Mag Checks and More

One of the readers of The Mooney Flyer wrote: I remember my instructor, an ex WW II pilot, giving me instructions on how to test the magnetos.  He advocated testing the magnetos in flight.  His premise was that in flight, the magnetos are under greater stresses and loads than they are during a run up.  If there were problems developing with the magnetos it would show up at that time rather than when they are lightly loaded.

REPLY: Frank Setzler, owner of Chandler Aviation, Inc. Chandler, AZ (a Mooney Service Center), agrees with Rich and his instructor from long ago. That’s because, when checking mags, it’s all about the pressure, stress and loads.

If you have an engine analyzer, you should watch the exhaust gas temperatures (EGT’s) to ensure that they rise about 50 degrees when the L and R mags are checked, compared to when the switch is in the “both” position.  With an even increase of all cylinder EGT’s, you can assume the magnetos are working internally at peak efficiency.

Pre-departure mag checks (Run-up)

These are more effective if they are checked under pressure by using a higher power setting (at least 15” of manifold pressure – or an RPM above 2,000).  Look at your individual EGT temps to verify that all injectors are firing properly by observing that all EGT’s are reading approximately the same.  One clogged injector reduces more than one cylinder worth of power!  Often times the operator must lean aggressively to keep from flooding the remaining good injectors with too much fuel.   Many “bad mag checks” can be caused by spark plug problems. A faulty spark plug affects the EGT drop on just one cylinder and one EGT bar. A faulty mag affects all EGT bars.

After the flight mag check

A high percentage of mag failures occur in flight. Prior to engine shutdown during taxi back, it’s a good idea to check that both mags are operating. Do this by selecting R/BOTH, then L/BOTH, while observing a slight drop in RPM.  There is no need to heat the engine up to run-up RPM’s at this point.  If you want to be on the safe side, Frank’s shop will always verify that when the MAG switch is in the OFF position, that indeed the engine will start to spool down.  This is only done at idle RPM.  To prevent backfire, it’s important to SLOWLY turn the key back to “R” from the OFF position.  You will notice the engine RPM coming back to life again, then “L”, and then back to “BOTH”.  If at any time the IGN Key can be removed from the mag switch in any position other then OFF, this is an unsafe condition and the IGN switch must be replaced.


Frank has observed engines running rough or with a slight miss at high power settings; often at high altitude.  If the normal engine controls don’t help the engine run smoother, try a mag check in the air.  If you find that by going to the L and R mags, that the engine performance improves, then there is an internal problem with the mag that is OFF.  Fix that mag problem when you get back on the ground!  NOW.  It won’t get any better.

After Shutdown

Frank also recommends that owners add one more post flight task – drain the Gascolator and both wing tank sumps. This will get the water and debris out of the system so it doesn’t remain in the tank or Gascolator for an extended time. Water and debris can cause corrosion issues that Frank often sees with inactive aircraft.


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