Definition
A removable component threaded into the cylinder head of a reciprocating engine that delivers a high-voltage electrical spark across a small air gap between two electrodes, igniting the compressed fuel-air mixture inside the combustion chamber. Aircraft spark plugs are typically shielded to prevent radio interference and use platinum or iridium electrodes for long service life under high temperatures and pressures.
Plain English
A small screw-in part that produces an electric spark to set fire to the fuel and air mix inside an engine cylinder. Without it, the engine has no way to start the burn that drives the piston down.
Context Anchor
Pilots encounter spark plugs during preflight discussions, before-takeoff engine checks, maintenance writeups, and troubleshooting a rough-running engine.
Derivation
From 'spark' (a small flash of electricity jumping a gap) and 'plug' (something that screws or fits into an opening). The name describes exactly what it does: a plug that produces a spark.
Why Pilots Care
Properly functioning spark plugs ensure reliable ignition, smooth engine performance, and help prevent power loss or engine failure.
Intuition Check
A spark plug does not power the engine by itself. It only starts the burning process at the right moment; the engine’s power comes from the fuel-air mixture burning in the cylinder.
Example Sentence 1
During the runup, the right magneto showed an excessive RPM drop, and the mechanic traced it to a fouled spark plug on the number three cylinder.
Example Sentence 2
Fouled spark plugs can cause the engine to run rough and lose power during flight.