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Magneto Grounding check & Engine idle check during run up.

Asked by: 1086 views Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

Hi everyone,

In our school, in the shut down checklist we instantaneously turn the key from both to off and back to both to check 'grounding' of the magneto. I am not sure why we do this but all I know is that there is a p-lead wire on primary coil that runs back to cockpit.

Secondly, after the run-up check, we pull the throttle  all the way idle and make sure RPM does not drop below 675RPM, do you know where does this number come from and why?

Lastly, how does high manifold pressure on engine can cause detonation?



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1 Answers

  1. Kris Kortokrax on Jun 03, 2016

    If you turn the mag switch to OFF and the engine does not die, then you have at least one P lead wire broken or a bad switch. Turning the switch to OFF grounds the magnetos and prevents them from creating a spark.

    The idle check (which should be done with carb heat on because of lower RPM) is to assure you that if you pull the throttle back in flight to simulate an engine failure or to perform a stall, for example, that your engine will not quit.

    Download a copy of the Pilot’s Handbook of Aeronautical Knowledge. Detonation is discussed on page 6-19. High manifold pressure, in and of itself, does not cause detonation, it is high manifold pressure coupled with a low RPM setting on the prop. This has led to the Old Wive’s Tale that one should not run an engine “over square”.

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