Pilot's tip of the week

The Correct Ignition Check

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Subscriber question:

"My airplane partner insists I check my right magneto before checking the left one. It seems like it shouldn't matter at all to me. Who's right?" — Dev S.

Doug:

“Mechanically, it doesn’t matter which magneto you check first. As long as you check both you’ve accomplished the task.

However, there’s a reason many manuals—and the Pilot Handbook of Aeronautical Knowledge—recommend checking the right magneto first, and then the left. Doing it in this order helps ensure both magnetos have been selected prior to takeoff.

When checking your mags, go two clicks to the left (which selects the right magneto), then back to both mags, then one click to the left (selecting the left magneto), and again back to both. This does not guarantee that you’ll be on both magnetos when you take off, but it guards against the error of thinking you switched to back to “Both” for takeoff when you actually switched one click from “Right” to “Left.”

When we check the mags, we’re checking three things. One is that the magneto grounding wires are still connected. If not connected, then there will be no drop in RPM when you select that magneto. Second is that the drop in RPM falls within the recommended limits as defined in the POH. This says the remaining mag can provide sufficient spark to keep our engine running, even if with less than full power. The third is that the differential drop between magnetos is within limits, which might clue us in to a mistimed or other mag issue we should check out before the flight.

With only one mag, we have less power for takeoff. On more than one occasion, I’ve had an applicant take off with only the left magneto selected. For one applicant in a Cessna 150, the combination of a hot, humid day and only the left magneto selected resulted in an anemic climb rate of less than 75 FPM.

Luckily, the applicant recognized the error before I had to end the test.”

Some aviation mechanic experts advocate checking your ignition in flight. Do you ever do this?

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