Definition
The system in a reciprocating aircraft engine that produces a high-voltage electrical spark at each spark plug, at the correct moment in the cylinder's compression stroke, to ignite the fuel-air mixture. In certified piston aircraft engines, ignition is supplied by two independent magnetos firing two spark plugs per cylinder, providing redundancy and more complete combustion.
Plain English
It is the part of a piston engine that creates the spark which lights the fuel-air mixture inside each cylinder, timed precisely so the engine runs smoothly. Aircraft piston engines use two separate spark systems for safety, so the engine keeps running even if one system fails.
Context Anchor
Seen in engine start procedures, before-takeoff engine checks, aircraft maintenance discussions, and troubleshooting for rough running or loss of power.
Derivation
Ignition comes from the Latin ignire, meaning 'to set on fire.' Reciprocating comes from the Latin reciprocare, meaning 'to move back and forth' -- a reference to the pistons sliding up and down inside the cylinders. Together, the term describes the spark-producing system used in back-and-forth piston engines, distinguishing it from the very different ignition arrangements used in turbine engines.
Why Pilots Care
Reliable ignition is essential for engine starting and continuous operation; failure can cause power loss or complete engine stoppage in flight.
Analogy
It is like the spark in a gas stove, but much more precisely timed. The fuel is already there; ignition is the spark that starts the burn at the right instant.
Intuition Check
Ignition does not mean only turning the key or pressing the starter. In this context, it means the spark system and spark timing that keep the fuel-air mixture burning correctly inside a piston engine.
Example Sentence 1
During the run-up, the pilot tested each side of the ignition system on the reciprocating engine and noted a small RPM drop on the left magneto, well within limits.
Example Sentence 2
The mechanic timed the ignition on the reciprocating engine so the spark occurred at the proper crankshaft position.