Definition
A small reservoir connected to a liquid-filled system that provides space for the liquid to expand into as it heats up, and to draw from as it cools and contracts. In airplanes, expansion tanks are commonly part of liquid-cooled engine systems, where coolant volume changes with temperature.
Plain English
A small extra container attached to a liquid system that gives the liquid somewhere to go when it gets hot and swells, and somewhere to come from when it cools and shrinks. It keeps the system from overflowing or running short.
Context Anchor
Seen during seaplane or airplane preflight discussions when inspecting fuel tanks, vents, floats, or other outside parts of the airplane.
Derivation
From Latin 'expandere' meaning 'to spread out.' Liquids spread out (take up more space) as they warm. The tank is named for the job it does -- it gives that expansion somewhere to go.
Why Pilots Care
Prevents pressure damage, fluid loss, or system failure from thermal expansion during flight.
Grounding Statement
A tank filled in cool weather can become too full after sitting in the sun because the fuel takes up more space as it warms.
Intuition Check
Do not think of an expansion tank as extra usable fuel. In this context, it is mainly space set aside to handle fluid expansion and protect the tank system.
Example Sentence 1
During preflight, the pilot opened the cowling and checked the coolant level in the expansion tank.
Example Sentence 2
After climb-out the fluid level rose in the expansion tank as the engine reached operating temperature.