Definition
A reciprocating engine in which the lubricating oil is stored in a reservoir (the sump) that forms part of the engine itself, typically at the bottom of the crankcase. Oil is drawn from this sump by a pressure pump, circulated through the engine to lubricate moving parts, and then drains back into the sump by gravity to be recirculated.
Plain English
An engine that keeps its oil supply inside itself, in a built-in pan at the bottom. The oil gets pumped around the engine to keep things slippery, then drains back down into the same pan to be used again.
Context Anchor
Seen in engine lubrication discussions, preflight oil checks, and descriptions of piston aircraft engine design.
Derivation
Sump comes from Middle Dutch somp meaning a swamp or low-lying spot where water collects. In an engine, the sump is the low point where oil naturally collects. Wet means the sump actually holds the oil, as opposed to a dry-sump engine where oil is stored in a separate tank.
Why Pilots Care
Determines oil quantity checking procedures and increases the chance of oil starvation during steep maneuvers compared with external-tank designs.
Analogy
Think of it like a lawn mower engine that keeps its oil in the bottom of the engine case. The oil is not stored in a separate tank; it sits in the low part of the engine and is circulated from there.
Intuition Check
“Wet” does not mean the outside of the engine is wet. It means the oil storage area inside the bottom of the engine contains the oil used for lubrication.
Example Sentence 1
The Cessna 172's wet-sump engine holds about eight quarts of oil in its crankcase, which the pilot checks through the dipstick during preflight.
Example Sentence 2
Wet-sump engines require the aircraft to sit level for an accurate oil reading on the dipstick.