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 critical pre-takeoff procedure for piston engines to assess ignition system health, noting RPM drop and engine roughness, with specific limits (e.g., 150 RPM total/50 RPM split).
Advanced diagnostics with a multi-probe engine monitor provide detailed EGT data to identify specific spark plug issues, and techniques like aggressive leaning can resolve plug fouling.
In-flight magneto checks offer a more demanding test of the ignition system, particularly at cruise power or lean-of-peak mixtures, and require specific throttle management precautions to prevent backfires.

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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.