After takeoff (no anti-icing required), what causes the bleeds to transfer from the APU to the engines?

• A. Thrust levers retarded from TOGA with gear up and flaps 20 degrees or less
• B. Autopilot engagement
• C. Increase in cabin pressure
• D. APU failure

Answer: (A)

Bleed air source preference switches based on the airplane’s pneumatic logic as you transition from takeoff to the climb. While you’re at TOGA thrust, the APU bleed can be supplying the system. Once you retract from TOGA — with the landing gear up and flaps at 20 or less — the engines are now delivering sufficient bleed air, and the automatic bleed control shifts the source from the APU to the engine bleeds. This keeps the pneumatic system efficient and minimizes APU load.

Autopilot engagement doesn’t trigger the transfer, and a rise in cabin pressure isn’t the trigger either. An APU failure would disable the APU bleed rather than cause a transfer to engine bleeds; the transfer described here is a normal consequence of changing thrust configuration after takeoff.