Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Posted (10 years 6 months 3 weeks 6 days 5 hours ago) and read 989 times:
B757
I'll try and think of something a little harder, but for now...
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??
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Sudden From Sweden, joined Jul 2001, 4127 posts, RR: 6 Reply 1, posted (10 years 6 months 3 weeks 6 days 5 hours ago) and read 981 times:
I would go for that he chould have switched it to ground fed. (or whatever it calls).
Am very not an expert on this, but this was the first that came to mind.
Dc10hound From United States of America, joined Dec 2000, 463 posts, RR: 6 Reply 3, posted (10 years 6 months 3 weeks 6 days 4 hours ago) and read 963 times:
Uh, you forgot to flip the switch to use the ground power. Just because the GND POWER AVAIL light is on, it doesn't mean that you are using it.
Yes, but Cdfmxtech said the ON light is illuminated....
The ON light means that the BPCU has closed the External Power Contactor.
"Eagles soar. But weasels never get sucked into jet intakes.."
AJ From Australia, joined Nov 1999, 2376 posts, RR: 27 Reply 5, posted (10 years 6 months 3 weeks 6 days 1 hour ago) and read 929 times:
Ground power must still be selected.... AVAIL means ground power is suitable for the aircraft, but not selected. Turning the APU switch off starts the shutdown cycle. When the APU gen starts to lose revs it trips off line, leaving you in darkness. Good try?
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Reply 6, posted (10 years 6 months 3 weeks 6 days ago) and read 918 times:
AJ - U nailed it.
I'm trying hard to think of B757 questions, but that damn airplane is so full of logic...it's hard to think of some without making it too hard.
It's also hard since so many carriers B757s differ so much.
But I'll keep 'em coming.
NormalSpeed From , joined Dec 1969, posts, RR: Reply 8, posted (10 years 6 months 3 weeks 5 days 21 hours ago) and read 885 times:
"The ON light means that the BPCU has closed the External Power Contactor."
Sorry, I only know 737-200 systems, and I misunderstood. On the 100/200, there is no GRD POWER ON light, Just a GRD PWR AVAIL light, and a GRD PWR off/on switch.
NormalSpeed From , joined Dec 1969, posts, RR: Reply 9, posted (10 years 6 months 3 weeks 5 days 21 hours ago) and read 883 times:
"Ground power must still be selected.... AVAIL means ground power is suitable for the aircraft, but not selected. Turning the APU switch off starts the shutdown cycle. When the APU gen starts to lose revs it trips off line, leaving you in darkness. Good try?"
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Reply 10, posted (10 years 6 months 3 weeks 5 days 21 hours ago) and read 879 times:
NormalSpeed
The B757 External Power Switch has 2 displays in it. The top portion says AVAIL and the bottom portion says ON.
When external power is hooked up to the aircraft and turned on (downstairs), but not switched on in the aircraft, the ON and AVAIL portions of the switch is illuminated. Once external power is switched on using this switch (momentray action tyoe switch), the AVAIL portion will go out.
This is totally different than the B737 external power switch. That lightstays illuminated regardless of whether power has been switched on or not.
Dc10hound From United States of America, joined Dec 2000, 463 posts, RR: 6 Reply 12, posted (10 years 6 months 3 weeks 5 days 3 hours ago) and read 814 times:
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.."
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 29 Reply 13, posted (10 years 6 months 3 weeks 5 days 3 hours ago) and read 809 times:
DC10hound
Damnn....I'm glad u caught that. I screwed that question up. Sorry 'bout that, I was going off the head. I had it flip-flopped. That changes the whole complexion of the question.
So here is the revision to the question:
If the above-mentioned scenario happens, which manual do you go to to figure out what is wrong, because this airplane is screwed up:
1). SRM
2). FIM
3). MEL
Dc10hound From United States of America, joined Dec 2000, 463 posts, RR: 6 Reply 14, posted (10 years 6 months 3 weeks 5 days 3 hours ago) and read 808 times:
I'll let someone else try that one.
"Eagles soar. But weasels never get sucked into jet intakes.."
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