After takeoff (no anti-icing required), what condition causes bleed air to transfer from engine bleed to the APU?

• A. Engine starting sequence begins
• B. Altitude reaches 10,000 feet
• C. APU start switch activated
• D. Thrust lever retarded from TOGA with gear up and flaps 20 degrees or less

Answer: (D)

Bleed air source selection is managed by the environmental control system, and it can switch from engine bleeds to the APU bleed when the airplane configuration indicates the engines no longer need to supply bleed air for takeoff and the APU can provide it instead. After takeoff, as you retard from TOGA and climb with the gear up and flaps at 20 degrees or less, the aircraft is in a phase where engine bleed demand for takeoff is reduced. Since anti-ice is not required in this scenario, the ECS automatically transfers bleed air supply to the APU bleed to feed the packs and cabin pressurization. This keeps engine bleeds free for other functions if needed and avoids overloading the engine bleeds.

Options that involve starting the APU bleed manually, altitude triggers, or engine start sequences don’t prompt this automatic transfer in the same way, and the automatic switch is specifically tied to the aircraft being configured for climb (retreat from TOGA with gear up and flaps 20 or less) when anti-ice is not required.