Definition
A thick, semi-solid deposit that forms inside an engine when oil mixes with combustion by-products, moisture, dirt, and oxidized oil compounds. Sludge accumulates in oil passages, sumps, screens, and around internal engine parts, where it can restrict oil flow and reduce cooling and lubrication.
Plain English
A gooey, dirty buildup that forms inside an engine over time. It comes from old, contaminated oil and can clog the passages oil needs to flow through.
Context Anchor
Seen during aircraft engine oil changes, filter or screen inspections, and maintenance checks for poor oil flow or engine condition.
Derivation
From Middle English 'slud' or 'sludge', meaning mud or mire. The everyday image of thick, sticky muck carries directly into the engine context — sludge inside an engine looks and behaves much like mud.
Why Pilots Care
Sludge can block oil passages, starve bearings and valve train components of lubrication, and accelerate engine wear if oil changes are neglected.
Grounding Statement
Picture clean oil turning into a thick dirty paste that settles where it should not be inside the engine.
Intuition Check
Do not treat sludge as just another word for dirty oil. In aircraft maintenance, sludge means a thick deposit or buildup that can collect and interfere with normal oil flow.
Example Sentence 1
During the oil change, the technician found sludge on the oil screen and recommended a closer inspection of the engine.
Example Sentence 2
Regular oil changes prevent sludge from restricting flow through the engine's narrow oil galleries.