Under which circumstances do the pneumatics switch from APU to engines during takeoff?

The transition of pneumatics from the Auxiliary Power Unit (APU) to the engines during takeoff primarily occurs when thrust is set out of TOGA, which stands for Takeoff/Go Around thrust. This is a critical moment in the flight as it signifies that the aircraft is in a high-power setting necessary for takeoff. At this point, the engines are effectively producing the required thrust, and the pneumatic system can switch from relying on the APU to utilizing the engines to support various systems such as air conditioning and pressurization.

In circumstances where the thrust remains at TOGA, the engines are fully engaged, which supports the pneumatic load requirements effectively. As the aircraft progresses through the takeoff phase, maintaining thrust is essential not just for performance but also for the transition of engine functions to manage pneumatic demands.

The other considerations like landing gear position or flap settings, while they play a role in overall aircraft configuration and safety, do not directly trigger the pneumatic switch from APU to engines. Thus, while those conditions are important for the proper conduct of takeoff, the primary switch mechanism is linked with the transition of thrust levels.