AOPA Pilot Magazine - July 2023 - 97

when re-tested after flying for 45 minutes.
No one should trust a test like that.
For another, the differential compression
test is not a valid test of cylinder
condition. A healthy cylinder can exhibit
poor compression readings, and a sick cylinder
can exhibit good compression readings.
Even if the test results were highly repeatable
(which they definitely aren't), it would
never make sense to base costly, risky, and
invasive maintenance decisions solely on
those results. Owners should insist on seeing
far more trustworthy evidence before
approving cylinder removal.
Finally, even if the compression test
is successful in identifying a sick cylinder
(which it occasionally is), that still doesn't
justify cylinder removal. Many cylinder ailments
can be remediated without cylinder
removal using minimally invasive procedures
that are less costly and risky, and
therefore obviously preferable.
Absurdly unreliable
One of the most convincing demonstrations
of just how unreliable the
compression test is came to me recently
from the longtime owner of a 1947 Piper
PA-12 Cruiser. He sent me his compression
readings from 2010 through 2022,
explaining that all but the last of these
compression tests were taken by the same
shop. He told me that no cylinder work had
been done during this period-not all that
unusual for small four-cylinder Lycomings.
Here are the numbers:
A graph of these readings makes their wild
gyrations a bit more obvious:
they're mostly noise with very little signal.
This agrees precisely with my own experience
with compression numbers over my
five decades as an aircraft owner.
I asked the owner how he managed to
Graphical depiction of the PA-12
compression history.
These readings were all over the place.
They gyrate up and down from one annual
to the next. If these readings actually
reflected cylinder condition, one would
expect to see a gradual deterioration over
time correlating with cylinder wear, but
no such trend is evident. Nor do the readings
for the various cylinders move up and
down together as one might expect if the
readings were taken by different mechanics
or under different conditions (e.g., hot
versus cold) from one year to the next.
The readings for cylinder number 1 are
particularly wild:
escape having the number 1 jug replaced at
the 2013 annual inspection when its compression
was recorded at 53/80. The owner
did not recall, but my theory is that his IA
was smart enough to realize the reading
was clearly bogus and wisely decided not
to do anything rash. Contrary to conventional
wisdom, there is no requirement
that a Lycoming cylinder be removed
simply because it exhibits a compression
reading below 60/80. Lycoming's guidance
on this subject can be found in Service
Instruction No. 1191A, which states in
pertinent part:
" If the pressure reading is below 60
psi or if the wear rate increases rapidly,
as indicated by appreciable decrease in
cylinder pressure, removal and overhaul
of the cylinders should be considered. "
[Emphasis mine.]
This less-than-proscriptive language
gives an IA considerable latitude to decide
whether to yank a low-compression jug.
And that's a good thing, because as you
can see these compression readings are
mostly garbage.
To understand why compression
test results provide so little insight into
actual cylinder health, we need to examine
the causes of poor compression results.
Virtually all low compression readings are
caused by air leakage past the compression
rings and/or air leakage past the exhaust
valve. Let's take a closer look at each of
these two phenomena.
Graph of compression history for cylinder
number 1.
A compression history for the Piper PA-12.
No cylinder work was done, and all but the
final year testing was conducted by the
same shop.
The annual compression readings for
the number 1 cylinder start at 75/80, then
drop to 64/80, then pop back up to 78/80,
then plummet to 53/80, then rise to 70/80,
then drop to 60/80, then shoot back up to
78/80. Do you really believe that this cylinder
kept getting sick, then healed itself,
then got sick again, then healed again? No,
that's not plausible.
So, what is one to make of these numbers?
The only plausible explanation is that
Leakage past the rings
Air leakage past the rings is largely an artifact
of the compression test rather than a
real problem with the cylinder. When the
engine is running at normal operating
temperatures, the piston has a very snug
fit inside the cylinder walls and so very little
exhaust gas is able to get past the rings.
By necessity, however, the compression
test is performed with the cylinder
and piston at a much lower temperature.
Since the aluminum piston contracts twice
as much as the steel cylinder barrel as both
cool down from operating temperature,
AOPA PILOT / July 2023
97
AOPA Pilot Magazine - July 2023

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