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 (sump) of the crankcase. A pump draws oil from this internal reservoir, circulates it through the engine, and the oil drains back into the sump by gravity to be used again.
Plain English
A lubrication setup where the engine stores its own oil inside the bottom of the engine, rather than in a separate tank. The oil keeps cycling through the engine and falling back into the same built-in pool.
Context Anchor
Seen in engine-system discussions, preflight knowledge, and oil servicing for many light training aircraft.
Derivation
‘Sump’ comes from a Middle English word meaning a pit or pool where liquid collects. ‘Wet’ here simply means the sump holds oil while the engine runs — the storage area is wet with the working oil supply.
Why Pilots Care
Affects engine simplicity, oil capacity, and the aircraft’s ability to operate in unusual attitudes without oil starvation.
Analogy
It is like a small fountain that keeps its water in the basin underneath. The pump sends the liquid up, and gravity brings it back down to the basin.
Intuition Check
“Wet” does not mean water is involved. In this term, it means the oil storage area is inside the engine and normally contains oil.
Example Sentence 1
The Cessna 172 uses a wet-sump oil system, so the pilot checks the oil level by pulling the dipstick on the engine itself.
Example Sentence 2
Mechanics check the oil level on the dipstick of a wet-sump oil system before each flight.