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

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.

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.