Definition
Engine-driven electrical generators that produce high-voltage current to fire the spark plugs in a piston aircraft engine. Magnetos operate independently of the aircraft's battery and electrical system, generating their own ignition current through a rotating magnet inside a coil. Most certified piston aircraft have two magnetos (a left and a right), each firing one of the two spark plugs in every cylinder, providing redundancy and improved combustion.
Plain English
Small self-powered generators on the engine that create the sparks needed to ignite fuel in the cylinders. Because they make their own electricity, the engine keeps running even if the battery fails.
Context Anchor
Seen during engine starting, preflight checks, magneto checks, shutdown, and especially hand propping safety discussions.
Derivation
From 'magneto-electric machine,' the original 19th-century name for a device that produces electricity using permanent magnets. The shortened form 'magneto' stuck. Knowing this helps because it reminds you the device is purely mechanical-magnetic — it doesn't need an outside power source to work.
Why Pilots Care
Both magnetos must be confirmed off before hand propping to prevent accidental starts, and each must be tested individually during preflight to ensure reliable ignition.
Grounding Statement
If the engine has fuel and air in a cylinder, and a magneto makes a spark while the propeller is moved, the engine can fire.
Intuition Check
Do not assume the engine needs the aircraft battery to start or keep running. Magnetos can make their own spark when the engine is turning.
Example Sentence 1
During the run-up, the pilot checked each magneto individually by switching from BOTH to LEFT, then BOTH, then RIGHT, watching for a small RPM drop on each.
Example Sentence 2
During the run-up, the pilot switched to the left magneto alone and noted a slight drop in RPM before returning to both.