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787 APU Running Inflight?  
User currently offlinezhutton From United Kingdom, joined Apr 2008, 44 posts, RR: 0
Posted (4 years 1 month 1 week 2 days 5 hours ago) and read 9043 times:

HI All,

Just looked at this fantastic photo of the 787 banking out of Farnbourgh and something caught my eye...

http://www.airliners.net/photo/Boein...Boeing-787-8-Dreamliner/1745844/L/

Correct me if im worong but is the APU intake open?, would this mean that the APU is running? is this allowed in flight? Are there any safety implications?

Thanks,

Zakk


A Landing you can walk away from is a good one ! - www.longmynd.com
34 replies: All unread, showing first 25:
 
User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 1, posted (4 years 1 month 1 week 2 days 5 hours ago) and read 8976 times:

It really is not that odd at all: many planes can start their APU in flight and will regularly do so to have more redundancy for approach and landing.

If I am not mistaken, the 747 can't start its APU in flight, but that's a quad, so redundancy is ample from its engines alone.

I should think that a twin, and especially the 787 with its electrical architecture is heavily dependant on the use of the APU under certain conditions, as an engine failure would otherwise leave it with just a single generator to feed all of its systems: not really a much wanted situation to be in.


User currently offlinerobffm2 From Germany, joined Dec 2006, 1117 posts, RR: 0
Reply 2, posted (4 years 1 month 1 week 2 days 5 hours ago) and read 8873 times:

On the left hand engine, what is that little exhaust pipe (?) and the smoke coming out of it?

User currently offlineoly720man From United Kingdom, joined May 2004, 6702 posts, RR: 11
Reply 3, posted (4 years 1 month 1 week 2 days 3 hours ago) and read 8634 times:

Quoting robffm2 (Reply 2):
On the left hand engine, what is that little exhaust pipe (?) and the smoke coming out of it?

It's an oil breather vent, afaik.

Something along the lines of

Smoke Out Of 747 Classic Engines? (by Medinaj Oct 14 2004 in Civil Aviation)



wheat and dairy can screw up your brain
User currently offlineflybaurlax From United States of America, joined Oct 2008, 638 posts, RR: 0
Reply 4, posted (4 years 1 month 1 week 2 days 2 hours ago) and read 8594 times:

Quoting zhutton (Thread starter):
is this allowed in flight?

I know for AS ETOPS cert that it is required for them to have the APU running for the majority of the flight. I forgot what the boundary lines are called, but basically as they head out over the ocean away from land they need to have the APU running until they're within a certain distance from land.



Boilerup! Go Purdue!
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (4 years 1 month 1 week 2 days 2 hours ago) and read 8571 times:

Quoting slz396 (Reply 1):
I should think that a twin, and especially the 787 with its electrical architecture is heavily dependant on the use of the APU under certain conditions, as an engine failure would otherwise leave it with just a single generator to feed all of its systems:

Each engine (and the APU) has two generators. Loss of an engine still leaves you with two (plus the APU once you start it).

Quoting robffm2 (Reply 2):
On the left hand engine, what is that little exhaust pipe (?) and the smoke coming out of it?

Oil vent. The core exhaust cones on this engine are closed; a lot of other engines vent this out the back of the core nozzle.

Quoting zhutton (Thread starter):
Correct me if im worong but is the APU intake open?

Yes.

Quoting zhutton (Thread starter):
would this mean that the APU is running?

Yes.

Quoting zhutton (Thread starter):
is this allowed in flight?

Yes. Encouraged even, and required in some cases.

Quoting zhutton (Thread starter):
Are there any safety implications?

It's a little safer, since you have more electrical capacity available with the APU running. However, the system is designed to transition seamlessly through power transfers, so it's not a particularly huge deal.

Tom.


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 6, posted (4 years 1 month 1 week 1 day 23 hours ago) and read 8469 times:

Quoting slz396 (Reply 1):
If I am not mistaken, the 747 can't start its APU in flight

Not sure about the 747 APU but I would think that it's just a matter of altitude.
No APU is going to start at 35.000 ft, like it is also very unlike to get an engine re-started at that altitude.



The truth lives in one’s mind, it doesn’t really exist
User currently offlinezeke From Hong Kong, joined Dec 2006, 9029 posts, RR: 75
Reply 7, posted (4 years 1 month 1 week 1 day 23 hours ago) and read 8457 times:

Quoting slz396 (Reply 1):

I should think that a twin, and especially the 787 with its electrical architecture is heavily dependant on the use of the APU under certain conditions, as an engine failure would otherwise leave it with just a single generator to feed all of its systems: not really a much wanted situation to be in.

