Definition
A series of gears inside a reciprocating or turbine engine that transmits power from the engine's crankshaft (or a turbine power takeoff) to drive the engine accessories such as magnetos, generators, fuel pumps, oil pumps, vacuum pumps, and tachometer drives.
Plain English
A set of connected gears inside the engine that uses some of the engine's power to spin the pumps, magnetos, and other components the engine and aircraft need to keep running.
Context Anchor
Seen in aircraft engine descriptions, maintenance manuals, and discussions of the engine accessory section.
Derivation
Accessory comes from the Latin accessorius, meaning something added or supplementary. Gear train describes a connected line of gears that pass motion from one to the next. Together it means the connected gears that drive the supplementary engine components.
Why Pilots Care
Proper function ensures reliable ignition timing and electrical power generation; damage can cause engine failure or loss of critical systems.
Intuition Check
Do not read accessory as “optional extra” here. In this context, engine accessories may be necessary for the engine and airplane systems to work properly. Do not read train as a vehicle. Here it means a series of connected gears.
Example Sentence 1
A broken gear in the accessory gear train caused the magnetos and the fuel pump to stop turning at the same time.
Example Sentence 2
A sheared pin in the accessory gear train can stop the alternator from charging the battery.