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I recommend performing
the ignition stress test on a regular
basis—I do it on most every flight,
generally at the end of the cruise phase
just before starting my descent.
during the preflight engine run-up, but many pilots
have never shut off a magneto in flight and are
uncomfortable with the idea. That’s a shame, because
the in-flight ignition stress test is far more exacting
and revealing of any ignition system problems than
the usual run-up. The typical preflight mag check can
detect only the grossest ignition system defects, while
the in-flight stress test will reveal much more subtle
ignition issues.
The best ignition system test you can perform is
an in-flight mag check with the engine leaned aggressively lean of peak (LOP) EGT. If your engine is not
capable of operating smoothly when LOP, then do the
test at the leanest mixture at which the engine does
run smoothly. We want the mixture to be as lean as
possible because the leaner the air-fuel mixture, the
more difficult it is to ignite. Therefore, if your ignition
system performance is marginal, it will show up during a lean (preferably LOP) in-flight test long before it
becomes apparent in any other phase of operation.
If your engine monitor has a user-programmable
sampling rate, it’s best to set it to the highest rate possible. For example, JPI EDM series monitors have a
default sampling interval of six seconds, but can be
programmed to use a two-second interval. When
you’re done with the flight-test profiles, you can reset
the sampling rate to the default value. (NOTE: This
applies to all of the flight-test profiles discussed in
this article.)
The ignition stress test should be performed in
normal cruise configuration with the mixture set as
lean as possible consistent with smooth engine operation (preferably LOP). Place your engine monitor in
“normalize mode,” which sets all the EGT bars to
mid-scale and increases the sensitivity of the bar-graph display. Now go through the usual mag-check
procedure: BOTH-LEFT-BOTH-RIGHT-BOTH for
singles, or shutting off one mag toggle switch at a
time (left to right) for twins.
Do not rush this procedure. Perform it very slowly,
making sure that you run the engine on each individual magneto for at least 10 engine monitor sample
