AOPA Pilot Magazine - February 2021 - 96

P&E

SAVVY MAINTENANCE
Savvy Maintenance coverage
sponsored by AIRCRAFT SPRUCE

OPINION

How mags work
The spark plugs in most piston aircraft engines are still
powered by 120-year-old technology
BY MIKE BUSCH

BENDIX D-3000
BENDIX S-20/S-200

BENDIX S-1200

BOTH THE FEDERAL AVIATION regulations
and their predecessor, the civil air regulations, require that certificated spark-ignition
reciprocating aircraft engines-the kind
most of us fly behind-have fully redundant
dual ignition systems. Part 33, Subpart C,
states that " each spark ignition engine must
have a dual ignition system with at least two
spark plugs for each cylinder and two separate electric circuits with separate sources
of electrical energy or have an ignition system of equivalent in-flight reliability. "
There's a good reason for this: Ignition
system failures are relatively commonplace. Without a properly functioning
ignition system, the engine could quit, the
airplane could fall out of the sky, and people
could get hurt. Spark plug failures happen
a lot, but the consequences usually aren't
serious-often they're not even noticeable-
precisely because we have two spark plugs
in each cylinder, and one is enough to keep
the cylinder producing power. Usually, the
only sign that a spark plug has failed in flight
is that the EGT on the affected cylinder rises
by 50 degrees Fahrenheit or so. Unless you
have an engine monitor installed and keep
it in normalize mode, you'll probably never

96 | AOPA PILOT February 2021

SLICK 4300/6300

notice. Often these failures are caused by
crud getting lodged in the spark plug electrodes, and often those failures self-resolve.
Even when they don't, spark plug failures
often aren't caught until the next preflight
magneto check, when the failed plug causes
an excessive magneto drop.
Magneto failures happen less often,
but when they do, the consequences can
be much more serious-or not, depending
on the failure mode. If the mag just quits
cold-say, because the breaker points fail,
the coil opens, or the condenser shorts-
then the consequences are relatively
benign: All cylinders continue to make
power in single-ignition mode, all EGTs
rise in unison, and you fly to your destination and get the bad mag fixed. No big deal.
A failure that affects the magneto's
timing can be a very big deal, particularly
if the timing is advanced-i.e., the spark
plugs fire earlier than they should. A mag
that fires 5 degrees early can send CHTs
right through the roof, and one that fires
10 degrees early can melt holes in pistons
and cause cylinder heads to separate. Ugly.
The worst kind of mag failure-and one
we're seeing disturbingly often-occurs

when the mag's plastic distributor gear
fails and starts shedding teeth. When this
happens, the magneto can start firing random spark plugs at random times, and all
hell breaks loose. The engine starts running
change-of-underwear rough, and unless the
pilot quickly throttles way back, the powerplant can start coming seriously unglued.
I counted six such magneto distributor
gear failures during a two-year period in a
fleet of roughly 300 piston GA airplanes.
That works out to an average of one failure per year per 100 airplanes. To my way
of thinking that's a pretty scary failure rate.
That's why the FAA requires that our
engines have two magnetos. Even if one mag
goes berserk, we've still got a healthy one to
get us home, right? Don't be so sure.
I investigated these six magneto distributor gear failures thoroughly. They
happened to all sorts of pilots, ranging
from newbies to veteran multi-thousandhour CFIs, and they occurred in phases
of flight ranging from pattern altitude to
Flight Level 210. Not once did the pilot
have the presence of mind to identify and
shut off the misfiring magneto! That's even
true of the failure that occurred at FL210,
where the experienced pilot had nearly half
an hour to troubleshoot the issue as he was
descending, power off, to an emergency
landing. In every one of these six cases-
high-time or low-time pilot, high altitude
or low altitude-the pilot declared an emergency, pulled the power back to near-idle,
and put the airplane on the ground at the
nearest airport. Fortunately, all the emergency landings were uneventful.
Had these pilots been taught to deal with
such a failure by identifying and shutting off
the bad magneto, their engines would have
resumed smooth operation and their airplanes could have continued uneventfully
to the planned destination, at which point
the bad magnetos could have been repaired
or replaced. But none of the pilots did that.
All treated the situation as a catastrophic
engine failure. Not one attempted to troubleshoot or resolve the problem, something
that easily could have been accomplished
simply by shutting off one magneto at a
time until the bad one was identified and
disabled.

AOPA Pilot Magazine - February 2021

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