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Here's the scoop from my previous mechanic, now retired. This gentleman REALLY knows turbocharged twins and singles, his was the premier shop for these type aircraft in our area until he retired. Here is what he had to say about in-flight mag checks:
"As to the in-flight mag check. It is a great deal of risk. If you have a Mag that is "dead" the engine goes from making 65% power to being driven by the prop, Negative 10%? The real problem is when it does that you take a chance of detuning the engine, i.e. damaging the counter weight bushings and pins. The procedure that I like to use to check mags in-flight is to reduce the power to about 25% power and check the mags, if both mags work at that power then increase the power to about 50% and do the same thing then increase to your cruise power and check it again. IF IN ANY OF THESE CHECKS IF THERE IS A "DEAD" MAGNETO, CUT THE MIXTURE TO THAT ENGINE AND WAIT ABOUT 2 TO 3 SECONDS AND TURN THE MAGS BACK TO BOTH AND THEN SLOWLY RETURN THE MIXTURE BACK WHERE IT WAS BEFORE THE TEST. Don't forget that at cruise speeds you may not see a drop on the in the rpm as the prop governor makes blade angle adjustments to compensate. Even is a Mag is "dead" the rpm may not droop very much. If you haven't got the message yet, this procedure is dangerous to the pocket book. You only use this procedure when you suspect that you have a single cylinder shutting down under high power, and in conjunction with your multi-probe EGT gauge you can determine what mag, what cylinder."
FWIW.
Kevin
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