The APU will start and operate at altitudes within which range?

• A. -1,000 to 20,000 ft
• B. 0 to 15,000 ft
• C. -5,000 to 25,000 ft
• D. 1,000 to 30,000 ft

Answer: (A)

APU operation is tied to the ambient air conditions it can reliably breathe and compress. The altitude range it’s certified to start and run defines its operating envelope: at lower altitudes there’s ample air density to start the turbine and deliver the needed bleed air and electrical power, even slightly below sea level. As you climb, the air becomes thinner, and beyond a certain point the compressor can’t maintain the required mass flow and pressure, so reliable starting and sustained operation aren’t guaranteed. That’s why the permissible range tops out around 20,000 ft. Values outside this envelope could lead to inability to start or keep the APU running properly.