When is the greatest demand placed on an APU?

The greatest demand on an Auxiliary Power Unit (APU) occurs when it is supplying bleed air. During processes such as engine starts, the APU must provide sufficient bleed air to crank the engines, especially in larger aircraft where the engines require substantial airflow to start. This demand peaks when both the turbine-driven generator and bleed air requirements are at their highest, particularly during high-demand circumstances like engine starting.

When supplying electrical power, while there is a notable load on the APU, it may not reach the same intensity of demand as when the APU is tasked with supplying bleed air. The aircraft systems draw upon the electrical power consistently but typically do not exhaust the APU at the same rate as during engine start.

During aircraft takeoff, the APU's role is generally diminished, as the main engines take over all necessary systems and functions, including power and environmental controls.

Thus, the role of the APU in supplying bleed air during engine starts represents the peak operational demand, making it critical to ensure that the APU is functioning effectively to meet these specific requirements.