The maximum altitude for starting the APU is __. It can be operated up to __.

• A. 37,000 ft / 41,000 ft
• B. 35,000 ft / 39,000 ft
• C. 40,000 ft / 45,000 ft
• D. 32,000 ft / 38,000 ft

Answer: (A)

APUs have different limits for starting versus running. Starting requires enough air density to let the pneumatic starter spin up the turbine and build the bleed-air pressure needed to bring the engines or systems online. As you climb, the air gets thinner, so there’s a practical ceiling beyond which the starter can’t achieve the necessary speed, which is why starting is limited to a lower altitude.

Once the APU is up and running, it simply has to sustain operation within its design envelope. The systems can function with the thinner air and the temperatures it encounters up to a higher altitude, so the operating ceiling is higher. That’s why the maximum altitude to operate is higher than the maximum altitude to start.

Thus, you can start the APU up to 37,000 ft, and once started, it can operate up to 41,000 ft.