Definition
On a piston aircraft ignition switch, LEFT (or L) is the position that energizes only the left magneto, allowing the engine to run on that magneto alone. Pilots select this position during the magneto check on runup to verify the left magneto and its associated set of spark plugs are firing correctly, while the right magneto is temporarily grounded out.
Plain English
A switch position that turns off one of the engine's two ignition systems so the pilot can test the other one by itself. With the switch on LEFT, only the left ignition system is firing the spark plugs.
Context Anchor
Seen on the ignition switch during engine start, shutdown, and especially during the engine run-up check before takeoff.
Derivation
Left comes from Old English words meaning the left-hand side. In this cockpit setting, it does not describe a turn or movement; it labels one side of the aircraft’s ignition system.
Why Pilots Care
Pilots verify both magnetos function independently to ensure reliable spark delivery to all cylinders.
Intuition Check
Do not read LEFT as an instruction to turn left or move something left. On the ignition switch, LEFT means only the left ignition system is selected and the right one is off.
Example Sentence 1
During runup, the pilot turned the ignition switch from BOTH to LEFT and noted the RPM drop was within limits.
Example Sentence 2
The left magneto continued to fire the engine after the right magneto was turned off.