Oil consumption is an even less important factor than compression when assessing if an engine is airworthy. Lycoming and TCM publish maximum acceptable oil consumption values, usually something
like 0.15 pounds/hour/horsepower; for a 300-hp engine that works out to a whopping 1. 5 quarts/hour!
the same speed if the grass wasn’t too tall.
But it got to the point that it was using a
quart of oil every hour, and was becoming
quite difficult to start. The compression
during start was so low (essentially nil)
that sometimes I had to spray ether into
the carb to get the engine to start. It also
started leaking combustion gases around
the head bolts, and would blow bubbles if I
sprayed soapy water on the head while it
was running. In fact, the mower became
somewhat useful as a fogger for controlling
mosquitoes. But it still made power and
would only foul its spark plug a couple of
times during the season when things got
really bad.
“Now keep in mind that this engine was
rated at just 8 hp and had just one cylinder
with displacement roughly the size of a
coffee cup, was using one quart of oil per
hour, and had zilch compression. Compare
that to an IO-550 with six cylinders, each
with a 5.25-inch bore. Do you suppose that
oil consumption of one quart per hour or
compression of 40/80 would have any
measurable effect on an IO-550’s power
output or reliability—in other words, its
airworthiness? Not likely.”
COMMON SENSE
I really like Bob Moseley’s common-sense
approach to aircraft engines. Whether
we’re owners or mechanics (or both), we
would do well to avoid getting preoccupied
with arbitrary measurements—such as
compression readings and oil consumption—that have relatively little correlation
with true airworthiness. Instead, we should
focus on the stuff that’s really important: Is
the engine “making metal”? Are there
cracks in the cylinder heads or crankcase?
Are there exhaust leaks, fuel leaks, or serious oil leaks? Most importantly, does the
engine seem to be running rough or falling
short of making full-rated power?
If the answer to all of these questions is
no, then we can be reasonably sure that our
engine is airworthy and we can fly behind
it with well-deserved confidence.
Mike Busch, 2008 National Aviation Maintenance
Technician of the Year, has been a pilot for 44
years, logging more than 7,000 hours. He’s a
certificated flight instructor and an airframe
and powerplant mechanic with inspection
authorization. E-mail questions to Mike at
mike.busch@savvyaviator.com.
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