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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?

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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.

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  • $\begingroup$ Whenever a product go through HALT/HASS testing there it will be socked in different temp ranges and even on the pressure as well. As its already handled in that testing why we are doing it again? When we do this kind of testing the product life will be reduced >50% (my college told me this) so why is it done again? or am i getting i wrong $\endgroup$ – Jeevan Nanaiah May 23 '18 at 11:30
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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:

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  • $\begingroup$ Welcome to Aviation.SE! $\endgroup$ – Federico Feb 15 '18 at 14:13
  • $\begingroup$ Whenever a product go through HALT/HASS testing there it will be socked in different temp ranges and even on the pressure as well. As its already handled in that testing why we are doing it again? When we do this kind of testing the product life will be reduced >50% (my college told me this) so why they do many times? or am i getting i wrong $\endgroup$ – Jeevan Nanaiah May 22 '18 at 9:27

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