Definition
A low point built into a fuel tank where water and sediment collect, fitted with a drain valve so the pilot can sample and remove contaminants before flight.
Plain English
The lowest spot in the fuel tank, with a small drain on it, where water and dirt settle so you can drain a bit out and check the fuel before you fly.
Context Anchor
Encountered during the before-flight inspection when the pilot samples fuel from the drain points under the wings or fuselage.
Derivation
Sump' comes from a Middle English word meaning a marsh or low pool — a place where liquid collects at the bottom. In aviation, it keeps that exact sense: the low collection point inside the tank.
Why Pilots Care
Draining the sump removes water and debris that could reach the engine and cause power loss or failure.
Intuition Check
A fuel tank sump is not a separate fuel tank or extra fuel supply. It is the low collection and drain point used to find and remove water or dirt from the fuel.
Example Sentence 1
During preflight, she drained a sample from each fuel tank sump and checked it for water and color.
Example Sentence 2
After heavy rain, extra care was taken to drain the fuel tank sump thoroughly to remove any moisture that had entered the system.