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“You’re the IA, so you have to make the
call. Which engine is airworthy?” Mose
will ask.
The correct answer is that Bonanza B’s
engine is airworthy, and Bonanza A’s is
unairworthy.
Bonanza B’s engine meets TCM’s
SB03-3 minimum compression specs (even
if just barely), and oil consumption of a
quart every three hours is high but still
well within specified limits. In contrast,
Bonanza A’s engine may have superb compression readings, but it’s clearly
unairworthy because it’s not making full
rated power—and that’s far and away the
most important airworthiness criterion for
any engine.
What’s wrong with Bonanza A’s engine?
Many things can cause an engine with
near-perfect compression readings to not
make full-rated power: improperly timed
ignition, misadjusted fuel-injection system,
collapsed lifters, incorrect pushrod length,
bad cam, or incorrect pistons ( just to name
a few).
In this case, Mose says that the
combination of high compression readings
and high oil consumption suggests that the
problem is most likely glazed cylinder
walls due to improper break-in procedure.
It’s possible that flying the engine at high
power for a few hours might cure the
problem; if that doesn’t do the trick, the
cylinders may have to come off for honing
and then the break-in repeated using the
proper procedure.
WHO HAS THE BEST ENGINE?
Now, just to make things more interesting,
Mose presents the roomful of IAs with
another hypothetical scenario.
“Four good-looking fellows, coincidentally all named Bob, are hanging out at the
airport. Remarkably enough, all four Bobs
own identical Bonanzas, all with TCM
IO-550 engines. Even more remarkably, all
four engines have identical calendar times
and operating hours.
“The four Bobs start comparing notes.
Bob One brags that his engine uses only one
quart of oil between 50-hour oil changes,
and his compressions are all 75/80 or better. Bob Two says his engine uses a quart
every 18 hours, and his compressions are in
the low 60s. Bob Three says his engine uses
a quart every eight hours, and his compressions are in the high 50s. Bob Four says his
compressions are in the low 50s, and he
adds a quart every four hours.
“Who has the best engine? And why?”
Many aircraft owners and mechanics have placed too much emphasis on compression test readings as
a measure of engine airworthiness. The truth is that an engine can have relatively low compression
readings while continuing to run smoothly and reliably and make full-rated power all the way to TBO
and beyond.
