APU Cold Soak means the APU has been in active or not operating in a cold environment for an extended period of time. Here, like any other piece of machinery and operating fluids are “cold soaked“, that is, has reached the ambient temperature of the environment. During extended flight at high-altitude, and APU would encounter this as it is not normally used during cruise flight. This does affect the ability of the unit to power up in the event of an emergency, as most machines don’t like being operated in extremely cold weather, and usually require a period of time in order to warm up.
The ETOPS Reliability program requires that in twin engines aircraft to periodically Check the ability of the APU to start in flight when “cold soak”, it means the temperature has decrease as much that the fluids and mechanical components become freeze.
To be representative, the “cold soak” can be achieved in flight after 2 hours of the APU shutdown and before the top of decent.