What is the exact meaning of "cold soak" of an Auxiliary Power Unit (APU)? Is it to do with long flights at high altitude and the fuel getting cold? Will it affect the ignition of the APU? Can anyone tell how this affects the function of APU?
It 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 term 'cold soak' is used, for example, when the aircraft is parked overnight in low temperature weather conditions. It is possible that the FADEC APU presents issues igniting the APU due to fuel low temperature.
For more info on aircraft cold soak tests, check the following videos:
The APU "cold soak" in flight start program is part of the ETOPS airworthiness requirements.
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.
You can find detail information in FAA AC 120-42, par.301m.