Auto APU Start - B777
 

Hi there,

FCOM states: "In flight, if both AC transfer busses lose power, the APU automatically starts, regardless of APU selector position. The APU can be shut down by positioning the selector to ON, then OFF."

Why APU starts automatically when AC tsx busses lose power?

Thanks for your time

If you lose BOTH AC transfer buses, then you have a major electrical problem and will lose a lot of essential equipment. The APU auto starts to ensure power is restored to the transfer buses. The RAT will also deploy and provide limited power until the APU generator comes on line.

Thank you!