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magneto info

Asked by: 1429 views Aircraft Systems

Why is there a differential drop in the magneto when your doing the run up 

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2 Answers



  1. Richard Eastman on Apr 22, 2020

    To “lift” from a 2002 AOPA discussion of magnetos …

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    More recently, various types of battery-driven ignition systems have come into wide use in experimental planes. Since these ignition systems are battery-driven (i.e. not self-generated charges from a self-driven coil), one generally does not see a differential drop in RPM between systems if both systems are electrical. If only one systems is electrical, than there will not be a drop in the electrical ignition side, but generally there is on the magneto-driven system. Recently, the FAA has begun certifying electronic ignition systems (see https://www.surefly.aero/ as an example).

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  2. Best Answer


    Kris Kortokrax on Apr 23, 2020

    AOPA info on electronic ignition doesn’t answer the poster’s question. The answer is found in the Aviation Maintenance Technician Handbook – Powerplant.

    “In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance. The drop in engine speed is a measure of the power loss at this slower combustion rate.”

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