frequencies of a
paint-metal structure, such as encountered in an aircraft lap joint.
Using numerical minimization techniques, this expression is used to
process the laser-ultrasonic data and produce thickness maps of both
the paint layer and the metal skin of a lap joint. Results from
standard samples with flat-bottom holes show that the laser-ultrasonic
technique can detect metal loss below 1% of the nominal thickness value
of the metal skin.
Introduction
The aircraft were relatively inexpensive and plentiful
when they were first introduced into military and civil services. But
their same characteristics are not found today. This is because the
modern aircraft are more complex. Many advanced systems have been
introduced in the aircraft that has reduced the number of aircraft in
the fleet. As a result, the costs for in-service support and initial
purchase have increased dramatically. National defense budget has been
reduced in many North Atlantic Treaty Organization (NATO) countries.
(Rudd, 1996) This reduced budget together with the increased costs of
aircraft will force aircraft to serve for a longer period, longer than
the period what was anticipated as its retirement period. It is found
that over 51 % of the aircraft used by the United States Air Force
(USAF) have served for more than 15 years. Among those 51 % aircraft,
44 % of the aircraft has served for more than 25 years. It is found
that some of the aircraft that are already overage are expected to
serve for more than 50 years such as C-135, B-52 and T-37 are supposed
to remain in service till 2015. At that time, they have been serving
for more than 50 years. The trend of using the aging aircraft is widely
recognized. Many countries and air forces are taking keen interest in
the matter of aging aircraft and the problems associated with them.
Thus, many aging aircraft programs are initiated that will deal with
the issues related to the maintenance of those aging aircraft. Research
and Development (R & D) program is one of the programs that are
initiated for addressing the issues related to aging aircraft. Research
and Development program deals with the integrity of aircraft structure.
Canada has also initiated some programs. An aging aircraft section is
found in the National Research Council of Canada (NRC). Although the
Research and Development funding is very limited in that section.
(Stoermer, 1990)
Corrosion is found to be the greatest threat for the structural
integrity of aging aircraft. There are many specific types of corrosion
but all of them result in the degradation of material. And as a result,
the structural integrity is greatly reduced. (Colavita, 2001)Two points
are vital in the aircraft maintenance program, corrosion control and
corrosion detection. Some forms of corrosion can be detected with naked
eyes but there are some special forms of corrosion that require
nondestructive testing (NDT) methods for the detection. In this method,
the parts are not dissembled and inspection is done without harming the
aircraft. Currently, eddy current and X-radiography are used for the
detection of corrosion. As new materials have been introduced in the
aircraft and so new types of problems are found in aging aircraft,
there is a great need for the introduction of some additional advanced
detection methods. Emerging technologies is the term that is used for
describing the newer methods used for the detection of corrosion.
(Moen, 1990) The budget for emerging technologies has been reduced
drastically within Department of National Defense (DND) and in Canada
in the past few years. So there is a greater need for the increase in
the budget for the emerging technologies so that newer techniques will
be identified and implemented for the detection of the problems related
to the aging aircraft.
Aging Aircraft
On March 13, 1958, two B-47 aircraft of the United
States Air Force (USAF) were lost due to the fatigue cracking in the
wing. This led to the establishment of the reality of aging aircraft
and the consequences of aging were recognized. At that time, a service
life for the B-47 was not established by the United States Air Force.
The design of the aircraft was based on the assumption that overload
was the only threat that could damage the structural integrity of the
aircraft. (Rudd, 1996) The cracking in the wing led to the
establishment of the Aging Aircraft Program and the United States Air
Force Structural Integrity Program. As the defense budget has been
reduced in many NATO countries, the existing aircraft will remain in
service for a period longer than their life. This sounds threatening to
the life of the aircraft as well as those of the pilots and passengers.
