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From CP57, Page 12 (October, 1988)

     The other day, Burt came in from a flight in his Defiant and reported a broken wire on the right rear magneto. He discovered this condition because, as has always been his habit, he conducted a magneto wiring check just before he shut the engine down.
     How many of us do this with any regularity? How many do it at all? If you have never done this check, you may possibly have a "hot" magneto, even though you have both mag switches turned off. This is a potentially dangerous situation. Anyone who moves the prop may suffer a prop strike. Many people during the history of aviation have been seriously hurt, even killed, by a "hot" magneto.
     The procedure to check if both of your magnetos are correctly grounded, is as follows: Just before you pull the mixture to shut down your engine after a flight (be sure the avonics master switch is off), momentarily flip both mag switches off and then back on. This only needs to take a second or so. The engine should instantly quit. If it continues to run, you have one or both magnetos "hot" or not grounded. Remember, a magneto is always hot unless it is connected to ground. Your mag switches should connect each magneto to ground when they are in the off position. Check the wiring at the magnetos or between the firewall and the magnetos. This is the most likely place for the wiring to fail due to the movement of the engine during start-up and shut down. Be sure to have adequate strain relief for the wires, and don't have the wires from the firewall to the engine too tight - you need adequate length to allow for the considerable movement of the engine relative to the airframe.
     Try to develop the habit of conducting this test each time you shut down; power to idle, avionics off, both mags off for a second, engine should abruptly quit, mags back on, engine should catch and run, then mixture to idle cut off as normal. Knowing, for a fact that your magnetos are indeed grounded and that anyone, including yourself, is not likely to get surprised by the engine suddenly firing when the prop is moved is very comforting.