Definition
A method of engine cooling in which liquid coolant is circulated through passages around the cylinders to absorb combustion heat, then routed to a radiator where the heat is transferred to the outside air before the coolant returns to the engine.
Plain English
The engine is kept from overheating by pumping liquid around its hot parts. The liquid carries the heat to a radiator, where moving air cools it down, and then it cycles back through the engine again.
Context Anchor
Seen in engine descriptions, aircraft system sections, and preflight discussions for aircraft that use a liquid cooling system instead of relying only on air flowing over the engine.
Derivation
The term comes from early engines that literally used water as the coolant. Modern systems usually use a water-and-coolant mixture rather than plain water, but the name has stuck.
Why Pilots Care
Water-cooled engines allow tighter temperature control and higher continuous power but add weight, complexity, and the need to monitor coolant levels and radiator function.
Intuition Check
Water-cooled does not always mean plain water is the only liquid used. In aircraft use, it usually means a water-based coolant mixture carries heat away from the engine.
Example Sentence 1
The Diamond DA42's diesel engines are water-cooled, so the preflight includes checking the coolant level.
Example Sentence 2
During climb, the water-cooled engine maintained stable cylinder-head temperatures thanks to its liquid cooling circuit.