Definition
A component fitted to the engine breather or crankcase ventilation system that removes oil mist from the air being vented overboard. The mixture of air and oil vapor is passed through internal baffles, screens, or a centrifugal element that causes the heavier oil droplets to coalesce and drain back into the engine, while the cleaned air is discharged to the atmosphere.
Plain English
A small device that catches the oil floating in the air being pushed out of the engine, sends the oil back into the engine, and lets the cleaned air escape.
Context Anchor
Seen in piston-engine aircraft maintenance, engine inspections, and discussions of oil loss or oil on the belly of the airplane.
Why Pilots Care
Reduces oil loss, prevents exhaust staining on the aircraft belly, and keeps the engine compartment cleaner.
Analogy
It works like catching spray from a mist before letting the air continue on.
Intuition Check
An air-oil separator is not an oil filter. It separates oil carried in air, not dirt carried in oil.
Example Sentence 1
After installing the air-oil separator, the owner noticed far less oil streaking along the belly of the aircraft after each flight.
Example Sentence 2
Oil on the lower cowling disappeared after the air-oil separator was serviced.