Which option represents the correct APU RPM thresholds for ground and flight to prevent start?

• A. Ground: 5%, Flight: 12%
• B. Ground: 6%, Flight: 12%
• C. Ground: 5%, Flight: 10%
• D. Ground: 4%, Flight: 12%

Answer: (A)

APU start prevention uses different minimum RPM values depending on whether the airplane is on the ground or in the air. The idea is to ensure the APU is spinning fast enough to start reliably and without causing issues, while also avoiding starts during flight when conditions are sensitive. A lower threshold on the ground allows quick, safe starts to provide ground power, whereas a higher threshold in flight helps prevent an unintended or unsafe APU start while the aircraft is in flight. This pairing reflects the safety and operational needs: easy ground starts and stricter control in the air. Other thresholds would push the ground-start timing too high or risk an in-flight start by being too low.