When do the bleeds switch back to the APU after landing?

The correct response indicates that the bleeds switch back to the Auxiliary Power Unit (APU) when the flaps are greater than 20 degrees during the landing phase. This timing is essential because it ensures that the aircraft systems relying on bleed air are properly managed for both safety and efficiency.

When the flaps are extended beyond 20 degrees, the aircraft is typically configured for landing, indicating that it is in the latter part of the approach phase. Transitioning to the APU at this moment allows for a smoother process in managing the aircraft's power and air conditioning systems once the engines become less efficient at providing that bleed air.

The choice to wait for this specific flap position takes into account the aerodynamic conditions of the plane, optimizing performance and stability. Other thresholds, such as landing gear deployment or coming to a complete stop, do not provide the same operational readiness for bleed air management immediately post-landing. Furthermore, only switching to the APU after engine shutdown would not utilize the benefits of bleed air during the critical moments leading up to and immediately after landing.