What is the limitation for extraction of bleed air from the APU for engine start and environmental air?

The limitation for the extraction of bleed air from the Auxiliary Power Unit (APU) for engine start and environmental air is set at 20,000 feet above ground level (AGL). This altitude is critical because, at higher elevations, the atmospheric pressure decreases, which could affect the performance of the APU and the efficiency of the bleed air system.

When the aircraft ascends, the APU's ability to provide sufficient bleed air for engine start and environmental control diminishes due to the thinner air at higher altitudes. At 20,000 feet and below, the APU can effectively operate, supplying the necessary bleed air without overwhelming the system or jeopardizing safety and operational efficiency. Above this threshold, the performance limitations and risks grow, necessitating the restriction on bleed air extraction to ensure the aircraft's systems function optimally during critical phases of operation such as engine start and climate control within the cabin.