During APU start on the ground, what is the purpose of the Avionics Master buttons?

During APU start on the ground, what is the purpose of the Avionics Master buttons?

• A. Disconnect navigation and communication equipment to prevent undesired voltage transients
• B. Enable full avionics operation on APU start
• C. Switch avionics to standby
• D. Isolate avionics from the bus

Answer: (A)

Voltage transients during ground APU start can couple into sensitive avionics and cause resets or damage. The Avionics Master switches are used to isolate the navigation and communication equipment from the electrical bus during that start sequence. By disconnecting the avionics from the bus, transients from the APU coming up won’t affect those systems, protecting them and preventing unwanted power-up or faults. Once the APU is stable and the bus is within limits, the Avionics Master is re-enabled to restore normal operation of the nav/com gear.

Voltage transients during ground APU start can couple into sensitive avionics and cause resets or damage. The Avionics Master switches are used to isolate the navigation and communication equipment from the electrical bus during that start sequence. By disconnecting the avionics from the bus, transients from the APU coming up won’t affect those systems, protecting them and preventing unwanted power-up or faults. Once the APU is stable and the bus is within limits, the Avionics Master is re-enabled to restore normal operation of the nav/com gear.