How many APU start attempts are allowed before a cooling period is required?

The correct answer is three attempts before a cooling period is required for an Auxiliary Power Unit (APU). This rule is in place to prevent damage to the APU, as repeated attempts to start it can lead to overheating and other mechanical issues. After three unsuccessful start attempts, the cooling period allows the APU components to return to a safe operating temperature, reducing the risk of failure or damage. It’s important to adhere to these guidelines for optimal performance and longevity of the equipment.