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.
Plain English
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.
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 out.
Why Pilots Care
Draining tank sumps removes water and debris that can cause engine failure or power loss.
Analogy
A tank sump is like the low point of a sink where unwanted material tends to collect near the drain. You check that low point because that is where problems are most likely to show up.
Intuition Check
Do not confuse tank sumps with fuel pumps or extra fuel tanks. Tank sumps are low drain points used to check and remove contamination from the fuel.
Example Sentence 1
During preflight, the pilot drained a small sample from each tank sump and checked it for water and the correct fuel color.
Example Sentence 2
After refueling, the pilot drained the tank sumps again to confirm the new fuel was clean.