Definition
The main housing of a reciprocating aircraft engine that encloses and supports the crankshaft, connecting rods, bearings, and other internal moving parts. It also provides mounting surfaces for the cylinders and accessories, and contains the engine's lubricating oil.
Plain English
The strong central case of a piston engine. It holds the crankshaft in place, supports the cylinders that bolt onto it, and keeps the engine oil inside.
Context Anchor
Seen in aircraft engine descriptions, maintenance inspections, oil leak checks, and discussions of piston engine construction.
Derivation
From 'crank' (the rotating shaft that converts piston motion into rotary motion) plus 'case' (an enclosing container). Literally, the case that houses the crankshaft.
Why Pilots Care
Damage or cracks in the crankcase can cause loss of engine oil pressure and lead to complete engine failure.
Intuition Check
Do not think of the crankcase as just an outer cover. In an aircraft piston engine, it is a major load-carrying part of the engine, not simply a shell.
Example Sentence 1
During the preflight, the pilot checked the crankcase for oil seepage around the seams.
Example Sentence 2
Oil was seeping from a small crack in the lower crankcase after the hard landing.