AOPA Turbine Pilot Magazine - March 2020 - 93

P&E

SAVVY MAINTENANCE
Savvy Maintenance coverage
sponsored by AIRCRAFT SPRUCE

OPINION

Risky business
Why it's is nearly impossible to install a cylinder properly
when the engine is on the airplane
BY MIKE BUSCH
A NEW LYCOMING
THROUGH BOLT has

threads that are
cadmium-plated
and in perfect
shape.

REUSED ONES have

threads that are
worn and damaged
with most of the cad
plating gone.

CYLINDER REPLACEMENT is a highly invasive and risky procedure with a long
history of causing catastrophic in-flight
engine failures. I have been involved with
at least a half-dozen of these maintenanceinduced catastrophic engine failures-as
an expert witness or investigator-where
the engine either threw a rod through
the crankcase or sustained the complete
separation of a cylinder from the engine,
resulting in a total loss of power. In some
cases, the pilot made a successful forced
landing; in others, the outcome was serious injury or death.
Cylinder replacement-and especially
replacement of multiple cylinders at once-
is a procedure that needs to be executed
perfectly. If it isn't, there can be dire consequences. Yet it's a procedure that most career
general aviation A&Ps perform routinely
without any apparent concern. Why aren't
these mechanics nervous? Undoubtedly
because they are convinced that they always
perform the cylinder transplant procedure
properly, and that only careless or incompetent mechanics screw it up.
That's wrong, and here's why.
IS "PROPERLY" IMPOSSIBLE?

Roger D. Fuchs-veteran A&P/IA, aircraft
engine overhauler, accident investigator, expert witness, FAA-designated

engineering representative, and really
smart guy-has done more research on
engine assembly practices and fastener
torque than anyone I know.
"My concern comes out of decades
of experience with Continental engines,
particularly the design of crankcases,
main bearings, fasteners, and assembly
practices," Fuchs said. "All my experience indicates that there is a major risk
to safety and airworthiness when performing cylinder replacement on mid- to
high-time Continental engines, not as a
result of improperly performed maintenance actions but rather as the result of
maintenance actions which experienced
mechanics are attempting to perform
properly according to the manufacturer's
recommendations as published."

According to Fuchs, the root cause of
spun bearings, thrown rods, and separated cylinders is simply "failure to
achieve sufficient preload in the assembled fasteners."
You may recall from my January 2020
column ("Savvy Maintenance: Tense
Bolts") that "preload" is the technical term
for the clamping force created by tightening a fastener, typically a threaded bolt or
stud, that holds assembled parts together.
Having sufficient preload is the key to a
strong and reliable bolted joint that will
not loosen, break, or shift under load. For
a bolted joint to be stable under cyclic
repetitive stress, the preload on the fasteners must be greater than the maximum
stress that is trying to pull the joint apart.
Unless this condition is met, the joint will

Cylinder replacement is a procedure that needs
to be executed perfectly. If it isn't, there can be
dire consequences.
Fuchs contends that there's a significant risk that an engine might come
apart after cylinder replacement-especially a top overhaul-even when the
work is performed exactly as prescribed
by the manufacturer. How can that be?

shift under load and the fasteners will ultimately fail from repetitive stress fatigue.
How do we obtain the desired preload
when installing a cylinder? In a perfect
world we'd tighten the cylinder base nuts so
that the deck studs stretched by about 0.005
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AOPA Turbine Pilot Magazine - March 2020

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