How many APU start cycles are allowed in one hour?

The maximum number of APU (Auxiliary Power Unit) start cycles allowed in one hour is three. This limit is typically established to ensure that the APU does not overheat or suffer from mechanical wear due to excessive starting attempts in a short period. The APU is designed to provide power and air conditioning to an aircraft when the engines are not running, and its start-up process involves significant energy and components working hard to get the unit operational.

Permitting no more than three start cycles in one hour helps to maintain the reliability and longevity of the APU, avoiding potential failures that could arise from constant cycling. This limit is grounded in operational safety and efficiency considerations, allowing maintenance teams to monitor the APU's performance effectively while ensuring adequate recovery time between cycles.