zhutton
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787 APU Running Inflight?

Thu Jul 22, 2010 1:02 pm

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
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slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 1:15 pm

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.
 
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robffm2
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 1:29 pm

On the left hand engine, what is that little exhaust pipe (?) and the smoke coming out of it?
 
oly720man
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 3:20 pm

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)
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flybaurlax
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 4:08 pm

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.
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tdscanuck
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 4:45 pm

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.
 
aviopic
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 7:04 pm

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.
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zeke
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 7:26 pm

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.
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aviopic
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 7:44 pm

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 ?
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zeke
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 7:50 pm

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.
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slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:36 pm

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]
 
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kanban
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:39 pm

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
 
aviopic
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:44 pm

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.
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slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:49 pm

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!
.
 
aviopic
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:55 pm

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.............
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slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 8:56 pm

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?
 
slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 9:03 pm

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!
 
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 9:06 pm

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.
 
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 9:19 pm

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.
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slz396
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 9:35 pm

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.
 
aviopic
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 10:24 pm

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.
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kanban
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 10:24 pm

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.
 
tdscanuck
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RE: 787 APU Running Inflight?

Thu Jul 22, 2010 10:38 pm

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.
 
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zeke
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 12:14 am

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.
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aviopic
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 5:42 am

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.
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slz396
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 6:55 am

Quoting Aviopic (Reply 20):
I didn't know the B74 had a physical limitation for an APU start.

You lean something new every day here!  
Quoting Aviopic (Reply 20):
From Steve I've learned this is due to ETOPS regulations

Careful, ETOPS is involved, but you're about to turn cause and event upside down, as the A340 can start its APU in cruise as well and as we all know, that a quad and thus freed of all ETOPS regulations...

Let's just say that if you have a twin and operating long haul routes (thus logically operate under ETOPS), you'll certainly have an APU that can be started, that's a fact, BUT if you don't have an ETOPS plane, it all depends on how good your APU is: like you say, the 737s APU isn't much to write home about hence it being limited in starting envelope, whereas the A340 can start it all throughout the flight, even in cruise, notwithstanding the fact ETOPS is of no factor here....

Quoting Aviopic (Reply 24):
What is the technical difference between an APU designed for ETOPS use and one that isn't

the difference between an APU able to start in cruise and one that isn't (BTW, get that ETOPS thing out of your head, it is not correct), is to be found in its ability to operate in thin air, which all comes down to its inlet and compressor stage.

The better that is built, the higher it can operate without surge, just like engines really....

Why have some engines seen increases in their max operating altitude in later versions?
Because fanblades and compressor have been aerodynamically improved...
Same on APUs really

Many manufacturers haven't bothered designing their APU so it is also able to be started in cruise for the simple reason they know they won't be used in flight anyway, whereas others know it might and thus have spent research and design money on it to improve the efficiency of the compressor stage... that's really all there is to it.
 
slz396
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 7:06 am

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.

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 rather used TOGA max?

If you max, you can hit EGT limits rapidly and it is considered a good operating practice to shed all possible load of the engines, regardless what engine that may be.

BTW, you're not saying N1 is not linked to N2 now here, are you, because for a second, that's how it sounded.
It that were to be the case, theoretically, it would be possible to design a superefficient engine with a high N1 (and thus high thrust) and an idle N2 (thus low fuel consumption): sadly, that obviously isn't possible, so saying N1 is not impacted by changes on N2 is obviously not fully correct.

Quoting zeke (Reply 23):
Offloading a shaft would improve EGT margin

It definitely would.

Just have a look at the EGT decrease when you switch a GEN off on an engine in TOGA max...

And the 787 is draining a lot of energy from its engines, due to its predominantly electrical concept, so doing that on one of its engines will definitely give you a considerable better EGT margin, which in turn allows you to get more from the engine running in TOGA max... when you're taking off from a short runway and especially if you're trying to impress with a short powerful take-off, that's what you're after.

[Edited 2010-07-23 00:14:36]
 
tdscanuck
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 11:11 pm

Quoting slz396 (Reply 26):
It needn't be hot at all to hit an EGT limit.

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 particular day, you won't be thrust limited by EGT.

Quoting slz396 (Reply 26):
If you max, you can hit EGT limits rapidly and it is considered a good operating practice to shed all possible load of the engines, regardless what engine that may be.

*If* you max...there was nothing in this case that would threaten even getting close to max. Reasonable ambient temperature, near sea-level altitude, nearly new engine = no EGT issues.

Quoting slz396 (Reply 26):
BTW, you're not saying N1 is not linked to N2 now here, are you, because for a second, that's how it sounded.

There's obviously a relationship, but they're not mechanically coupled. Since they're controlling on TPR (not N1, correction credit to Zeke), increasing power offtake from N2 has no direct impact on N1 (or TPR)...the engine will just increase fuel flow to get back to the target TPR. Thus you only have a thrust shortfall *if* you hit the EGT limit and can't add any more fuel.

