Definition
An aircraft air conditioning system that cools cabin air by using the cooling effect produced when compressed air is expanded through a turbine. Hot, high-pressure bleed air from the engines is passed through heat exchangers and an expansion turbine, which drops its temperature and pressure before the air is delivered to the cabin. It uses air itself as the refrigerant, with no chemical refrigerant or vapor-compression cycle.
Plain English
A system that cools cabin air by squeezing hot engine air, removing heat from it, and then letting it expand rapidly so it gets cold. The cold air is then sent into the cabin. There is no freon or chemical refrigerant -- the air does the cooling work itself.
Context Anchor
Seen in aircraft environmental control and cabin air-conditioning systems, especially on larger or turbine-powered aircraft.
Derivation
Called 'air-cycle' because the working substance going through the cooling cycle is plain air, in contrast to a 'vapor-cycle' system, which uses a refrigerant that changes between liquid and vapor.
Why Pilots Care
Maintains safe and comfortable cabin temperatures at high altitude where outside air is far below freezing.
Grounding Statement
When compressed air is allowed to expand quickly, it cools sharply -- the same reason air rushing out of a tire feels cold. An air-cycle system uses that effect on a large scale to chill cabin air.
Intuition Check
Air-cycle cooling does not mean simply blowing outside air into the aircraft. It means cooling air through a controlled process of compression, heat removal, and expansion.
Example Sentence 1
The flight crew noticed rising cabin temperatures after the left air-cycle cooling system pack tripped offline.
Example Sentence 2
During preflight the technician checked the air-cycle cooling system for turbine bearing wear.