The APU will not start if the APU RPM exceeds certain speeds on the ground and in flight. What are these limits?

• A. 5% on the ground; 12% in the air
• B. 12% on the ground; 5% in the air
• C. 0% on both
• D. 8% on ground; 10% in air

Answer: (A)

APU start sequencing uses how fast the APU is already turning to decide if a start is appropriate. If the APU RPM is above a small threshold, starting the APU would fight against it, which isn’t safe or effective. On the ground, that threshold is 5% of maximum RPM, because even light windmilling or external drive can spin the APU to a few percent and starting it in that condition isn’t recommended. In flight, the threshold is higher—12%—since the aerodynamics and system interactions in the air make a higher pre-start speed acceptable before starting would be attempted. So the limits are 5% on the ground and 12% in the air.