Definition
A ground check performed at low RPM to verify that the magneto P-leads are properly grounded. The pilot momentarily switches the ignition from BOTH to OFF and immediately back to BOTH. The engine RPM should drop noticeably during the brief OFF position, confirming both magnetos are being grounded by the switch. If the engine continues running normally with the switch OFF, one or both P-leads are broken or disconnected, meaning the magnetos are 'hot' and the propeller could fire if turned by hand.
Plain English
A quick test done on the ground to make sure the ignition switch can actually turn the magnetos off. The pilot flicks the switch to OFF for a split second; the engine should stumble. If it doesn't, the magnetos are still live even with the switch off — which makes the propeller dangerous to handle.
Context Anchor
Used during engine run-up, after ignition maintenance, or as part of a shutdown check on aircraft with magneto ignition systems.
Derivation
Magneto comes from magnet, because a magneto uses magnets to make the electricity for the spark plugs. Safety check is literal here: the check is meant to confirm that the ignition system is safe when the switch is placed OFF.
Why Pilots Care
Prevents propeller strikes or injuries on the ramp by confirming the magnetos are safely disabled.
Intuition Check
Do not confuse a Magneto Safety Check with the normal magneto performance check. The safety check asks, “Will the ignition shut off?” not “Does each magneto run smoothly?”
Example Sentence 1
After the run-up, he performed the magneto safety check at idle and confirmed a slight RPM drop when the switch was cycled to OFF.
Example Sentence 2
Failing to perform the magneto safety check could result in an unexpected propeller rotation during maintenance.