When the APU bleed air is selected on, what effect does this have on the other bleed valves?

When the APU bleed air is selected on, the X-bleed valves are automatically opened to allow for the proper distribution of bleed air throughout the aircraft’s pneumatic system. This function is crucial because it enables the use of air from the Auxiliary Power Unit (APU) to serve systems that require pneumatic pressure, such as engine starting or environmental control systems.

Opening the X-bleed valves facilitates airflow between the left and right bleed air systems, allowing for redundancy and ensuring a consistent airflow. This is especially important during engine start procedures, as it can help maintain the necessary pressure levels if one engine's bleed system experiences a drop in performance or demand.

While the answer indicates that options involving manual closure or shutdown of other valves might seem like plausible considerations, they do not apply directly when the APU bleed air is selected on. The automatic opening of the X-bleed valves optimally utilizes the APU and maintains essential system functionality.