Definition
A large gear with teeth cut around its circumference, used in aircraft engines to transfer rotational force. In a reciprocating engine, the ring gear is bolted around the outside of the flywheel or crankshaft hub and meshes with the starter motor's pinion gear to crank the engine during start. In a planetary gear system, the ring gear is the outer gear with teeth cut on its inside surface, surrounding and meshing with the planet gears.
Plain English
A big circular gear with teeth around its edge. In most piston aircraft engines, it's the gear the starter motor grabs onto to spin the engine and start it.
Context Anchor
Seen in aircraft engine starting-system discussions, maintenance write-ups, and inspections of starter engagement problems.
Derivation
Called a 'ring' gear because it is shaped like a ring — a large circular band with teeth, encircling the part it drives or is driven by.
Why Pilots Care
Damage or wear on the ring gear teeth can prevent the starter from engaging, causing engine starting failures that may strand the aircraft.
Intuition Check
Do not think of a ring gear as just a round part or a spacer. The important point is that it has gear teeth, and those teeth let the starter turn the engine.
Example Sentence 1
When the pilot turned the key to START, the starter motor's pinion engaged the ring gear and began turning the crankshaft.
Example Sentence 2
During the annual inspection the mechanic checked the ring gear teeth for chips or excessive wear.