Definition
A liquid circulated through a liquid-cooled aircraft engine to absorb heat from the engine and carry it to a radiator or heat exchanger, where the heat is released to the outside air. Coolant is typically a mixture of water and ethylene or propylene glycol, which raises the boiling point, lowers the freezing point, and inhibits corrosion.
Plain English
The fluid that flows through a liquid-cooled engine to keep it from overheating. It picks up heat from the engine and dumps it overboard through a radiator.
Context Anchor
You may encounter coolant during a preflight inspection when checking engine fluid levels, looking for leaks, or inspecting the area around a liquid-cooled engine.
Derivation
From 'cool' plus the suffix '-ant,' meaning 'a thing that does the action.' A coolant is literally 'a thing that cools.' Knowing this makes the role of the fluid obvious: its whole job is to cool the engine.
Why Pilots Care
Maintaining proper coolant level prevents engine overheating and potential in-flight power loss or damage.
Intuition Check
Coolant does not mean a liquid that is always cold. It means a liquid whose job is to remove heat from the engine.
Example Sentence 1
During preflight, the pilot checked the coolant reservoir level and inspected the radiator hoses for leaks.
Example Sentence 2
Rising coolant temperature in flight prompted the pilot to reduce power and land at the nearest suitable airport.