In which situation is it advised to operate the APU at maximum load?

Operating the Auxiliary Power Unit (APU) at maximum load is advised during ground operations primarily due to the APU's design functions. The APU provides electrical power and pneumatic pressure necessary for various ground operations, including starting the main engines and powering onboard systems while the aircraft is on the ground.

When the aircraft is on the tarmac, the APU can safely handle the maximum load without the risks that might be associated with other scenarios, such as during flight or emergency situations. In these instances, operating at maximum load ensures that adequate power is supplied for essential systems without overtaxing the APU or impacting safety.

During flight maneuvers or emergency landings, the engine systems take precedence, and reliance on the APU decreases significantly, making it inappropriate to operate at maximum load. Additionally, during routine maintenance checks, the APU may be operated at various loads for diagnostic purposes but does not require maximum load to perform its essential functions. Therefore, ground operations create the ideal conditions for maximizing the APU's capacity safely and effectively.