Final answer:
Prior to or immediately after starting the APU, retracting external power and air conditioning units is important to avoid damage and ensure a smooth transition to APU power.
Step-by-step explanation:
Aircraft Systems and Operations: APU Start
Prior to or immediately after starting the APU (Auxiliary Power Unit), it is important to retract the aircraft's external power and air conditioning units. This is done to minimize the risk of damaging these external systems when the APU begins to generate power and provide air conditioning.
Retracting the external power and air conditioning units also ensures that the aircraft transitions smoothly from relying on external power to using the APU's internal power source. As the APU starts, it gradually assumes the responsibilities of generating electrical power, providing compressed air, and supporting other systems on the aircraft.
It's worth noting that the specific procedures for retracting external systems may vary depending on the aircraft model and the airline's operating procedures. However, the general principle remains the same: to transition from external power to APU power without causing damage or interruptions.