Definition
A mechanical drive system in which a roller chain transmits rotational power between sprockets, typically from an engine or accessory drive shaft to a driven component such as a magneto, supercharger, or accessory section in certain reciprocating aircraft engines.
Plain English
A chain-and-sprocket setup, like the chain on a bicycle, used inside some aircraft engines to turn one part by power from another.
Context Anchor
Seen in aircraft maintenance and systems descriptions for chain-driven mechanisms, such as some control, drive, or positioning systems.
Derivation
From 'chain' (a series of connected metal links) and 'gear' (a toothed mechanical component used to transmit motion). Together the term describes a drive system where a chain rides on toothed sprockets rather than gears meshing directly.
Why Pilots Care
On engines that use chain-driven accessories, chain wear or stretch can affect timing and accessory performance, which is why these systems are inspected and adjusted at maintenance intervals.
Analogy
Think of the chain on a bicycle: the pedals turn one sprocket, and the chain transfers that motion to a sprocket at the rear wheel. A chain gear in an engine works the same way, just inside a sealed, oil-lubricated housing.
Intuition Check
Do not confuse chain gear with landing gear. Here, gear means a toothed mechanical wheel, not the wheels and struts used for takeoff and landing.
Example Sentence 1
The mechanic inspected the chain gear for stretch during the engine's scheduled overhaul.
Example Sentence 2
Older radial engines often used chain gear to drive magnetos and pumps from the crankshaft.