|Quoting HAWK21M (Reply 8):|
Ground power unit to supply 115vAC 400Hz to the Aircraft on Ground with APU shutdown.
Exactly. If you have a look at the plug itself, it has four heavy duty prongs that provide the 115V 400Hz three phase power to the aircraft. There are also two smaller prongs that are part of the aircraft's 28V DC system as well. The 28V DC prongs are shorter than the four AC
prongs for safety reasons. The relay that activates the ground power will not function unless the 28V DC prongs are fully engaged in the plug. That means that the AC
prongs will also have to be fully engaged as well. This stops ground power from being activated from the flight deck with the plug only partially engaged preventing shorts and possible injury or death to ground staff who are connecting the aircraft up! This can make it quite difficult on the 737 if the battery is flat. No 28V DC means no ground power! Time for a battery change before power can be applied.
On the 737, the access panel that houses the ground power socket also has two headset jacks. One is the flight interphone used to speak to the crew on a turnaround or provide a method of communication from the tug to the flight deck during towing operations. The other is the service interphone used in conjunction with the other interphone locations on the aircraft for maintenance purposes. In the access panel, there are also two lights. On our 737's, one white light shines when ground power is NOT in use so we know it is safe to disconnect. The other light is red. On when the brakes are set, off when they're not. Handy for a turnaround so you can visually confirm the brakes have been released before commencing a pushback.