Reliability-Centered Maintenance, Part 3
TO PROPERLY APPLY RELIABILITY-CENTERED maintenance (RCM) principles to the maintenance of our piston aircraft engines, we need to
analyze the failure modes and failure consequences of each major
component part of those engines.
Last month, we looked at the issue of catastrophic failures of piston aircraft engines and saw that the predominant risk of such
failures is greatest when the engine is young, not when it’s old. This
month, we’ll examine the critical components of these engines, how
they fail, what consequences those failures have on engine operation
and safety of flight, and what sort of maintenance actions we can
take to deal with those failures effectively and cost-efficiently.
It’s hard to think of a more serious piston engine failure mode than a
crankshaft failure. If it fails, the engine quits.
Yet crankshafts are rarely replaced at overhaul. Lycoming says its
crankshafts often remain in service for more than 14,000 hours and
50 years. Teledyne Continental Motors (TCM) hasn’t published this
sort of data, but TCM crankshafts probably have similar longevity.
Crankshafts fail in three ways: 1) infant-mortality failures due to
improper material or manufacture; 2) failures following unreported
prop strikes; and 3) failures secondary to oil starvation and/or bearing failure.
We’ve seen a rash of infant-mortality crankshaft failures in recent
years. Both TCM and Lycoming have had major recalls of crankshafts
that were either forged from bad steel or were physically damaged
during manufacture. Those failures invariably occurred within the
first 200 hours after a newly manufactured crankshaft entered service. If a crankshaft survives the first 200 hours, we can be confident
that it was manufactured correctly and should perform reliably for
Unreported prop strikes seem to be getting rare because owners
and mechanics are becoming smarter about the high risk of
operating an engine after a prop strike. Both
TCM and Lycoming state that any incident
that damages the propeller enough that it
has to be removed for repair warrants an
engine teardown inspection. This applies
even to prop damage that occurs when the
engine isn’t running. Insurance will pay for
the teardown and any necessary repairs, no
questions asked, so it’s a no-brainer.
Crankcases are also rarely replaced at major
overhaul, and they often provide reliable
service for many TBOs. If the case stays in
service long enough, it will eventually crack.
The good news is that case cracks propagate
slowly, so a detailed annual visual inspection
is sufficient to detect such cracks before they
pose a threat to safety. Engine failures
caused by case cracks are extremely rare.
CAMSHAFT AND LIFTERS
The cam/lifter interface endures more pressure and friction than any other moving
parts in the engine. The cam lobes and lifter
faces must be hard and smooth in order to
function and survive. Even tiny corrosion
pits (caused by disuse or acid buildup in the
oil) can lead to rapid destruction (spalling)