This maybe a performance trick for the 787, unload the engine generators by using the APU generators to give better takeoff performance.

Quoting Aviopic (Reply 6):
No APU is going to start at 35.000 ft

The A330/A340 APU can be started in lfight above FL410.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 8, posted (4 years 1 month 1 week 1 day 23 hours ago) and read 8448 times:

Quoting zeke (Reply 7):
The A330/A340 APU can be started in lfight above FL410.

Yes, most APU's I know can be started at any altitude but they just won't run due to the lack of oxygen.
I remember a KL 737 Captain telling me that the his operations manual said not even to try above 18.000 ft.
Whether this is a Boeing or just KL procedure I do not know.

I am not familiar with A330/40 systems.
Does the A330/340 have a special feature to make the APU run at that altitude ?



The truth lives in one’s mind, it doesn’t really exist
User currently offlinezeke From Hong Kong, joined Dec 2006, 9029 posts, RR: 75
Reply 9, posted (4 years 1 month 1 week 1 day 23 hours ago) and read 8435 times:

Quoting Aviopic (Reply 8):

Yes, most APU's I know can be started at any altitude but they just won't run due to the lack of oxygen.

Incorrect it will run at FL410, but will only provide electrical power at that altitude.

Quoting Aviopic (Reply 8):
Does the A330/340 have a special feature to make the APU run at that altitude ?

No, all Airbus aircraft as far as I am aware have APUs that will start and run to the maximum certified level for the airframe.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 10, posted (4 years 1 month 1 week 1 day 22 hours ago) and read 8407 times:

Quoting tdscanuck (Reply 5):
Each engine (and the APU) has two generators. Loss of an engine still leaves you with two

Interesting.

Do you happen to know if the full electrical system can run on a single GEN too?

Quoting tdscanuck (Reply 5):
Yes. Encouraged even, and required in some cases

Indeed, seems like a sensible thing to do to me, especially here....

Quoting zeke (Reply 7):
This maybe a performance trick for the 787, unload the engine generators by using the APU generators to give better takeoff performance.

Given the runway is quite short and the plane flew back to the USA, it could probably use every bit of thrust available, so a bleedless take off seems to be a good option. Whether air bleed or just electrical bleed, the tricks to get more out of the engines are still very much the same: shift (some of) their load to the APU.  

[Edited 2010-07-22 13:38:44]

User currently offlinekanban From United States of America, joined Jan 2008, 3497 posts, RR: 27
Reply 11, posted (4 years 1 month 1 week 1 day 22 hours ago) and read 8406 times:
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Quoting zhutton (Thread starter):
would this mean that the APU is running?



don't they need to have an airflow to cool it down after use and purge any flammable vapors? granted it wouldn't take very long to accomplish


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 12, posted (4 years 1 month 1 week 1 day 22 hours ago) and read 8397 times:

Quoting zeke (Reply 9):
No, all Airbus aircraft as far as I am aware have APUs that will start and run to the maximum certified level for the airframe.

Thanks but I am completely mystified.
Does it carry Hydrazine on board like an F.16 does, that would surely do the trick.
Can't image though.
http://en.wikipedia.org/wiki/Hydrazine

Hope some else know's how that works and were the oxygen is coming from.



The truth lives in one’s mind, it doesn’t really exist
User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 13, posted (4 years 1 month 1 week 1 day 22 hours ago) and read 8391 times:

Quoting Aviopic (Reply 12):
Hope some else know's how that works and were the oxygen is coming from.

How do the engines run at altitude you think?

An APU is just a small jet engine, burried in the tail of the aircraft, no need for hydrazine whatsoever!
.


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 14, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8386 times:

Quoting slz396 (Reply 13):
How do the engines run at altitude you think?

An APU is just a small jet engine, burried in the tail of the aircraft, no need for hydrazine whatsoever!

Sigh...........  
I know how they run, you've missed the point.
When started at a reasonable altitude they will keep running, that's not the problem.
The question is how to start at high altitude.............



The truth lives in one’s mind, it doesn’t really exist
User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 15, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8384 times:

Quoting Aviopic (Reply 6):
Not sure about the 747 APU but I would think that it's just a matter of altitude

I've looked it up, and the 747 APU is only certified for ground starts just as I thought it was and it may be left running till FL200 only.

Okay, the 747 is a quad, but still, that's quite a limited APU, don't you agree?


User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 16, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8381 times:

Quoting Aviopic (Reply 14):
The question is how to start at high altitude.............

It must surely be just me, but I still don't get the question really.

Why would there be a particular problem to start an APU at say fly FL250 or FL300?

It's just a matter of having sufficient air flow through the APU to help it reach its self-sustained speed.

