Definition
A vented tube or fitting connected to the engine crankcase that allows internal pressure to escape to the outside atmosphere. As the engine runs, combustion gases leak past the piston rings into the crankcase (called blow-by), raising pressure and adding moisture and vapor. The breather lets this pressure and vapor vent overboard so the crankcase stays near atmospheric pressure and seals are not pushed out.
Plain English
A small vent line on the engine that lets pressure and vapor escape from the inside of the engine, so pressure does not build up and damage seals or push oil past them.
Context Anchor
Seen during preflight inspection, engine servicing, and checks around the engine cowling or lower engine area.
Derivation
The word breather comes from the idea of letting something breathe -- in this case, the crankcase. Just as a person needs to exhale, the engine's internal cavity needs a way to release pressure that builds up while it runs.
Why Pilots Care
A clogged or frozen breather can cause crankcase pressure to rise, forcing oil past seals or creating a fire hazard from leaking oil.
Intuition Check
An oil breather is not an oil filler, oil drain, or oil filter. It is a vent path that lets pressure and oily vapor escape from inside the engine.
Example Sentence 1
During preflight, the pilot checked the oil breather tube under the cowling to make sure it was clear and not damaged.
Example Sentence 2
After an extended flight in cold weather the pilot inspected the oil breather for ice buildup that could restrict venting.