Hang on a minute...
So when the external power is hooked to the aircraft both the ON and AVAIL lights come on?
No. The AVAIL light illuminates when external power is connected to the external power receptacle.
Momentary action on the EXT PWR switch turns on the system. When this action occurs, the Bus Power Control Unit (BPCU) closes the External Power Contactor(EPC). The ground for the external power ON light comes directly from the EPC when that contactor is closed. The ON legend will illuminate. The BPCU also trips the Auxiliary Power Breaker (APB), removing the APU generator from the buses.
The scenario that started this discussion:
You've just pulled into the gate after taxi. The APU was started a couple of minutes ago. and APU Gen was selected on. You shut down both engines. The ground crew then hooks up external power(the ON and AVAIL lights are illuminated on the switch).As soon as you see the lights, you take the APU switch to the OFF position. 90 seconds later, all AC power on the aircraft is lost. What went wrong?
1. When the AVAIL and ON legends are illuminated, external power is supplying the aircraft buses. The APU generator may be producing power, but it is not supplying any buses because the APB is tripped.
2. Unless something else is wrong, somebody must push the EXT PWR switch to get the ON legend illuminated.
3. The time delay for APU shutdown is 60 seconds. If the APU speed is above %85 for 20 seconds more than that, it will shutdown using the protective auto shutdown.
The above scenario should not occur as written.
"Eagles soar. But weasels never get sucked into jet intakes.."