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cancidas
Topic Author
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APU Use Inflight

Wed May 05, 2004 12:14 pm


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i thought that APUs are shut down inflight. at least that's what the EMB-145 and CRJ-200 pilots i talk to tell me. not to say that you're wrong tim, i honestly don't know the truth as to DL's ops specs in terms of running the APU. i was under the impression that it need not be kept running once both engines are operational since the engines are able to keep all systems neccesary running. am i right or wrong? can anyone help?
"...cannot the kingdom of salvation take me home."
 
b752fanatic
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RE: APU Use Inflight

Wed May 05, 2004 12:36 pm

The APU is not used inflight, only in case of a power loss, in order to restore power and turning the engines back on again.

Some APU's have altitude restrictions, for turning them on.

"Truth is more of a stranger than fiction." Mark Twain
 
AJ
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RE: APU Use Inflight

Wed May 05, 2004 4:33 pm

APUs are used in flight for various reasons, including a u/s engine driven generator, ETOPS requirements (ie 737) or other maintenance procedure.

That being said the stream seen from the tail of this aircraft is most likely the air conditioning pack exhaust condensing and following the relative airflow out past the tail. I've noticed it following many aircraft types, including the 747.
 
RyDawg82
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RE: APU Use Inflight

Wed May 05, 2004 4:52 pm

Some airlines require that the APU be kept running on flights of a hour or less. The theory being why shut it off only to start it again shortly on the ground. Save the cycle and extend APU useful life. I'm not sure why Delta would keep it running on a 767-400 as I'm sure its not a short hop.

Can't say that and APU is not used inflight. It may not be supplying power to anything, however it can run and could be used....
-R
 
atomother
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RE: APU Use Inflight

Wed May 05, 2004 5:21 pm

I thought about that when I saw the contrail and after further inspection it seems that the trail is offset a bit to the right and not coming out of the exhaust of the APU. I am wondering if this is just condensation coming off of the APU's inlet? I also notice that there seems to be a fair amount of other vapor in the wake coming off the wings too.
 
wing
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RE: APU Use Inflight

Wed May 05, 2004 6:19 pm

First of all I have to say I am amazed with your careful observation.To me its just an airplane picture.  Big thumbs up

My previous company which operates 737's had a procedure to keep APU running during the flights less than an hour.Also we used to keep APU runing until 10000' after TO,turn it on again at 10000 during descent.

Keeping the APU running during short flights will increase the life of APU as well as they were leased not with working hour but with the number of start cycles.So keeping it running will save some costs too.

Although its not a widely used procedure keeping APU on during TO and landing is used for a substitude in case of an engine or generator failure during the most critical phase of the flight.
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SDFOH
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RE: APU Use Inflight

Wed May 05, 2004 8:39 pm

On two eng a/c there are three generators. In the event of one of the engine gens being out the apu is run for the backup generator. On CRJs there MUST be at least 2 working gens at all times. If 2 generators go out in flt then the plane will divert to the nearest airport until it can be fixed.
 
XFSUgimpLB41X
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RE: APU Use Inflight

Thu May 06, 2004 12:52 am

Sdfoh- we still shut down the APU before takeoff and dont turn it back on unless we have to.
Chicks dig winglets.
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: APU Use Inflight

Thu May 06, 2004 1:10 am

COuld be on for many reasons. Maybe just an APU Inflight Start Verification.
 
atomother
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Joined: Sat May 22, 1999 8:47 am

RE: APU Use Inflight

Thu May 06, 2004 2:00 am

Oh yeah, I forgot to add that I am pretty sure this bird was doing a SLC to ATL run. Maybe even to MCO? Thats a pretty long flight.
 
greasespot
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RE: APU Use Inflight

Thu May 06, 2004 2:18 am

On the 727 the apu cannot be used in flight. On the 737 if the CSD is disconnected and the generator is therefore inop then the APU is used the entire flight to provide power...

GS
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
A/c train
Posts: 674
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RE: APU Use Inflight

Thu May 06, 2004 7:49 pm

I think APU has to be available for Autoland status aswell. CAT 3 i believe.
 
buckfifty
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RE: APU Use Inflight

Fri May 07, 2004 3:12 am

I think AJ gives a plausible explanation. Could be the exhaust from the packs themselves (they do run quite hot).
 
JBirdAV8r
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Joined: Sun Jun 24, 2001 4:44 am

RE: APU Use Inflight

Fri May 07, 2004 12:51 pm

Due to the position of the contrail relative to the aircraft, I believe it is, in fact, pack exhaust.
I got my head checked--by a jumbo jet
 
AAR90
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Joined: Fri Jan 21, 2000 11:51 am

RE: APU Use Inflight

Fri May 07, 2004 3:00 pm

I think APU has to be available for Autoland status aswell. CAT 3 i believe.

Depends entirely upon the aircraft, the airline and its authorized configurations. No AA aircraft requires APU use in flight during any normal operation... including CAT-III approaches.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: APU Use Inflight

Tue May 11, 2004 3:24 am

On the F28s, sometimes we'd leave them running for fear that they wouldn't start at an outbase. That would get it through the day until we could fix it that night.
 
dl757md
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Joined: Mon May 24, 2004 9:32 am

RE: APU Use Inflight

Mon May 31, 2004 5:52 pm

Could be the packs as there is a similar less noticeable trail on the other side of the tail. Could it possibly be water from a galley or lav drain? Not sure.
757 Most beautiful airliner in the sky!
 
SpeedbirdHeavy
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Joined: Wed Jan 14, 2004 3:39 pm

RE: APU Use Inflight

Thu Jun 03, 2004 2:20 am

I thought there would be enough bleed from one engine to re-start another in the air. If the running engine is at cruise speed, isn't that enough to do an in air cross-bleed start?

Or does the APU have to be switched on to do an engine restart?
China Airlines...Come fry with us!
 
av757
Posts: 603
Joined: Wed Apr 14, 2004 5:49 am

RE: APU Use Inflight

Thu Jun 03, 2004 5:13 am

On the Boeing 757/767 the APU is primarily used for ground operations for electrical power generation and to supply bleed air for air conditioning packs and engine start operations; in flight APU bleed air is available up to approximately 17.000ft.

It is also certified and designed to operate continuously, and to be started in flight up to the maximum operating ceiling of the aircraft to provide electrical power as a back up in case of an engine failure or an engine driven generator.

Maximum operating ceiling:

Boeing 757 42.000ft.
Boeing 767 43.100ft.

You may dispatch the aircraft for ETOPS 120 or less with an inoperative engine driven generator using the APU generator continuously in flight, and the APU is mandatory for dispatch and must be operative for ETOPS 180 or more, to serve as a backup power source for an engine failure or an engine driven generator failure in flight.

You do not need the APU for a crossbleed start inflight, the operating engine has enough bleed capacity for this case inflight.

Regards:
AV757
 
fly727
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Joined: Tue Jul 29, 2003 8:27 am

RE: APU Use Inflight

Mon Jun 07, 2004 2:47 am

I thought there would be enough bleed from one engine to re-start another in the air. If the running engine is at cruise speed, isn't that enough to do an in air cross-bleed start?

Good point.

At least on the 727 which I fly, you have enough flow to do a cross-bleed start. Some aircraft allows the APU to be operating while flying; on the 727 due to desing it can't be used in the air (lack of cooling air).

RM  Smile
There are no stupid questions... just stupid people!

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