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Magneto check

Asked by: 4619 views , ,
Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating, Light Sport Aircraft, Private Pilot, Student Pilot

I happened to read the following, but it did not make any sense. "During the runup, when switching magnetos from "BOTH" to "RIGHT", you notice no RPM drop. This indicates the left P-lead is not grounding or the engine has been running only on the right magneto because the left magneto has totally failed" Here's what I don't understand: If the left magneto has totally failed and only the right magneto is being used, wouldn't that instead cause a decrease in RPM? Since dual ignition causes a complete and even combustion of the fuel/air mixture, which results in a higher engine performance and power output.

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1 Answers



  1. Bidochon on Dec 19, 2014

    When your key is on BOTH, because the left magnetos is not working, in reality you are only using the RIGHT magnetos….when you switch to Right magnetos, well you are only using the RIGHT magnetos…so BOTH and RIGHT have identical outcomes. So no RPM drop. And you are right, there is a decrease of RPM compare to the case where both magnetos work but you won’t be able to notice that because only one is working.

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