When would the pneumatics transition from engines to APU on landing?

The correct answer highlights the operational protocols concerning the transition of pneumatic systems from the engines to the Auxiliary Power Unit (APU) during landing. Specifically, this transition occurs when the flaps are set to a position greater than 20 degrees.

The rationale for this is grounded in aircraft operational procedures: extending the flaps to a greater angle indicates that the aircraft is configured for landing, signaling a change in aerodynamic and power demands. At this point, the pneumatic system can switch to the APU to conserve engine resources during landing and taxi phases. This transition also ensures that engines are not unnecessarily taxed during low-speed operations, where they would not be as efficient.

Additionally, keeping the engine's pneumatic systems engaged at lower flap settings or with the landing gear retracted could lead to inefficiencies or complications, as fewer resources are required for aerodynamic control and stabilization at those phases of flight.

In summary, the pneumatic transition occurs when flaps are set to greater than 20 degrees because this configuration optimizes aircraft performance, resource management, and follows established operating protocols, ensuring that the aircraft is properly prepared for landing.