Abstract
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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