Sure, air is thinner at altitude, but provided you have a well-shaped inlet which is sufficiently large to provide enough air volume per second, there's really no problem and definitely no need for tricks like hydrazine, god forbid!

I'd be scared to start the APU if that were to be the case!


User currently onlineTristarsteve From Sweden, joined Nov 2005, 4000 posts, RR: 34
Reply 17, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8376 times:

Quoting Aviopic (Reply 8):
I remember a KL 737 Captain telling me that the his operations manual said not even to try above 18.000 ft.

The B737 was not designed at the start as an ETOPS aircraft. The APU was not designed to start above 18000ft.
Aircraft that are designed for ETOPS have APUs that will start at cruise altitude. Part of ETOPS airworthiness is to check that this actually works every few months.
But most APUs will only provide electrical power at altitude. The air bleed closes above around 25000 ft depending on aircraft.

Quoting Aviopic (Reply 6):
Not sure about the 747 APU but I would think that it's just a matter of altitude

No its the design. The electrical interlocks stop B744 APU starting in flight, and if you take off with it running, it will shut itself down at quite a low altitude (14000ft?).
Older B747s sometimes had APUs you could start in flight, and some Flight Engineers knew how to do it, but it was not normal.

Quoting kanban (Reply 11):
don't they need to have an airflow to cool it down after use and purge any flammable vapors?

Yes, it takes about two minutes after you select APU off, until the door closes.

Quoting Aviopic (Reply 12):
Thanks but I am completely mystified.

I hope you are not any longer. If Concorde engines could operate at 58000ft without hydrazine (or anything else except ambient air), I am sure you understand that an APU will operate happily at 41000ft, as long as it was designed to do so.

Quoting slz396 (Reply 1):
should think that a twin, and especially the 787 with its electrical architecture is heavily dependant on the use of the APU under certain conditions,

The B777 has two generators on each engine. One of them is small and only used with the loss of another generator.
The B777 can be despatched ETOPS with an inoperative APU.


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 18, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8363 times:

Quoting Tristarsteve (Reply 17):
I hope you are not any longer. If Concorde engines could operate at 58000ft without hydrazine (or anything else except ambient air), I am sure you understand that an APU will operate happily at 41000ft, as long as it was designed to do so.

Thanks.
I knew they would keep working at altitude just didn't know they could be started at high altitude.
Guess I've missed the ETOPS thingy.
Still I would like to know what the design differences are.



The truth lives in one’s mind, it doesn’t really exist
User currently offlineslz396 From , joined Dec 1969, posts, RR:
Reply 19, posted (4 years 1 month 1 week 1 day 21 hours ago) and read 8345 times:

Quoting Aviopic (Reply 18):
Still I would like to know what the design differences are.

You mean why some planes can start and operate APUs at altitude, whereas others can't?

it's redundancy.

Electricity is extremely important to have, and so you really don't want to be flying on just a single GEN.

Traditionally, there's 1 GEN per engine, so quads have 4, meaning in case of an engine failure, they loose just 1 GEN, so no big deal and no real need to start an APU to get its GEN online.

Twins however, if they have only 1 GEN per engine, are down to just 1 operative GEN, so here it is really important to get a second GEN online by starting the APU, which is why that possibility will have be made available.

Even if you have a twin with 2 GENs per engine, an engine failure will cut the number of GENs available in half and it is not even sure that you may not have been dispatched with one of those 2 already inop prior to the flight, so even those planes with increased redundancy will normally allow an APU start in cruise....

How the APUs on planes allowing in flight starts at cruise levels differenciate from those who don't?
Probably in the size and shape of their inlet, as well as the aerodynamic efficiency of the compressor of the APU, as it needs to cope with much thinner air. Nothing too dramatic though, it really isn't rocket science either to start an APU at FL380 or above.


User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 20, posted (4 years 1 month 1 week 1 day 20 hours ago) and read 8321 times:

Quoting slz396 (Reply 19):
Electricity is extremely important to have, and so you really don't want to be flying on just a single GEN.

Aircraft electrons have been my job for the last 30 years and still are.  
Quoting slz396 (Reply 19):
You mean why some planes can start and operate APUs at altitude, whereas others can't?

Not really.
I didn't know the B74 had a physical limitation for an APU start which the B73 doesn't have.
Still the B73 APU can't be started at high altitude.
From Steve I've learned this is due to ETOPS regulations which does make sense, redundancy is part of my daily life.
So suppose their is an ETOPS B73(no idea whether or not there is one) and there is one normal B73.
According the post's in this thread one APU will start at high altitude and the other one won't.
Now what would be the design difference to make the ETOPS version able to start at high altitude ?

