"You need to tackle your mechanic"

What happens when you shut off the mags in flight? Mike, Paul, and Colleen answer this question, plus Cirrus cylinder woes, lapping technique, and more.

Email podcasts@aopa.org for a chance to be on the show.

Join the world's largest aviation community at aopa.org/join

Full notes below:

Jeff is questioning the part of his checklist that calls for turning off the mags just before shutting down. He remembers doing this accidentially in flight training and his instructor becoming somewhat panicked. The hosts explain that turning off the mags before shutting down is safe and determines if the P-lead is intact. Accidentally turning them off in flight is a different story. Unburned fuel in the exhaust will ignite rapidly and probably ruin the exhaust if you were to immediately turn them back on. If this happens, Paul recommends leaning the mixture completely, counting a few seconds, turning the mags back to both and then slowly brining up the mixture again. By doing this you'll hopefully allow the collected fuel to dissipate.

Brian asks about Cirrus cylinder problems after a recently failure caused his partner to execute an emergency landing. The hosts explain that Continental has been having issues with cylinders on certain big bore Continentals. There's an AD that requires the cylinder exhaust ports to be filed, and while they've seen many cracks where the AD had supposedly complied with, Mike said the issue is that most mechanics aren't filing away enough material. However, Paul has also seen cases where the AD was properly complied with and the cylinder still cracked, and even new cylinders that aren't covered by the AD are cracking. He recommends doing a careful visual inspection for cracking near the exhaust port anytime the cowling is off.

Yuriy wonders how much exhaust valve sidestepping is too much. Mike said he's had big sidestepping in many of his valves with no adverse effects. They recommend continuing to borsescope a few times a year to ensure the valve isn't burning. As Yuriy's compressions continue to come down the hosts recommend being proactive with lapping. Otherwise they recommend to keep flying as much as possible.

Randall wonders how much lapping is enough lapping. He's looking for clarity on when the job is done. Paul invokes the TLAR method (That Looks About Right) while Mike additionally says to invoke the TFAR method (That Feels About Right). They say 4 times with new compound is often enough, but that it's about feel and look of the valve. Mike does recommend a compression check before putting everything back together as a gut check, and that most people new to the procedure often don't go far enough.