How many starts or start attempts are permitted for the APU starter motor in one hour?

The correct answer indicates that the APU (Auxiliary Power Unit) starter motor is allowed a maximum of three starts or start attempts within one hour. This limitation is primarily established to prevent overheating and potential damage to the starter motor.

The manufacturer's recommendations for the APU's starter motor operation take into account factors like cooling periods between attempts and the overall reliability and longevity of the equipment. After three attempts, the APU starter should be allowed sufficient time to cool down before any further attempts are made, which ensures that the components within the starter do not sustain excessive wear or heat-related damage.

Understanding this limitation is crucial for operating the aircraft safely and effectively, especially when dealing with on-ground operations where the APU is frequently used for power and air conditioning. Pilots and maintenance crews must adhere to these guidelines to ensure the APU remains functional and to reduce the risk of in-flight or ground operational issues.