What happens to the fuel flow during on-speed operations of the APU?

During on-speed operations of the Auxiliary Power Unit (APU), the fuel flow is regulated to maintain a constant RPM. This regulation is essential for ensuring that the APU operates efficiently and reliably. When the APU reaches its optimal operating speed, it becomes crucial to manage the fuel flow precisely to avoid any fluctuations in RPM that could lead to inefficiencies or potential operational issues.

This process includes controlling the fuel injection to match the power demand without exceeding necessary levels, ensuring that the APU can provide continuous and stable output. By maintaining a constant RPM, the system can deliver necessary power for aircraft systems while maximizing fuel efficiency.

The other options suggest approaches that would not support stable APU operation or would be undesirable during on-speed operations. For example, completely cutting off fuel flow would disrupt APU functionality, pumping at maximum capacity could lead to over-pressurization or overload, and switching to an alternate fuel source could introduce variability that undermines the stability needed during on-speed operations. Thus, regulating fuel flow is the optimal approach to maintain consistent APU performance.