Ask Paul: Why are Piper and Cessna mag checks different?

Q: During a checkout in a Piper Cherokee, (I had only Cessna experience), I asked the CFI why Piper has me check the mags at 2,000 rpm and Cessna at 1,700 rpm on what is, essentially, the same engine?

My opinion is that it is not a deal breaker to not have exactly the rpm in question, just close, and that it is the percentage of change during the mag check that is what is really important. Can you help us understand this?

DAVE RICE, via e-mail

A: This is a question that may have crossed many a pilot’s mind as they transition from one aircraft type to another. First of all, the most important place to check for the proper procedure for the specific aircraft you are going to fly is to review the airframe manufacturers Pilot’s Operating Handbook (POH).

There may be a chance that the particular airframe manufacturer has a specific rpm range it has established during the testing and certification of the aircraft. Therefore, those guidelines should be followed.

The above may be the rule for certificated aircraft, but what do you do if you’ve got a homebuilt aircraft? The best thing to do is review Lycoming Service Instruction 1132A or its latest revision. This publication provides guidelines on how to conduct a magneto check by either power reference or by engine speed. Regardless of the method you choose, the drop in rpm, or magneto drop-off, must not exceed 175 rpm on all Lycoming engine models, except helicopters, which are permitted no more than a 200 rpm drop-off. Regardless of engine model or airframe application, the difference in magneto drop-off between both magnetos should not exceed 50 rpm.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:

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