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I was ignorant once too, both about wing
fatigue and what I could do to an aircraft before "destroying" it
due to pulling G's. It took a tragic mishap and loss of life to
educate myself and the aircraft community I was flying in at the
time about wing fatigue.
Wings, being made of metal, will flex
went loaded up with G's. They even flex in 1 G level flight. Next
time you are on a commercial jet watch the wings as the aircraft
approaches liftoff and you can see how much they flex up near the
wingtips. How much a wing flexes depends on how much G is applied at
the time. Sooner or later, depending on how much and how often the
wing is flexed, the wing will fail. Want a demonstration? Bend a
paper clip back and forth. If you bend it 90 degrees each time it
will fail quickly. Bend it 10 degrees and it will still fail but you
have to bend it many more times. What else did you notice? I am
willing to bet the paper clip failed not at the extreme but at some
intermediate point where it wasn't flexed all the way. Was there any
visual indication the paper clip was getting ready to fail? Probably
not, all of a sudden the last couple of flexes seemed easier, then
there it was in two pieces. Get the point? Metal, when flexed, will
eventually fail if flexed repeatedly. Every time you apply G in your
T-34 you are flexing the wing and spar and bringing it that much
closer to failure. The T-34 wing and spar are not indestructible.
They can and will fail. What determines when it will fail is how the
aircraft is operated prior to the failure.
Many years ago in another life I was a
military pilot. My aircraft community lost an aircraft when the wing
separated and the aircraft crashed. The aircraft had used up all the
calculated fatigue life available and fatigue life expenditure (FLE)
had exceeded well over 100 percent. Since I had information of a
privileged nature concerning this mishap I am not at liberty to
discuss the aircraft type, location, date, or any other mishap
details but I can summarize some of the lessons learned.
-- The aircraft was being operated
within the published flight envelope. It was not exceeding airspeed
or G limitations prior to or during the wing failure. We knew it was
operating within the G limitations because it carried counting
accelerometers that would register a "hit" whenever 4, 5, 6, or even
7 G's were reached. We weren't getting high G hits outside the
published.
-- Fatigue life was monitored using the
accelerometers. Readings were pulled monthly and fed to the
engineers who calculated remaining fatigue life. When the fatigue
life expended (FLE) reached certain levels G restrictions were
imposed on the aircraft. When the 100% threshold was met the
aircraft was grounded.
-- FLE on the mishap aircraft was
improperly calculated for many reasons. All data was still available
so FLE was re-calculated and it was discovered the aircraft had FLE
well over 100%. The wing had been "bent" so many times that it
failed just like a paper clip fails when bent.
-- The aircraft was actually in 1 G,
wings level flight when the wing failed. You don't have to be under
high G loading to cause the wing to fail at some given point in
time. The damage was already done. We knew the aircraft was in level
flight because another squadron aircraft in the same flight
witnessed the breakup.
-- Sadly, up to this mishap, we the
operators had a collective mentality that the aircraft could never
be broken. Following this mishap we began to operate much more
intelligently regarding when and how we pulled G's.
FLE vs. G's pulled is not a
straight-line curve, it is more exponential. At 6 G a much higher
percentage of fatigue life will be expended than at 4 G. FLE is also
gross weight dependent. Most, if not all, military aircraft have an
absolute G limit which is most likely at a significantly lower gross
weight than takeoff weight. As gross weight increases the amount of
G one can pull is reduced. So if you are operating your aircraft in
a high G environment vice straight and level you are consuming a far
higher percentage of available wing life each flight and pushing
that wing closer to failure.
Every time you hit a certain G level the
same amount of fatigue life is expended regardless of how long you
are pulling the G. A five second 4 G pull uses as much fatigue life
as a split second pull. It is possible to instantaneously pull a
very high G load within the normal airspeed flight envelope without
the body noticing it. Even if pulling less G and you flew through
wake turbulence or other turbulence you could instantaneously hit a
higher G load. Your body wouldn't feel it but that higher load would
consume more FLE nonetheless. What's important in wing fatigue and
calculating FLE is the fact you hit the level, not how long you were
at that level. Bend your paper clip and leave it bent-- it won't
fail. Bend it repeatedly and it will fail.
Flight hours flown is irrelevant
regarding FLE. What does matter is how many G's were pulled and how
many times the G's were applied. This would account for why a single
owner/operator aircraft probably being operated in a more benign
environment would last longer than an aircraft being leased/flown in
an air combat or out of control flight environment that repeatedly
subjects it to high stresses. The squadron with the mishap described
above took delivery of a brand new aircraft from the factory.
Operating within the normal flight envelope the wing was used up in
less than three years of normal operations and had less than 2,000
hours on it when restrictions were imposed. The aircraft was
operated in a training environment and had a greater number of
higher G hits than comparable aircraft in the fleet even though it
had fewer flight hours.
What does this mean to the average T-34
operator? Hopefully a great deal. Do you know how your aircraft was
operated in the past? Do you know how many G's were pulled and how
many applications of those G's were made? Do you have any idea how
much fatigue life has been expended over the years? Do you know the
condition of your spars? Is there any corrosion in the spars? How do
you intend to operate your aircraft?
You should be asking yourself the above
questions in regarding what fix to apply to your aircraft.
Personally I would opt for a fix relative to the type of flying I
would be doing. The military flying club I am associated with
operates two T-34B aircraft. The flying club program prohibits
aerobatics, air combat, and out of control flight training so we
opted for the least expensive AMOC we could find. But if I were
operating in a higher G arena like flight demonstration and air
combat I would personally opt for a beefier fix, such as new spars
or a doubler plate. But that is a personal choice.
Since the original wing separation
mishap the FAA has imposed many restrictions on the aircraft. Many
operators probably felt those restrictions were too harsh and too
restrictive. The latest mishap proves one very big point-- the FAA's
positions have been validated. Hopefully this will get everybody's
attention out there. There has been much discussion about whether
the Texas Air Aces aircraft was being operated within the FAA's
restrictions concerning the wing. The investigation will tell the
story, let it run its course and we can all read about. But one
thing should be painfully obvious-- the FAA has been proven right in
a very bad way. Let's not do it again.
--
About the author. Dave Marshall is a former Navy carrier pilot and
graduate of the Navy's aviation safety officer school. He is also a
licensed pilot and former manager of a military flying club
operating, among other aircraft types, two T-34B aircraft.
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