Which power sources are listed as options for seamless UPS power transfer in the described system?

• A. Ground power, APU, and engines
• B. Ground power and external power only
• C. APU and engines only
• D. External power only

Answer: (A)

Seamless UPS power transfer relies on having multiple approved sources available so power can be kept steady as conditions change. In this system, the UPS can take power from three sources: ground power when the aircraft is at a gate, the auxiliary power unit when the engines aren’t supplying power, and engine-driven generators during flight or engine operation. The transfer logic switches between these sources so the load never experiences a gap, preserving essential systems throughout all phases of operation.

Other options would miss one or more essential sources, risking a loss of UPS power during key phases—on the ground without ground power or APU, or in flight without engine-derived power.