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2 Answers

Run ups

Asked by: 1508 views Aircraft Systems, Student Pilot

While performing a mag check during a run-up what if I notice an abnormal drop in RPM and the engine is running rough, what actions should I take both on the ground and in the air?

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2 Answers



  1. Dauntless Aviation - www.dauntless-soft.com on Jul 28, 2015

    Why would you leave the ground if you were unsure of what this entails during a ground check? Your question seems like you are doing a take-home pre-solo test somewhere…

    From the Pilot’s Handbook of Aeronautical Knowledge:

    A malfunctioning ignition system can be identified during the pretakeoff check by observing the decrease in rpm that occurs when the ignition switch is first moved from BOTH to RIGHT, and then from BOTH to LEFT. A small decrease in engine rpm is normal during this check. The permissible decrease is listed in the AFM or POH. If the engine stops running when switched to one magneto or if the rpm drop exceeds the allowable limit, do not fly the aircraft until the problem is corrected. The cause could be fouled plugs, broken or shorted wires between the magneto and the plugs, or improperly timed firing of the plugs. It should be noted that “no drop” in rpm is not normal, and in that instance, the aircraft should not be flown.

    from there, this article in flight traiing magazine should give you an idea if you suspect (and when to suspect) fouled plugs:

    http://flighttraining.aopa.org/magazine/2007/December/200712_Features_Lean_It.html

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  2. Russ Roslewski on Jul 28, 2015

    If you are a student pilot, you should have your CFI show you, but basically this is often caused by some temporary fouling of the plugs, typically caused by not leaning the mixture for taxi.

    The typical way to get rid of this is to run the engine at higher rpm that the usual runup, while leaning aggressively for a little while. Might only be a few seconds, might be a minute. Then try the normal mag check again. This “burns it off”.

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