EAA Sport Aviation - December 2010

Most engine monitors have a “normalize mode” that levels all the EGT bars at mid-scale and increases the sensitivity of the display. It’s a good idea to use this mode during mag checks because doing so will make any ignition anomalies much more obvious. Bring the engine up to the manufacturer-specified run-up rpm (commonly 1700 for direct-drive TCM engines, 1800 to 2000 for Lycomings), place the engine monitor in normalize mode, perform the mag check ( BOTH-LEFT-BOTH-RIGHT-BOTH), then return the engine monitor to its default mode (sometimes called “percentage mode”).

IN-FLIGHT MAG CHECK

The usual preflight mag check is a relatively non-demanding test and will only detect gross defects in the ignition system. To make sure your engine’s ignition is in tip-top shape, I suggest performing an in-flight mag check at cruise power and a lean mixture— preferably a lean-of-peak (LOP) mixture.

An in-flight LOP mag check is a far more demanding and discriminating way to test your ignition system because a lean mixture is much harder to ignite than a rich one. A marginal ignition system can pass the normal preflight mag check, but it takes one in excellent shape to pass an in-flight LOP check.

The in-flight mag check is performed at normal cruise power and an aggressively lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 to 20 seconds while watching the engine monitor in normalize mode. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag. Don’t expect any rpm drop, at least if you have a constant-speed prop. Focus primarily on the EGTs, and secondarily on any perceptible engine roughness when running on one mag.

If you see a falling or unstable EGT, write down which cylinder and which mag, so you or your mechanic will know which plug is the culprit. If you don’t write it down, I guarantee you’ll forget the details by the time you get back on the ground. (Don’t ask me how I know this.)

BAD MAG CHECKS

If you perform a mag check (ground or
flight) and don’t like what you see, then

PHOTOGRAPHY COURTESY OF MIKE BUSCH

LYCOMING’S REVISED
GUIDANCE ON MAG CHECKS

On June 18, 2010, Lycoming issued Service Instruction No. 1132B, revising its guidance on how preflight mag checks should be performed. Some highlights of this new service bulletin:

● Engines with fixed-pitch props should conduct the mag check at approximately 1800 rpm (2000 rpm maximum). Those with controllable-pitch propellers should use 50-percent to 65-percent power as determined from the manifold pressure gauge (unless otherwise directed by the POH).

● Maximum allowable mag drop is 175 rpm
for each magneto, and 50-rpm difference
between magnetos.
● If mag drop exceeds 175 rpm, lean the
engine to peak rpm and then repeat the
mag check at the newly leaned mixture.

For a link to the full Revised Service Instruction 1132B, visit www.SportAviation.org.

When performing a mag check, I recommend you focus primarily on your digital engine monitor, not your tachometer.

what? How can you tell what’s wrong, and
what you should do to correct it?

To begin with, the phrase “mag check” is
a bit misleading. The vast majority of “bad
mag checks” are caused by spark plug
issues, not magneto issues. We should really
call it an “ignition system check.” Using the
EGT method, it’s usually easy to tell
whether a bad mag check is due to a spark
plug problem or a magneto problem: A
faulty spark plug (or, more rarely, a faulty
ignition lead) affects only one cylinder (i.e.,
one EGT bar on your engine monitor), while
a faulty magneto affects all cylinders (and all
EGT bars).

MAG TIMING ISSUES

During a pre-takeoff mag check, if you get an excessive rpm drop when you switch to one mag but all EGTs rise and the engine runs smoothly, chances are that it’s not a bad magneto but rather retarded ignition timing (i.e., spark too late). This is sometimes caused by mechanic error in timing the mags during maintenance (especially annual inspections), but it can also be caused by excessive magneto cam follower wear ( possibly due to inadequate cam lubrication) or some other internal mag problem. Retarded ignition timing also results in higher-than-usual EGT indications. Mildly retarded timing is not a serious problem, but it does cause some loss of performance, so it should be addressed.

Conversely, advanced ignition timing (i.e., spark too early) results in lower-than-usual EGT indications and higher-than-usual cylinder head temperature (CHT) indications. Advanced timing is a much more dangerous condition because it can lead to detonation, pre-ignition, and serious engine damage. If you observe low EGTs and high CHTs after an aircraft comes out of maintenance, do not fly until you’ve had the ignition timing rechecked.