What is the correct priority order for power supply to AC BUS 1 / AC BUS 2?

What is the correct priority order for power supply to AC BUS 1 / AC BUS 2?

• A. Onside (Same side generator), Inside (APU), Offside (Other side generator), Outside (GPU)
• B. Inside (APU), Onside (Same side generator), Outside (GPU), Offside (Other side generator)
• C. Onside, Offside, Inside, Outside
• D. Outside, Onside, Inside, Offside

Answer: (A)

Power to AC BUS 1 and AC BUS 2 is kept by a fixed source priority that favors internal, closest sources before external power. The first source used is the generator on the same side as the bus (onside) because it feeds the bus directly with the lowest impedance, keeping the distribution straightforward and reliable. If that source isn’t available, the APU generator is used next; it provides an independent, internal backup that can power the buses even when engine-generated power isn’t present or sufficient. If the APU is also unavailable, the generator on the opposite side (offside) serves as a secondary internal source. Finally, ground power from a GPU is used only when the aircraft is on the ground and no internal sources are usable. This hierarchy—onside, then APU, then offside, then GPU—matches the described sequence.

Power to AC BUS 1 and AC BUS 2 is kept by a fixed source priority that favors internal, closest sources before external power. The first source used is the generator on the same side as the bus (onside) because it feeds the bus directly with the lowest impedance, keeping the distribution straightforward and reliable. If that source isn’t available, the APU generator is used next; it provides an independent, internal backup that can power the buses even when engine-generated power isn’t present or sufficient. If the APU is also unavailable, the generator on the opposite side (offside) serves as a secondary internal source. Finally, ground power from a GPU is used only when the aircraft is on the ground and no internal sources are usable. This hierarchy—onside, then APU, then offside, then GPU—matches the described sequence.