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Today's Pop Quiz - 10/31/02  
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Posted (11 years 12 months 1 day 20 hours ago) and read 1305 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??
=================================



14 replies: All unread, jump to last
 
User currently offlineSudden From Sweden, joined Jul 2001, 4130 posts, RR: 6
Reply 1, posted (11 years 12 months 1 day 20 hours ago) and read 1297 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.  Smile



When in doubt, flat out!
User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 2, posted (11 years 12 months 1 day 19 hours ago) and read 1289 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.

'Speed


User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 3, posted (11 years 12 months 1 day 19 hours ago) and read 1279 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.."
User currently offlineMandala499 From Indonesia, joined Aug 2001, 6927 posts, RR: 76
Reply 4, posted (11 years 12 months 1 day 17 hours ago) and read 1254 times:

the airport had a blackout ! LOL

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineAJ From Australia, joined Nov 1999, 2395 posts, RR: 24
Reply 5, posted (11 years 12 months 1 day 16 hours ago) and read 1245 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?

User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 6, posted (11 years 12 months 1 day 15 hours ago) and read 1234 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.


User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 7, posted (11 years 12 months 1 day 13 hours ago) and read 1219 times:

So when the external power is hooked to the aircraft, both the ON and AVAIL lights come on?

User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 8, posted (11 years 12 months 1 day 12 hours ago) and read 1201 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.

'Speed



User currently offlineNormalSpeed From , joined Dec 1969, posts, RR:
Reply 9, posted (11 years 12 months 1 day 12 hours ago) and read 1199 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?"

But isn't that what I said?

'Speed


User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 10, posted (11 years 12 months 1 day 12 hours ago) and read 1195 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.



User currently offlineSudden From Sweden, joined Jul 2001, 4130 posts, RR: 6
Reply 11, posted (11 years 12 months 1 day 2 hours ago) and read 1162 times:

So I was correct, sort of.


When in doubt, flat out!
User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 12, posted (11 years 12 months 18 hours ago) and read 1130 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.."
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 13, posted (11 years 12 months 18 hours ago) and read 1125 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
 Wink/being sarcastic


User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 14, posted (11 years 12 months 18 hours ago) and read 1124 times:

I'll let someone else try that one.
 Big grin



"Eagles soar. But weasels never get sucked into jet intakes.."
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