How many times can the APU start sequence be attempted in one hour?

The APU (Auxiliary Power Unit) start sequence can be attempted up to three times within one hour. This limitation is primarily based on the thermal and mechanical considerations of the APU. Each start attempt generates heat and mechanical stress; thus, allowing only three attempts in one hour helps prevent overheating and damage to the APU. This protocol ensures that the unit operates reliably while maintaining safety standards.

Attempts beyond the allowed sequence could lead to complications such as reduced performance or potential failure of the APU. Therefore, understanding these limitations and the rationale behind them is crucial for effective ground operations and maintenance of the aircraft.