PUBLIC SUBMISSION | As of: February 08, 2009 Tracking No. 805f3998 Comments Due: June 10, 2008 Late comments are accepted |
Docket: FAA-2007-0051
2007-NE-37-AD
Comment On: FAA-2007-0051-0001
Airworthiness Directives; Teledyne Continental Motors (TCM) IO- 520, TSIO-520, and IO-550 Series Engines with Superior Air Parts, Inc. (SAP) Cylinder Assemblies Installed
Document: FAA-2007-0051-0016
Anonymous
This NPRM concerns me on several topics.
I am an aircraft structural engineer and Airframe and Powerplant Mechanic. As
such, this makes me uniquely qualified to comment on what has been published
in this NPRM.
First, it appears that the method you referenced to detect the initial crack has little
merit. Differential pressure checks are intended to identify wear related leaks,
such as those due to worn, or broken piston rings, or burned or unseated valves.
Differential pressure checks cannot identify any cracks, which has already started,
but has not yet traversed through the entire wall thickness of the cylinder. As
such, it is a completely useless inspection technique for identifying anything other
than an already cracked-through cylinder. By the time a crack actually has
proceeded to that point, the problem usually is obvious. By then, the engine will
manifest symptoms like leakage of oil, which may smoke or catch fire, a drop in
manifold pressure resulting in a loss of power and changes in RPM, or a generally
rough running engine due to the pressure imbalance of that cracked cylinder
relative to the others. This inspection therefore appears to be nothing more than
a “feel-good” action on the part of the FAA, possibly designed to protect Superior’s
financial situation. It does not adequately address the real safety concern posed
by these under-designed cylinders.
The second concern I have with your NPRM is with your 50 hour inspection cycle.
It is absolutely essential prior to setting up your inspection interval to know your
crack propagation rates. The inspection approach in this NPRM seems to mimic
a Damage Tolerance analysis method where you assume a crack exists in the
material at the beginning of the inspection interval and based on your crack
propagation rate you have to position your inspection intervals at a point prior to
failure of the part plus any margin to account for unknowns. If the crack initiates
at the beginning of your 50 hour period and propagates at a faster rate than the
NPRM procedure predicts, then the part will fail. In the past, failure of cylinder
heads has a demonstrated history to be fatal. Crack growth rates can vary
depending on the severity of the conditions. Unfortunately the way a PMA head
gets tested, the guidance does not allow an endurance test to impose both the
normal and adverse conditions encountered in actual flights. For example, a pilot
could neglect to manage his mixture appropriately or exclusively conduct touch-
and-go landings. Both of these real world conditions could greatly accelerate
crack growth rates and result in total failure of the part in just a few hours. These
are truly adverse conditions, but are examples where it would be hard to predict
the higher growth rates on a PMA cylinder head that is significantly thinner than
the OEM cylinder head without actual flight test data. When you ask Superior’s
thinner PMA part to accommodate the same load as a thicker OEM part, the
stresses for the thinner part will go up resulting in the fatigue life being reduced.
This is an engineering fact. The lower fatigue life may or not be an issue for the
cylinder head depending on where this life falls in the TBO continuum.
Unfortunately, the PMA cylinder head in this NPRM has failed on at least two
dozen occasions in the exact same area that is thicker on the OEM part, which
has not failed. This PMA head is not equal to or better than the OEM head so it
should not be eligible for a PMA. As such, this cylinder head should never be
allowed to go to full TBO.
The OEM typically is not handed a full TBO at initial engine certification. This is
something that has to be demonstrated after several engines were flown in actual
flight environmental conditions. The engines are torn down and inspected at
various points along the way to full TBO. Current FAA certification practice
indefensibly does not require the PMA cylinder heads to earn their TBO like OEM
type certified engine manufacturers. The mind set could be that since a PMA part
replaces the OEM part and both are virtually identical, then testing would be
redundant. It is very difficult for a reverse engineered cylinder head to be identical
to the OEM part. Unfortunately, if the PMA holder shows that they meet the
minimum endurance requirements, including a typical engine endurance test on a
test stand, then they will be handed the full manufacturer’s TBO. In effect, the
PMA holders are not held to the same high standards that the OEMs are being
required to follow. This is not adequate since the PMA holders clearly have less
experience than the OEM. By the FAA neglecting to require the PMA holders to
demonstrate the same level of developmental testing and service experience,
through actual flight tests, that their cylinder head can make it to TBO, the FAA is
doing an injustice to the flying public. Time and time again PMA cylinders have
failed at a lower threshold than OEM cylinders and this cylinder is one more case
in point. If a problem was encountered in the OEM TBO flight testing, the TBO
would not be granted beyond that point. Why is the FAA now proposing in this
NPRM that this PMA holder be allowed to continue to fly with cylinder heads that
have already been demonstrated in the real world flight environment to prematurely
fail? This is a violation of the FAA’s congressionally mandated requirement for
safety. This further appears to be another example where the FAA is making a
conscious effort to be cozy with their applicants, who they call “customers”. The
flying public is the FAA’s primary customer and I for one do not want to fly on a
single engine aircraft that has cylinders that have a demonstrated probability to fail
even knowing that 1 hour previous they may have passed a meaningless
differential compression test. This AD is a farce and should be laughed out of
existence. Anyone in the FAA approving this AD should be dismissed for
malfeasance of their congressionally mandated duty. Shame on you!