| 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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