Quoting slz396 (Reply 26):
And the 787 is draining a lot of energy from its engines, due to its predominantly electrical concep

It's actually draining a lot *less* energy from it's engines, due to it's predominantly electrical concept. Electrical offtake requires less power removal to start with, and is also a far more efficient way to extract power. The 787 isn't using any less total power at the end of the day, it's just pulling more electrical power and (much) less pneumatic power.

Quoting slz396 (Reply 26):
so doing that on one of its engines will definitely give you a considerable better EGT margin, which in turn allows you to get more from the engine running in TOGA max... when you're taking off from a short runway and especially if you're trying to impress with a short powerful take-off, that's what you're after.

Having more margin does nothing to give you more power...it's only when you run out of margin that you have to start cutting back. When you're taking off from a short runway and want to be impressive, you go for full thrust. The engine is certified to produce full thrust at full electrical output at a considerably higher ambient temperature than Farnborough...as a result, reducing electrical load by running the APU does nothing to increase available thrust.

Tom.
 
Viscount724
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RE: 787 APU Running Inflight?

Fri Jul 23, 2010 11:31 pm

Quoting slz396 (Reply 25):
the A340 can start its APU in cruise as well and as we all know, that a quad and thus freed of all ETOPS regulations...

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 certainly not freed of all ETOPS regulations. That's why (at least in the US) the T in ETOPS no longer stands for "Twin". ETOPS is now simply short for "Extended Operations".
 
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RE: 787 APU Running Inflight?

Sat Jul 24, 2010 12:00 am

Quoting tdscanuck (Reply 22):
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.

I thought hot coffee was essential?  Wow!
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tdscanuck
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RE: 787 APU Running Inflight?

Sat Jul 24, 2010 12:39 am

Quoting Starlionblue (Reply 29):
I thought hot coffee was essential?

"No food...no data, no coffee...no way!"

Tom.
 
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HAWK21M
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RE: 787 APU Running Inflight?

Sat Jul 24, 2010 10:40 am

APUs are restricted by Pneumatic supply by Altitude limits not Electrical supply.
regds
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DocLightning
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RE: 787 APU Running Inflight?

Tue Jul 27, 2010 11:44 pm

Quoting flybaurlax (Reply 4):

I know for AS ETOPS cert that it is required for them to have the APU running for the majority of the flight

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 scoop that it is.

If it were to be running the majority of the time that the aircraft is operating, then wouldn't you expect the intake to be built like the PACS intakes? Why would you want to include a door that can be closed (with all the extra weight and complexity of the machinery to do so) when it's essentially going to be open all the time?

Quoting slz396 (Reply 25):

Careful, ETOPS is involved, but you're about to turn cause and event upside down, as the A340 can start its APU in cruise as well and as we all know, that a quad and thus freed of all ETOPS regulations...

The A340 is essentially a quad A330. Airbus made some compromises on both airframes so that there was as much commonality between the two types as possible. Thus, if the A330 is going to be ETOPS compliant, then the A340 will have the same systems, even if they aren't technically necessary.

And before people start braying on about how ETOPS is really no longer the standard, it was at the time these aircraft were designed.
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RE: 787 APU Running Inflight?

Wed Jul 28, 2010 1:33 am

Quoting DocLightning (Reply 32):
If it were to be running the majority of the time that the aircraft is operating, then wouldn't you expect the intake to be built like the PACS intakes? Why would you want to include a door that can be closed (with all the extra weight and complexity of the machinery to do so) when it's essentially going to be open all the time?

Different speeds require different airflows?
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tdscanuck
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RE: 787 APU Running Inflight?

Wed Jul 28, 2010 5:33 am

Quoting DocLightning (Reply 32):
Quoting flybaurlax (Reply 4):

I know for AS ETOPS cert that it is required for them to have the APU running for the majority of the flight

Then why have a door for the intake like that?

AS flies 737's...737's have a NACA inlet for the APU.

Quoting DocLightning (Reply 32):
That door is going to be a major hit to the aerodynamics of the aircraft, sticking out there like the scoop that it is.

When the APU is running, it's sucking air in so the drag isn't as bad as it may first appear. But for aircraft built for ETOPS from the get-go, you're only supposed to need the APU in flight when you're on a single engine, at which point drag is the least of your worries.

Quoting DocLightning (Reply 32):
If it were to be running the majority of the time that the aircraft is operating, then wouldn't you expect the intake to be built like the PACS intakes?

They do, for aircraft that are expected to run it all the time.

Quoting DocLightning (Reply 32):
Why would you want to include a door that can be closed (with all the extra weight and complexity of the machinery to do so) when it's essentially going to be open all the time?

Because it's not going to be open all the time, and having a flush area is better for drag than having a perpetual NACA scoop that you're not using, like the 737 has.

Tom.

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