Magneto Checks

Tuesday, June 22, 2021 at 11:17 AM ET

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This cutaway view of a Bendix magneto shows a little bit of what's going on inside. Magneto manufacturers typically recommend 500-hour service intervals, and an owner's choices include an outright overhaul, inspect and repair as necessary (IRAN) or exchange and replacement. Some are easily rebuildable while it may be attractive to simply replace others.

Key Takeaways:

Magneto checks are a crucial pre-takeoff procedure for spark-ignition engines, assessing ignition system health by noting RPM drop (typically max 150 RPM drop, 50 RPM split difference) and engine roughness.
On the ground, failed magneto checks can often be remedied by leaning the mixture or increasing RPM to burn off spark plug fouling, with engine monitors providing detailed EGT data for identifying specific cylinder issues.
In-flight magneto checks offer a more rigorous test for the ignition system, especially at cruise or lean-of-peak settings, but require pilots to retard the throttle to idle if a mag fails to prevent backfiring.

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Performing a magneto check during the before-takeoff checklist is a time-honored task for those of us flying with spark-ignition (gasoline-fueled) piston engines. The idea is to check their health, both individually and by comparing them to each other. The checklists typically call for noting the rpm drop but engine roughness also is something that should be assessed during the procedure

Checking magnetos also can be done in the air, and in many circumstances can pose a tougher test for your ignition system than what the before-takeoff checklist demands. Still, there are some do’s and don’ts, plus some tricks to consider.