Final answer:
The exact number of APU start attempts allowed in 1 hour depends on the aircraft's manufacturer guidelines and model, typically allowing a few attempts with mandatory cooling periods in between to prevent APU damage.
Step-by-step explanation:
The number of APU start attempts that are allowed in 1 hour can vary depending on the aircraft manufacturer's guidelines and the specific model of the aircraft. Typically, for most turbine aircraft, you are allowed a certain number of start attempts followed by a cooling period. For example, you might be allowed three attempts to start the APU, with each attempt followed by a 1-minute cooling period if the APU does not start. If after three attempts the APU has not started, a longer cooling period, often around 30 minutes, is required before additional attempts are made. This cooling period serves to prevent overheating and potential damage to the APU. However, without the specific aircraft model information, providing an exact number of allowed APU start attempts in 1 hour is not feasible. Here, options A (2 attempts), B (3 attempts), C (4 attempts), and D (5 attempts) present possible scenarios, but the correct answer would typically be in the aircraft's operating manual or SOP (Standard Operating Procedure).