Discussions have begun about aging aircraft and the requirement of some
urgent aging aircraft policy is felt. Following are the problems
associated with the aging aircraft. (Lincoln, 2001)
Fuel System Problems
The working group determined that the three
most common fuel system problems encountered by jet pilots are leaks,
fuel-filter clogging and inability to shut down the engine. Major fuel
leaks can result in engine fire, engine flameout or, eventually, in
fuel exhaustion. Engine instrumentation will indicate only leaks that
are downstream of the fuel flow meter. A leak between the tanks and the
fuel flow meter can be recognized only by comparing fuel usage between
engines or by comparing actual usage to planned usage. On a long
flight, one might see a fuel imbalance. (Sampath, 1996)
The working group has said that it is the crew’s responsibility to
isolate the leaks if a major leak occurs. This should be done in order
to prevent fuel exhaustion that can lead to f ire. The chances for a
major leak to lead to fire are greater in two cases. First, if the
plane is stationary and second, the altitude is low. It is the crews’
responsibility to request for the emergency services that should be
available at the landing time even if there is no fire. If the fuel is
heavily contaminated with rust, water, algae etc., there are chances
for the observation of multiple fuel filters by pass indications.
Fuel-filter clogging results from debris in the fuel line. Typically
this comes from severe fuel contamination either off the truck or
following tank maintenance. In any case, clogging usually will be
observed at high power settings when the fuel flow through he filter
(and the pressure drop across the filter) is greatest. Usually, the
fuel system plumbing will bypass a clogged filter and send fuel
directly to the engine in an attempt to keep the fire lighted. However,
one should anticipate problems with fuel control and flow as the
contaminant goes into the engine fuel system. (Barnaby and Marlies,
1986) With fuel contamination, there is potential for multiple-engine
flameout. Fly the airplane and follow the AFM or Aircraft Operating
Manual (AOM). Shutting down an engine using normal procedures may not
be possible if the engine-fuel shut-off valve malfunctions. Stopping
fuel flow to the engine can be accomplished by pulling the fire handle,
but the shutdown may take a bit longer than usual as fuel runs out of
the plumbing between the valve and the engine.
Oil System Problems
The oil system is monitored by a number of
sensors -- pressure, temperature, quantity and filter clogging. A
general failure is confirmed by the presence of multiple abnormal
indications, but a single abnormal indication may or not be a valid
indication of trouble. And, because there is considerable variation
between failure progressions in the oil system, the symptoms will vary
from case to case. Nevertheless, the working group suggests the
diagnostics that follow. First, oil system problems may occur in any
flight phase and generally progress gradually. (Rudd, 1996) They
eventually may lead to severe engine damage if the engine is not shut
down. Leaks will cause a reduction in oil quantity, down to zero
(though there still will be some usable oil in the system at this
point). Once the oil is exhausted completely, the oil pressure will
decrease to zero, followed by the low-oil pressure light. Maintenance
error has caused leaks on multiple engines; therefore, the crew should
monitor oil quantity on all engines. Rapid change in the oil quantity
indication after thrust lever movement may not indicate a leak -- the
change may be caused by oil flow fluctuations as more oil flows into
the sumps. (Barnaby and Marlies, 1986) Bearing failures will be
accompanied by an increase in oil temperature and vibration. Audible
noises and filter clog messages may flow; if the failure progresses to
severe engine damage, low-oil quantity indications and low-oil-pressure
indications may be observed. Oil pump failure will be accompanied by
low-oil-pressure indications and a low-oil-pressure light, or by an
oil-filter clog message. Oil system contamination -- by carbon
deposits, cotton waste, improper fluids, etc. -- generally will lead to
an oil-filter-clog indication or an impending-bypass indication. This
indication may disappear if thrust is reduced, because the oil flow and
pressure drop across the filter also will decrease. (Sampath, 1996)
Thrust Lever Response
Thrust lever problems on modern jets can be
subtle -- so subtle that crews can miss them altogether -- with
disastrous consequences. The working group explains the phenomenon this
way: If an engine slowly loses power -- or if, when the thrust lever is
moved, the engine does not respond -- the airplane will experience
asymmetric thrust. This may be concealed by the autopilot's efforts to
maintain the required flight condition. If, in the absence of external
visual references, the crew does not recognize the situation until the
autopilot drops out, an unrecoverable airplane upset can result.
