Subscriber question:
"I'm renting at a new location and their in-plane checklists have a 'dead-mag check' after startup and a 'live-mag check' during shutdown. Then there's also a mag check during runup. Is there a reason to do all these mag checks? It seems excessive." —Bill N.
David:
“A Dead Mag check looks for a magneto that should be functioning but isn’t (that is, it’s dead). A Live Mag check looks for a magneto that shouldn’t be functioning—but is. The dead mag check will just save you some aggravation, but the live mag check could save a life.
The Dead Check happens right after startup. Select each mag in turn. There should be a small drop in RPM, but no other changes. If a mag is dead or dying, it will be readily apparent and the flight is over before leaving the tiedown. Sure, you would have found it at the runup (and you’ll still do that runup check later) but a quick check now could avoid an unnecessary taxi out and back.
The Live Check happens just before shutdown. Remember that magnetos will generate a spark whenever the prop is rotated. The only thing that prevents a spark inside the cylinder is the P-lead, a wire grounding the Mag output to the airframe. If that circuit has broken anywhere—inside the mag, along a wire, or at the ignition—the mag is live, even with the keys in your pocket. Anyone turning the prop could potentially cause the engine to start and cause grievous injury or death.
Before you shutdown with the mixture control, switch both mags off just long enough for the engine to stumble but not die. Switch the mags back on, and then proceed with your normal engine shutdown procedure. Don’t delay in switching the mags back on. Excessive delay could result in a backfire that can destroy muffler baffles. Obviously, if the engine continues to run with the mags supposedly shut off, get a mechanic to check it out before the airplane flies again.
After the engine is shut down but before you remove the keys, you should also give the key a gentle pull at the both and left and right positions. If the key comes out in any position except ‘off,’ the switch should be replaced immediately.
One last point: Even with both dead and live mag checks, it’s still best to consider a prop ready to kill you whenever you touch it.”
