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A330 single pack ops with APU = fuel saving?

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A330 single pack ops with APU = fuel saving?

Old 6th Nov 2008, 07:14
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A330 single pack ops with APU = fuel saving?

Morning all

The company I work for has recently instituted a new fuel-saving SOP on the A330, requiring us to switch off either pack 1 or 2 when we are on ground with APU bleed running.

Now as we all know, the system design is such that pack flow is set to "high" when APU bleed is on, regardless of what pack flow selection is made. Surely this means that once you've switched APU bleed on, deselecting one pack isn't going to affect the fuel flow at the APU itself?

Furthermore I have watched the APU page on start and as I carry out the procedure. There's no APU fuel flow meter of course, but you can clearly see a large APU EGT rise when the APU is running for electrical generation only, and then you select bleed on. Once you've done that, then any further selection of one / two / or zero packs on, doesnt cause APU EGT to budge by even a single degree.

So, is it a fuel saving or not, and if it is, can someone please explain to me how and why?
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Old 6th Nov 2008, 08:54
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Ajax, on the 717 I fly, engineer advice is that only one pack is better on the ground, because APU airflow isn't sufficient to run two packs at sufficient speed to be as efficient as they can be; internal labyrinth seals not sealing properly.

Assuming there's some commonality in system operation between the types, possibly its a matter of cooling/heating efficiency as opposed to straight fuel usage.
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Old 6th Nov 2008, 10:26
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According to a fuel conservation simpsium,

selecting one pack off on ground will reduce fuel consumption for some airplanes and increase for others.

We use the same technique for the A320. In the A320 it reduces EGT about 60 to 80C. That`s a clear sign of reduced fuel flow, so its OK.
If EGT doesn`t change, I think its worthless.

Cheers
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Old 6th Nov 2008, 11:03
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Stupid SOP's

Concerning A330,its the most stupid thing to do.

You are just stressing one pack to work hard to keep the air at 22 degrees to fill the balloom. No change at all in the APU EGT.

But if you want to destroy the pack keep doing it. Its stupid rule made by idiots who sit in a desk and don't look out.

In my opinion using flap 3 for landing together with rev idle its another stupid thing concerning safety and the brake wear when you are close to MLW either on the -200 or -300. The company i used to work loves to do stupid idiot rules for us to follow. Another one is to taxi a widebody with only on engine runing.


They are the same that order boeing to install a brake fan for the B777-300ER,can u imagine that?For sure they receive a no no answer from boeing because it will cost a lot of millions to change the whole f. thing.
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Old 9th Nov 2008, 18:24
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As per my experience, no EGT decrement switching one pack off.
No apu fuel flow available on my screen nor alpha code.
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Old 9th Nov 2008, 19:29
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On the 737NG the APU fuel flow increases when operating only 1 pack compared to when both packs are running. Why i'm not quite sure.
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Old 9th Nov 2008, 22:22
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Made a particular note of EGT on the 717 APU the other day whilst parked on the bay. 1 pack and electrical load 77%. Second pack on saw the EGT rise to about 83% then slowly drop to 80%.

Now what fuel flow increase that involves I don't know.

As an aside, prior to engine start one needs to confirm that APU bleed is at least 27PSI. With only 1 pack on bleed usually runs at 30 to 37 psi, whereas with two on it can be as low as 20psi per side. The FCOM specifically says to check bleed with only one pack on, so generally, I operate on the ground with just the one pack.
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Old 10th Nov 2008, 05:24
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Hi All.

I am 100% sure, at those of my days with B 737-800, there was a maintenance page where APU fuel flow can be seen on ground and I had confirmed that 2 pack fuel flow is less than 1 pack fuel flow. And it was a reccomendation to use 2 packs for less fuel consumption.

KRUGERFALAP,

Probably we are flying at some company and I 100% agree with you. Most of the time I do think they are not reading what they write to SOP; there are so many illogical statements and discrepancies. They are sitting on the desk and writing the SOP as unaware of real operational environment. That is why SOP is changing so freqeuntly. Isn`t it funny this company is flying with Airbus so many years though, still not a sure what correct SOP is, and changing so often? The last I heard today, they changed the call "what to say before raising the gears after T/O".

Last edited by JABBARA; 10th Nov 2008 at 09:56.
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Old 10th Nov 2008, 10:01
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There are two basic designs of APU. You can either bleed air from the compressor, or have a separate load compressor.
The Garret 85 in old B737 DC9 etc had a bleed from the engine. In this type of APU the APU works harder the more you bleed off it. So no packs on is the lowest fuel useage.
The ST6 on the Tristar had a load compressor. This is a separate turbine and compressor bolted onto the engine. It spins all the time and produces air. If the APU bleed valve is closed then this air is dumped overboard through a dump valve. On the Tristar the lowest fuel flow is with the APU set to min bleed, and a couple of packs on. Turn the packs off and the APU works harder, because the air is being fed into a closed duct.

So look and see what design of APU you have.
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