Definition
Electrical components threaded into the cylinders of a piston aircraft engine that produce a high-voltage spark across a small gap to ignite the compressed fuel-air mixture at the correct moment in the combustion cycle. Aircraft engines use two spark plugs per cylinder, fired by two independent magnetos, for redundancy and more complete combustion.
Plain English
Small devices screwed into each engine cylinder that make a tiny electrical spark to set the fuel and air on fire at just the right moment, which is what makes the engine run.
Context Anchor
Seen in engine-system lessons, maintenance records, and pre-takeoff engine checks when the pilot is checking that the engine runs smoothly.
Derivation
The word "spark" refers to the small electrical discharge the device produces, and "plug" refers to its threaded body that screws (plugs) into a hole in the cylinder. The name simply describes what it does and how it fits.
Why Pilots Care
Worn or fouled spark plugs cause rough running, power loss, or complete engine failure.
Intuition Check
Do not think of spark plugs as casual car parts that happen to be in an airplane. In an aircraft piston engine, they are flight-critical ignition parts, and many aircraft engines use two spark plugs per cylinder for reliability.
Example Sentence 1
During the runup, a slight RPM drop on one magneto is normal, but a large drop may indicate a fouled spark plug.
Example Sentence 2
Replacing the spark plugs restored smooth engine operation on the training flight.