FAA Aviation Mechanic General (AMG)Ground Operations and ServicingEasy
A technician wants to conserve the aircraft battery while performing avionics checks and decides to use a ground power unit (GPU). What is the correct procedure for connecting the GPU to the aircraft?
- ATurn the GPU output switch ON first, then connect the cable to the aircraft receptacle
- BTurn on the aircraft battery first to verify GPU output voltage before connecting
- CConnect the cable to the aircraft receptacle with the GPU output switch OFF, then switch the GPU output ON
- DConnect the cable while the aircraft avionics are powered up to confirm proper voltage transfer
Show answer & explanationAnswer & explanation
Correct answer: C. Connect the cable to the aircraft receptacle with the GPU output switch OFF, then switch the GPU output ON
The GPU output switch must be OFF when the cable is connected to prevent an arc at the connector and a voltage transient to the aircraft; the output is switched on only after the connection is secure.
Why the other options are wrong
- A. Energizing the GPU before connection risks arcing and voltage spikes at the receptacle.
- B. The GPU voltage should be verified independently, not by energizing the aircraft battery first.
- D. Avionics should be off during any external power connection or disconnection to prevent damage.
GPU Connection Procedure
The sequence of steps used to safely connect and disconnect external ground power to an aircraft.
- Verify voltage/frequency compatibility first
- Output switch OFF before connecting cable
- Monitor gauges continuously during use
- Output switch OFF before disconnecting cable
Memory trick: Plug first, power second — never the reverse.