AOPA Pilot Magazine - January 2024 - 97

says it's " due " my airplane would never fly
and I'd need to take out a second mortgage.)
Almost all this manufacturer's " when
to " guidance is only a suggestion, not a
requirement. The FAA doesn't require
noncommercial (Part 91) operators to
comply with TBOs or pay any attention to
other manufacturer-prescribed inspection,
maintenance, overhaul, or replacement
intervals. With only two exceptions, it's
up to the aircraft owner whether to comply
with such " when to " guidance. In
most cases, it makes sense to ignore such
guidance and only perform maintenance
when it's demonstrably necessary based on
actual condition.
One huge problem with TBOs and
other manufacturer's " when to " guidance
is that it's a " one size fits all " guidance
that ignores how regularly the aircraft is
operated, whether it lives in a hangar or
outdoors, whether it lives in an area of
high or low corrosion risk, and other factors.
Since the guidance has to work for
the entire fleet, it's always calibrated for
the worst-case aircraft. Since your airplane
is probably not the worst-case, following
the manufacturer's " when to " guidance is
almost certainly excessive.
If your Cessna 182 flies 40 hours a year
and spends most of its life tied down outdoors
in South Florida, then it might make
sense to overhaul your O-470 every 12 years
and your prop every six years. On the other
hand, if your Bonanza flies 140 hours a year
and lives in a hangar in Denver or Boise or
Missoula, it would be crazy to do that.
Generally, aircraft components should
be maintained " on-condition " rather than
on a fixed timetable. There are a handful
of exceptions (like magnetos) whose
condition cannot be determined without
taking them apart, or whose TBOs are
FAA-mandated by airworthiness directives
or airworthiness limitations. Except
for those few things, when your shop tells
you " your xxx is due " your response should
be, " No thanks, I'll pass. "
5. Not running to failure
The purpose of preventive maintenance
is to prevent failures. (Corrective maintenance
is what we do after something has
failed.) Preventing failures sounds like a
good thing. But often it doesn't make sense.
We should only approve preventive
maintenance that's worth doing. To do that,
we need to think about the consequences
of failure. If the failure of a component is
an " acceptable failure " -one that doesn't
compromise safety of flight and isn't likely
to cause us to be stuck somewhere we don't
want to be-then it doesn't make sense to
spend money trying to prevent it. The optimal
maintenance strategy for components
whose failure is acceptable is to run the
component to failure and then repair or
replace it when it fails.
My Cessna 310 has two expensive
400-series vacuum pumps, one mounted
on each engine. The pumps are required to
operate my deice boots and used to drive
my vacuum operated gyro instruments
(before I got rid of them and installed a
pair of Garmin GI 275s). Now, the manufacturer
says these pricey vacuum pumps
should be replaced every 500 hours. In my
view, doing so would be ridiculous because
the failure of a vacuum pump is a totally
acceptable failure-the remaining pump
is perfectly capable of operating the boots
and spinning the gyros. Because the vacuum
system is fully redundant, the only
sensible strategy is to run each pump until
it fails and then replace it. The same logic
would apply to a single-engine airplane
equipped with a standby vacuum system.
Similarly, the Cirrus SR22 has a fully
redundant electrical system with dual
alternators, dual batteries, and dual
electrical buses. The manual calls for alternator
replacement every 500 hours, which
doesn't make much sense given that the
failure of one alternator is an acceptable
failure in this aircraft. If I owned an SR22,
I would run the alternators to failure.
6. Accepting " shotgunning "
Picture this: You're on a cross-country
flight when your engine suddenly hiccups.
It really gets your attention. You
instinctively push the mixture full-rich
and switch tanks. The engine runs normally
and the problem doesn't recur.
Upon returning home, you put your airplane
in the shop and tell your A&P what
happened. What does he do? In all probability,
he cleans and gaps all your spark
plugs, removes and cleans your fuel injector
nozzles, checks your gascolator and
fuel control unit filters for contamination,
and checks your mag timing. Maybe he
installs new spark plugs or sends out your
magnetos for overhaul. What did your A&P
just do? Did he diagnose your engine problem
and then fix it? No, what he did was
to work on everything he could think of
that might have been wrong, hoping to get
lucky. In the industry, we call this " shotgunning "
and it's a huge cause of costly and
unnecessary maintenance.
Diagnosis needs to be based on data.
For an engine hiccup, the best diagnostic
approach would be to dump and analyze
the digital engine monitor data for
the incident flight. Other good tools for
gathering data would be a borescope
inspection of the cylinders and perhaps an
oil filter inspection. All too often, however,
mechanics skip the diagnosis phase and go
straight to the therapy phase without really
knowing what's wrong and instead relying
on guesswork. Don't let this happen.
If your mechanic tells you something
like " I think we should overhaul the magnetos "
or " I think we should install new
spark plugs, " your response should be,
" What evidence do we have that the mags
(or plugs) caused the problem? " Unless
your mechanic can answer this question
in a clear, convincing, data-driven fashion,
you should probably say no and try to
refocus him on gathering diagnostic data.
Owner in command
These six common owner mistakes represent
failures of the owner to stay in control
of maintenance, and abdication of maintenance
decision making to their mechanic
or shop. An aircraft owner who has mastered
the art of staying in control of their
maintenance is one I like to call an " owner
in command. " If you own an aircraft, that
what you should aspire to be.
mike.busch@savvyaviation.com
savvyaviation.com
Ask the A&Ps /
Mike Busch, Paul New, and Colleen
Sterling answer your toughest
aviation maintenance questions on
our Ask the A&Ps podcast. Submit
questions to podcasts@aopa.org
aopa.org/ask-the-a-and-ps
AOPA PILOT / January 2024
97
http://www.savvyaviation.com http://www.aopa.org/ask-the-a-and-ps
AOPA Pilot Magazine - January 2024

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2024