Indications of thrust lever problems may include: (Rudd, 1996) •
Multiple system problems such as generators dropping off-line or engine
low-oil pressure; • Unexplained airplane attitude changes; • Large
unexplained flight control surface deflections (autopilot on) or the
need for large flight control inputs without apparent cause (autopilot
off); and, • Significant differences between primary parameters from
one engine to the next. The working group said that if there is a
chance for the asymmetric thrust to occur, the appropriate rudder input
or trim input should be done as the first response. If the autopilot is
disconnected without the performance of the appropriate control input
or trim, there is a great chance to observe a rapid roll. (Colavita,
2001)
Vibration
From the beginnings of powered flight, pilots have
listened for vibrations with all their senses to gauge the health of
their engines. Vibration detection remains a useful technique in
troubleshooting, but it is not easy to identify the cause of vibration
without other indications. (Rudd, 1996) Hence, a crew must study the
engine instrumentation to discover what is causing the vibrations.
Turbine engine vibrations can result from many causes, including: • Fan
imbalance at assembly; • Fan-blade friction or shingling; • Water
accumulation in the fan rotor; • Blade icing; • Bird ingestion/FOD; •
Bearing failure; • Blade distortion or failure; and • Excessive fan
rotor-system tip clearances.
While vibrations certainly should be recorded in the maintenance log
and the offending engine should be observed closely during the
remainder of the flight, the working group reminds pilots that
vibrations in and of themselves are not particularly dangerous. It is
not necessary that vibration damage the aircraft even if the vibration
is very sever due to some failures on the flight deck. It is advised
that no action should be taken only on the basis of an indication of a
vibration. So, scan the engine instruments for clues. Shut down the
engine if dictated by the failure mode. Remember, a damaged engine may
continue to vibrate even after shutdown due to an unbalanced fan wind
milling close to the airframe's natural frequency. Changing airspeed or
altitude may reduce the vibration. (Colavita, 2001)
Corrosion
As the airplane fleet is aging, corrosion has been
recognized as a serious problem in maintenance of these aircraft
(Wallace, 1985). A particular corrosion inspection problem is the
detection of hidden corrosion in lap joint structures. A lap joint is
formed by at least two metallic skins joined together by fasteners. The
presence of corrosion between the two skins will lead to thinning of
the metal skin as well as pillowing (bulging) of the surface of the lap
joint (caused by the presence of corrosion by-products). When the
thinning of the metal skin reaches a specified level, normally 10% of
the nominal skin thickness, the section of the lap joint must be
replaced. Presently, this type of corrosion is detected mainly by
visual inspection, e.g., by observing the pillowing of the surface when
a beam of light (flash lamp) is directed onto the lap joint at a
grazing angle. This method of detection is tedious, time consuming,
very dependent on the operator as well as mainly qualitative in nature.
Quantitative methods are needed if the aerospace industry wants to
shift from a reactive mode toward corrosion (i.e., "find and fix") to a
managed approach (i.e., "predict and plan") (NATIBO, 1998)
Previously a novel method has been represented based on
laser-ultrasonic for a rapid and quantitative detection of hidden
corrosion in a lap joint structure (Choquet, 1998). This method
consists of analyzing the frequency spectrum of a wide-band
laser-ultrasonic signal obtained from the lap joint structure. Based on
a multi-layer ultrasonic model (Levesque & Piche, 1992) the
frequency analysis allows us to determine areas where the top skin of
the lap joint is "acoustically" separated from the rest of the
structure. For these areas, the analysis of the position of the
resonance peaks in the laser-ultrasonic frequency spectrum leads to a
very accurate measurement of the residual metal skin thickness.
Initially, the presence of a thin layer of paint on top of the metal
skin was considered to have no impact on the residual metal thickness
measurement. However, a more detailed analysis has shown that even a
paint layer of a few tens of microns in thickness has a strong effect
on the values of the resonance frequencies. The multi-layer model
predicts these frequency shifts, if we considered the paint layer
bonded to the top metal skin. For a simple two-layer structure, such as
a paint layer on a metal skin, the multi-layer model can be simplified
to yield a simple characteristic equation that gives the positions of
the resonance peaks in the laser ultrasonic frequency spectrum. This
characteristic equation can then be used to determine the thickness of
the two layers using the measured position of the resonance peaks in
the ultrasonic spectrum and a standard numerical optimization method.
