PSA Airlines Canadair Regional Jet Systems (PSA-CRJ) Practice Exam

When starting the APU in flight, the APU door opens based on which factors?

1. Temperature and Pressure

2. Airspeed/Mach number and Altitude

3. Engine RPM and Fuel Flow

4. Flight Phase and APU Load

The correct answer is: Airspeed/Mach number and Altitude

The correct choice indicates that the APU door opens in response to airspeed/Mach number and altitude. This relationship is crucial for ensuring the APU operates effectively and safely while the aircraft is in flight. When the aircraft is airborne, the APU door's operation is closely monitored by the aircraft's systems to optimize airflow and maintain performance. The airspeed or Mach number is important because it influences the aerodynamic conditions around the aircraft, affecting the amount of air entering the APU. Additionally, altitude plays a significant role since as the aircraft ascends or descends, the air density changes, which can impact the APU’s performance and cooling needs. Opening the APU door according to these factors ensures that the unit receives adequate airflow for combustion and cooling, preventing any potential overheating or performance issues. The other factors listed do not pertain to the specific conditions necessary for the APU door operation during flight. For example, engine RPM and fuel flow may relate to overall engine performance but do not dictate the APU door's opening mechanism. Similarly, while flight phase and APU load are important considerations, they are not the primary determinants for the door's operation during the specific act of starting the APU in flight.