The ACPC power distribution priority is ______.

The priority of AC power distribution in the CRJ550 aircraft system is important for ensuring that the essential electrical loads are powered efficiently and reliably. The correct sequence is for the On-side Integrated Drive Generator (IDG) to provide power first when available, as it is typically the most reliable and preferred source of electrical power during normal operations. The APU (Auxiliary Power Unit) serves as the backup power source and is utilized next if the On-side IDG is not available. Lastly, the Off-side IDG is their third option for supplying power.

This hierarchy is designed to keep the aircraft’s systems operating smoothly. Starting with the On-side IDG maximizes the efficiency and reliability of the aircraft's electrical system, as the IDG converts engine power into electrical power very effectively. The APU serves as a ready source of power if the main generator is not operational, allowing continued functionality without relying on external sources. The Off-side IDG comes last, as it is typically less favored due to potential transfer time and complexities associated with switching power sources.

The other options do not align with the operational priorities established for aircraft power distribution, as they suggest different power sourcing sequences that would not optimize reliability and safety during flight operations.