Research previously carried out at IMI has shown that broadband
ultrasonic spectral analysis can be used to identify areas of suspected
corrosion in metal lap-joint structures and then to measure in those
areas the amount of metal loss due to corrosion. The method assumes
that when corrosion is encountered, the top skin of the lap joint is
detached from the rest of the structure. If no paint is present on top
of the metal skin, simple ultrasonic resonance analysis could then be
used to obtain a very accurate thickness measurement of the residual
metal skin. However, if the skin is painted, previous research at IMI
has shown that the paint and its adhesion characteristics can severely
affect the estimate of the metal loss, even for very thin paint layers
(thickness <50jim). Since aircraft inspections are generally done
with minimal modifications to the aircraft surface, in most cases,
corrosion detection would have to be made with painted surfaces.
(Chapman, & Marincak, 1996)
Common System Problems
Someone once said that 99 percent of
electrical problems are really mechanical problems, and experience
seems to bear that out. One of the more common occurrences is a
generator failure — typically a mechanical failure of the moving
components. The most common problem technicians face is with electrical
connectors. Whenever there is a failure of an electric component, there
is always some mechanical problem behind it. Sometimes, the failure of
electromechanical components occur because of mechanical parts, such as
an autopilot that is under operation all the time Just like mechanical
systems, electrical systems wear, age and degrade, and that translates
to poor performance and occasional failures. (Wiring Integrity
Analysis, 2000)
As aircraft age, so do their electrical systems, and that can make
for shocking surprises. The crew of Boeing 727 got just such a surprise
one day right after take off. White smoke came billowing out of the
cabin vents, obscuring visibility and sending a bolt of fear through
passengers and cabin attendants alike. Fortunately the crew was able to
quickly dump fuel in return for a hasty emergency landing before the
situation got out of control. The problem appeared to be chaffed
electrical power cables that had shorted out. The excessive heat caused
the plasticized wire insulation to melt and fuse together, emitting the
white smoke and fumes. The maintenance manager explained that the
insulation start cracking with the passage of time when the wiring
becomes old and this leads to the corrosion of the terminal ends.
Sometimes, the corrosion is formed under the insulation especially in
the case of aluminum wiring. The corrosion forms in such a way under
the insulation that it can not be seen and electrical resistance is
increased due to it. Grounds also become corroded with old electrical
systems. Sometimes it happens that rotating beacon or a nav light stop
functioning. The problem is identified as the bad ground. When the bad
ground is cleaned, rotating beacon or nav light start functioning.
Deterioration of the electrical system can cause a number of anomalies,
some of which are exasperatingly difficult to sort out. One of the most
prevalent problems is chafing and degradation of wire insulation caused
by vibration, improper modifications and environmental contaminants.
One result of this degradation can be arcing----either between wires,
or between wires and the aircraft structure---resulting in situations
like that experiences by the 727 crew. There is a chance for the wire
bundles to chafe and wear if the joints of the wiring are not secured
properly and as a result, the wires become exposed. In fact, degraded
wiring can cause any number of erroneous instrumentation readings,
including faulty caution and warning indications. (Down to the Wire,
2001) Pilots have reported that they had called maintenance for
checking and identifying the problems before their departure because
they were unable to get the engine fire detection system. Mechanics
checked the system and found out that wires were bared due to the
chaffed wiring in 12 locations. A study conducted by Boeing of 81
in-service aircraft and six recently retired aircraft determined that
wiring degradation is not necessarily related to the age of the
aircraft, environmental conditions or type of wiring, but is more a
function of maintenance and modifications performed over the life of
the aircraft. In particular, the areas that need increased emphasis are
removal of accumulated contaminants from time to time and inspection of
wiring for critical airplane systems. Dirt, oil and many other
contaminants should not be allowed to accumulate on the bundles of wire
because they result in arcing and plane can catch fire.
