Plain English Meaning

Low spots at the bottom of each fuel tank with a drain valve. Water is heavier than fuel, so it settles into these spots, where the pilot drains a small amount during preflight to check for water or dirt.

Full Definition

The lowest points inside an aircraft fuel tank where water and other contaminants collect, fitted with a drain valve so the pilot can sample and remove that fluid during preflight.

Context Anchor

Encountered during the preflight inspection when checking the aircraft fuel system before flight.

Derivation

‘Sump’ comes from the Middle Dutch ‘somp’ meaning a swamp or low pool. In machinery, it has long meant the low point where liquid collects. The fuel tank sump is exactly that — the low point where unwanted liquid settles .