Definition
Components mounted on an aircraft engine that are driven by the engine but are not part of its primary function of producing power. Typical examples include magnetos, generators or alternators, starters, fuel pumps, vacuum pumps, and tachometer drives.
Plain English
Extra parts attached to the engine that the engine turns. They don't make the engine run, but the engine runs them so they can power other systems on the aircraft.
Context Anchor
Seen in aircraft maintenance, engine descriptions, equipment lists, and logbook entries.
Derivation
From the Latin 'accedere', meaning 'to approach' or 'to be added to.' An accessory is something added on — not the main thing, but attached to it. That fits the aviation use exactly: parts added onto the engine, driven by it, but not part of its core power-producing role.
Why Pilots Care
Failure of critical accessories can remove electrical power, instrument air, or fuel pressure, directly affecting flight safety and requiring immediate pilot action.
Intuition Check
Do not read “accessories” as optional decorations or convenience items. In aviation, accessories are working support parts connected with a main aircraft system.
Example Sentence 1
The mechanic removed the engine accessories before pulling the powerplant for overhaul.
Example Sentence 2
Loss of the vacuum pump accessory caused the attitude indicator to fail during the instrument approach.