When does the APU Line Contactor close?

When does the APU Line Contactor close?

• A. At 90% RPM
• B. Immediately on start
• C. At 95% RPM
• D. At 95% RPM plus 7 seconds

Answer: (D)

The APU Line Contactor closes only after the APU is ready to supply stable electrical power and won’t connect to the electrical bus during start. It waits for the APU to reach 95% RPM, which shows the unit has reached near steady-state operation and can deliver proper voltage and frequency. After that, a 7-second delay allows transient effects to settle and confirms the power is stable before connection. When both conditions are met, the contactor closes to tie the APU to the aircraft buses. If the APU doesn’t reach 95% RPM or doesn’t hold for the 7-second window, the contactor stays open to protect the electrical system.

The APU Line Contactor closes only after the APU is ready to supply stable electrical power and won’t connect to the electrical bus during start. It waits for the APU to reach 95% RPM, which shows the unit has reached near steady-state operation and can deliver proper voltage and frequency. After that, a 7-second delay allows transient effects to settle and confirms the power is stable before connection. When both conditions are met, the contactor closes to tie the APU to the aircraft buses. If the APU doesn’t reach 95% RPM or doesn’t hold for the 7-second window, the contactor stays open to protect the electrical system.