What is the minimum ambient temperature for starting a cold soaked APU on the ground?

The minimum ambient temperature for starting a cold soaked Auxiliary Power Unit (APU) on the ground is indeed -40°C. This specific temperature limit is critical because it ensures that the APU can start reliably under extreme cold conditions. At this threshold, the necessary systems and components within the APU are designed to function, preventing potential damage or failure during the start-up process.

When temperatures drop significantly, various factors like oil viscosity and battery performance can affect the starting capability of the APU. Therefore, -40°C is established as a safety guideline to ensure that operational integrity is maintained and to avoid unnecessary wear or operational issues. This temperature limit reflects the engineering design considerations aimed at ensuring safe and reliable aircraft operation under harsh environmental conditions. Higher temperature limits may impair the APU's ability to start or could risk damaging components due to the cold conditions.