What are the cooling time limitations for APU starts on the ground?

The correct answer outlines the specific cooling time limitations for Auxiliary Power Unit (APU) starts while the aircraft is on the ground. When starting an APU, the procedures are designed to ensure the unit is not overstrained from consecutive attempts, which could lead to premature failure or damage.

The initial guideline allows for a one-minute cooling period between the first two start attempts. This brief interval helps prevent overheating and allows the APU to stabilize. After the second attempt, if a third start is necessary, the cooling period increases significantly to 20 minutes. This longer duration is crucial because it allows the APU to cool sufficiently and reduces the risk of thermal stress, ultimately preserving the integrity and longevity of the unit.

Understanding these limitations is essential for maintaining the effectiveness of the APU and preventing potential operational issues. Following these cooling times is a matter of adhering to manufacturer guidelines and ensuring safety during ground operations. The other options do not reflect the structured cooling process needed for optimal APU performance, particularly after multiple unsuccessful start attempts.