Definition
The ignition switch position in which both the left and right magnetos are energized simultaneously, firing both spark plugs in each cylinder. This is the normal operating position for engine start (after priming, where applicable) and all phases of flight. Using BOTH produces more complete combustion, slightly higher power, and smoother engine operation than running on a single magneto.
Plain English
The switch setting that turns on both ignition systems at the same time, so both spark plugs in every cylinder fire. This is the setting you fly with.
Context Anchor
Seen on the ignition switch in piston-engine aircraft, especially during engine start, the engine check before takeoff, and normal engine operation.
Derivation
Both comes from Old English words meaning “the two together.” That fits the cockpit use: BOTH means the two ignition sources are operating together.
Why Pilots Care
Operating in BOTH provides ignition redundancy and ensures smooth engine performance; selecting only one magneto during flight can cause power loss or roughness.
Intuition Check
BOTH does not mean a vague “all-purpose” setting here. On the ignition switch, it specifically means the left and right magnetos are both operating.
Example Sentence 1
After completing the magneto check, the pilot returned the ignition switch to BOTH before advancing the throttle for takeoff.
Example Sentence 2
After landing, the pilot returned the switch to BOTH before shutting down the engine.