Definition
An aircraft engine in which heat from combustion is removed by a liquid coolant circulated through passages around the cylinders, then routed to a radiator where the coolant releases its heat to the outside air before returning to the engine.
Plain English
An engine kept cool by pumping fluid around the hot parts, which then flows through a radiator to dump the heat into the air, much like a car engine.
Context Anchor
Seen when discussing engine cooling, engine temperature limits, and damage outside the airplane that could affect cooling parts.
Derivation
Liquid-cooled simply means cooled by a liquid. The term is used in contrast to air-cooled, where outside air alone carries heat away from the cylinders. Knowing the contrast is the whole point of the name.
Why Pilots Care
Liquid-cooled engines provide more even temperature control, which can reduce the risk of detonation and improve reliability in varying flight conditions, though they add weight, complexity, and maintenance items such as coolant checks and radiator inspections.
Analogy
Cooling works the same way as a car: a pump pushes coolant through the engine to soak up heat, then through a radiator where airflow carries the heat away.
Intuition Check
Liquid-cooled does not mean the engine ignores outside air. The liquid moves heat away from the engine, but outside air is still needed to remove that heat from the radiator.
Example Sentence 1
Because the trainer had a liquid-cooled engine, the preflight included checking the coolant level along with the oil.
Example Sentence 2
In high ambient temperatures the liquid-cooled engine maintained steadier operating temperatures than the air-cooled engines on other training aircraft.