Definition
An engine lubrication system in which the oil supply is carried in a reservoir that is part of the engine itself, typically the lower portion of the crankcase (the sump). A pump draws oil from this internal reservoir, circulates it through the engine to lubricate and cool moving parts, and returns it directly to the same sump by gravity.
Plain English
The engine stores its own oil inside itself, in a built-in pan at the bottom. Oil is pumped up through the engine, does its job, then drains back down into that same pan to be used again.
Context Anchor
Seen in aircraft engine oil system discussions, preflight oil checks, and Pilot’s Operating Handbook information for many small training airplanes.
Derivation
‘Sump’ comes from a Middle English/Low German word meaning a pit or pool where liquid collects. ‘Wet’ here means the sump actually holds the oil supply — as opposed to a ‘dry-sump’ system where the sump is kept empty and oil is stored in a separate external tank.
Why Pilots Care
Simpler design with fewer components, but can lead to oil starvation during extreme maneuvers or steep climbs.
Analogy
Similar to the oil pan in a car engine, where oil stays pooled at the bottom and is picked up as needed.
Intuition Check
“Wet” does not mean water is involved. In this term, it means the engine’s lower oil reservoir contains oil, rather than the oil being kept in a separate tank.
Example Sentence 1
The Cessna 172 uses a wet-sump system, so I check the oil level by pulling the dipstick on the engine itself during preflight.
Example Sentence 2
Most light training aircraft use a wet-sump system because it requires less maintenance than alternatives.