Looking forward for the answer, tomorrow.
I am off   



The truth lives in one’s mind, it doesn’t really exist
User currently offlinekanban From United States of America, joined Jan 2008, 3497 posts, RR: 27
Reply 21, posted (4 years 1 month 1 week 1 day 20 hours ago) and read 8320 times:
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Quoting Tristarsteve (Reply 17):
Quoting kanban (Reply 11):
don't they need to have an airflow to cool it down after use and purge any flammable vapors?

Yes, it takes about two minutes after you select APU off, until the door closes.

So that's why the bloody door is open... problem solved.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (4 years 1 month 1 week 1 day 20 hours ago) and read 8315 times:

Quoting Aviopic (Reply 6):
No APU is going to start at 35.000 ft, like it is also very unlike to get an engine re-started at that altitude.

All ETOPS aircraft can start the APU up to maximum certified altitude. It's part of required testing.

Quoting zeke (Reply 7):
This maybe a performance trick for the 787, unload the engine generators by using the APU generators to give better takeoff performance.

It doesn't work...the engine is controlling on N1, which isn't impacted by electrical offload (on the N2 spool). The only time you'd get a limit from power offtake is if you hit the EGT limit, but it's not nearly that hot in Farnborough.

Quoting slz396 (Reply 10):
Do you happen to know if the full electrical system can run on a single GEN too?

Not the entire system, but everything you need will run on one generator. You'd start to lose non-essentials, like galleys, at some point.

Quoting slz396 (Reply 10):
Given the runway is quite short and the plane flew back to the USA

It stopped in Shannon, Ireland for fuel.

Quoting slz396 (Reply 10):
it could probably use every bit of thrust available, so a bleedless take off seems to be a good option. Whether air bleed or just electrical bleed, the tricks to get more out of the engines are still very much the same: shift (some of) their load to the APU

But offloading electrical doesn't impact the engine nearly as much as offloading bleed...that's part of the whole point of the more-electric architecture.

Quoting Aviopic (Reply 14):
The question is how to start at high altitude.............

It's hard...APU's have a much worse design space than big jets. But they do it, because they have to.

Tom.


User currently offlinezeke From Hong Kong, joined Dec 2006, 9029 posts, RR: 75
Reply 23, posted (4 years 1 month 1 week 1 day 18 hours ago) and read 8276 times:

Quoting tdscanuck (Reply 22):

It doesn't work...the engine is controlling on N1, which isn't impacted by electrical offload (on the N2 spool). The only time you'd get a limit from power offtake is if you hit the EGT limit, but it's not nearly that hot in Farnborough.

Thr Trent would be a triple spool engine with EPR control. Offloading a shaft would improve EGT margin. Does not need to be hot for people who want to show off.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineAviopic From Netherlands, joined Mar 2004, 2681 posts, RR: 42
Reply 24, posted (4 years 1 month 1 week 1 day 13 hours ago) and read 8198 times:

Quoting tdscanuck (Reply 22):
All ETOPS aircraft can start the APU up to maximum certified altitude. It's part of required testing.

Yes, that's what Steve said as well.
And I believe all of you.

Quoting tdscanuck (Reply 22):
It's hard...APU's have a much worse design space than big jets. But they do it, because they have to.

Haha, I am trying to learn something here.
The question was "how" ?
What is the technical difference between an APU designed for ETOPS use and one that isn't.



The truth lives in one’s mind, it doesn’t really exist
25 Post contains images slz396 : You lean something new every day here! Careful, ETOPS is involved, but you're about to turn cause and event upside down, as the A340 can start its AP
26 slz396 : It needn't be hot at all to hit an EGT limit. Remember Farnborough has a very short runway, so how big the chance they didn't flex on take-off but ra
27 tdscanuck : Usually true, but when you're talking about a FADEC engine certified to produce full rated thrust at a temperature well above ambient on that particu
28 Viscount724 : Not sure about Europe, but US FAA ETOPS rules have covered 3 and 4 engine aircraft since 2007, with different conditions in many cases, but they're c
29 Starlionblue : I thought hot coffee was essential?
30 tdscanuck : "No food...no data, no coffee...no way!" Tom.
31 HAWK21M : APUs are restricted by Pneumatic supply by Altitude limits not Electrical supply. regds MEL.
32 DocLightning : Then why have a door for the intake like that? That door is going to be a major hit to the aerodynamics of the aircraft, sticking out there like the
33 Starlionblue : Different speeds require different airflows?
34 tdscanuck : AS flies 737's...737's have a NACA inlet for the APU. When the APU is running, it's sucking air in so the drag isn't as bad as it may first appear. B
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