Aging Aircraft’s Wiring and Firm Standards
With more than 2,000
commercial passenger planes in the U.S. still flying beyond their
original design life, the federal government will soon announce a
program requiring airlines to rigorously monitor aircraft wiring
systems in order to catch age-related electrical failures before they
result in fatal disasters. Until now, airplane manufacturers and the
airlines have not considered the aging of electrical wires and other
non-structural components to pose serious safety threats, mainly
because of the existence of backup systems. But the Federal Aviation
Administration has assembled a team of engineers and maintenance
specialists in the wake of the 1996 explosion of a Boeing 747 on TWA
Flight 800 off Long Island and more recent red flags raised over
abrasion on wiring insulation found during inspections this year of
Boeing 737 aircraft that have accumulated the most flight hours. (Down
to the Wire, 2001) The FAA report, representing an expansion of the
agency's aging aircraft program, is expected to be forwarded this month
to the White House Commission on Aviation Safety and Security. In
addition to wiring issues, the program will cover pumps and other
electro-mechanical systems, and fuel, hydraulic and pneumatic lines,
said FAA spokesman Les Dorr Jr. A source in the FAA's transport
standards office said that certain individuals were responsible for the
increase in the wiring problems many years ago. But now attention is
given to this matter and it is under progress. The source said the
report urges regular inspections of wiring with a special focus on the
susceptible areas of the aircraft. The agency does not know about the
type of wiring installed in all the planes. Each plane had different
sort of wiring in it. The agency just guesses about the type of wiring
used. (Review of Federal Programs, 2000)
There are about 150 miles of wire on a commercial jetliner.
Inspections this year found abrasion of varying degrees on the
protective insulation of wires on about two-thirds of older 737
wing-fuel tanks that were inspected. In some cases, the abrasion
exposed bare wire, raising the potential for electrical "arcing" and a
burn-through of the conduit that encases the wire bundles. A leading
theory in the Flight 800 accident suggests that an arc occurred near
the jet's center fuel tank, sparking an explosion that ripped apart the
plane. The cause of the accident is still under investigation. (Down to
the Wire, 2001) FAA officials declined to discuss the impending
report's contents or to say whether inspections will be increased, a
suggestion that has been made by aviation watchdogs.
The Tribune reported in May that many older airliners contain wire
insulation that the U.S. military stopped using 20 years ago because of
concerns about reliability. Beginning in 1978, the Defense Department
documented about the abnormal insulation aging that resulted in the
cracking of wire coatings called Poly-X and Kapton, which were removed
from fuel tank areas of fighter planes by the late 1980s, Pentagon
records show.
The FAA said there is no evidence that Poly-X, Kapton or any wire
insulation pose risks in commercial aircraft, which are exposed to
fewer rigors than military planes. (Review of Federal Programs, 2000)
Although investigators have not closed in yet on the probable cause of
the Sept. 2 crash of Swissair Flight 111 off the coast of Nova Scotia,
the plane, an MD-11 that contained Kapton wire insulation, experienced
some unspecified electrical problems during its seven-year lifetime,
according to maintenance records of the MD-11 cited by the Canada
Transportation Safety Board. Chief crash investigator Vic Gerden has
said that an electrical system failure is one of a number of leads
being studied. The plane's captain reported smoke in the cockpit and
objects recovered from the cockpit are reported to show signs of smoke
damage.
Ed Block, a former wiring expert for the Pentagon who has publicly
disclosed problems with several kinds of wiring insulation, said
chafing and flammable insulation on the electrical systems of aging and
high-use aircraft is a widespread problem and may have caused a number
of aircraft fires and fatal accidents in recent years, including
possibly TWA Flight 800 and the 1990 fuel-tank explosion of a
Philippines Airlines 737. (Review of Federal Programs, 2000) It is
found from the upcoming reports that FAA is showing its concern and
attempt for holding a whirlwind.
The hot button issue is concerned with the type of wiring used. It
will check whether all the wires are same. This is has been some
fallacious contention of the FAA and it will check if the wire can be
replaced that has been susceptible to chafing, stress and breakdown. If
this is not the case, then the plane will need the retirement.
Suggestions
Some suggestions related to aging aircraft are listed
below:
• Attention should be paid to technical obsolescence
• The
system should be upgraded
• The unexpected mission requirements should
be changed that were found during the design specification and
development.
• Attention should be paid towards the great increase in
the maintenance costs
• The safety is decreasing as aging aircraft will
be used beyond their life limit.
• The readiness of fleet will be
impaired.
• The third line repair facilities are unavailable. Attention
should be paid on getting those facilities.
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