At what altitude is the APU maximum for ECS bleed operations?

The maximum altitude for Auxiliary Power Unit (APU) bleed operations in an aircraft is critical for ensuring proper engine performance and cabin pressurization. The correct altitude for APU bleed operations is 15,000 feet. This limitation is established primarily due to the decreasing efficiency of the APU at higher altitudes, which affects the bleed air's ability to meet the environmental control system (ECS) demands.

At this altitude, the APU can still supply adequate bleed air to maintain cabin conditions, support the environmental systems, and operate normally. Operating above this altitude could lead to insufficient air pressure and temperature control in the cabin, impacting passenger comfort and safety.

Understanding these limitations is crucial for flight crews to ensure compliance with operational procedures and to maintain safe flight operations. The other options provided either exceed or fall short of the operational limits as outlined for APU bleed usage during flight.