Definition
The procedure of forcing oil through the internal passages and bearings of an aircraft engine before it is started, typically after overhaul, long storage, or installation of a new or rebuilt engine. Oil is pumped under pressure into the engine's oil system — usually through an external pressure source connected to an oil gallery port — until oil appears at the rocker arms or other end points, ensuring all bearing surfaces are coated before the engine is cranked.
Plain English
Pushing oil through every part of an engine before you start it, so no metal surface is dry when the engine first turns over.
Context Anchor
Seen in aircraft engine maintenance, especially before the first start after an engine has been overhauled, installed, or left unused for a long time.
Derivation
From the prefix 'pre-' (Latin prae, meaning 'before') and 'oiling.' Literally: oiling beforehand. The derivation reinforces the timing — oil is in place before the engine ever turns.
Why Pilots Care
Prevents dry-start damage to bearings and other components, reducing wear and the risk of premature engine failure.
Intuition Check
Preoiling does not just mean checking that the engine has oil in it. It means making sure oil has reached the places inside the engine that need lubrication before the engine is run.
Example Sentence 1
After installing the overhauled engine, the mechanic spent ten minutes preoiling it before attempting the first start.
Example Sentence 2
Preoiling is required on a freshly overhauled engine to avoid metal-to-metal contact during the initial rotation.