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AF330

APU ON but engines OFF!

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Good evening,

 

I was in training today at CDG....

I saw an A380 Emirates. We could see that the APU was on (the air was coming out behind, kind of air "waves").

But the engines were not turning!!!! ;)

I asked my teacher why the engines were not turning and he said the starter was not started (or compressed air not going to the accessory gear box - AGB). But I think that there was not enough compressed air (N not at 100%)...

 

1) Is my "teacher" right? Because the APU can not work if the starter is not ON!

 

2) Where is the compressed air taken? Is it after the APU BLEED Valve or are there other pipes from the compressor to the AGB?

 

So If you start APU (+starter), you start automatically the flow of compressed air from compressor to AGB? Who was right in this story?

 

Thanks,

A.Roy

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The apu can run without the engine starter engaged.  The apu may just be running to provide air to the packs or electric power.  The bleed air from the apu was just going to the ac packs.  The apu bleed is open, after this valve, it goes to the the bleed system, through the isolation valve, and then to the packs.  Once you engage the starter, the packs automatically shut off, the air then goes to the engines to start.

 

Hope this helps,

1251452

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Hello!

 

The APU is electrically started. It will provide electrical power and peneumatic power to use as required. APU bleed may be used just for air conditioning (packs) until engines are started. there is a valve that controls if (and how much sometimes) air will be bled from the APU's compressor stages. Each engine has its own starter valve (& bleed valve). The pneumatic system works differently on each aircraft type, but as a general rule you need to close the pack valves to have all the air available for the engine pneumatic starters (all big enough jet engines are pneumatically started). Pilots will command the start valve of engine X to open only when needed. having it continuously fed by whatever is available in the pneumatic lines will be a trmendous source of wear and tear, and waste of pneumatic energy once the engine is running (the starter may now act as a compressor since it is now powered and may extract air out of the system ... not good).

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Thanks,

 

But to get the pneumatic and electrical power, you need to turn the compressor/turbine. To turn the turbine/compressor, you need the starter.

 

So how can you get bleed air with starter's off?

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Thanks,

 

But to get the pneumatic and electrical power, you need to turn the conpressor/turbine. To turn the turbine/compressor, you need the starter.

 

So how can you get bleed air with starter's off?

 

None of this makes sense. The APU is started electrically from ground source, it then provides it's own bleed air and electrical power to other components in the aircraft. What's difficult about this?

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Isn't this the teen that keeps asking how things do this or that for only Airbus aircraft?

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I'll add some words to your statement to make it correct:

 

 

 


But to get the pneumatic and electrical power from the engine, you need to turn the compressor/turbine. To turn the turbine/compressor, you need the starter.

 

The part I think you're missing is that the APU itself can provide bleed air and electrical power to the airplane, completely independent of any engines running.

 

It is completely normal to see an airplane sitting on the ramp with its APU running, but without any movement in the engines...

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I think we need to clarify one detail here:

 

The APU is an engine on itself!! The OP maybe thought the APU was a generator running on pneumatic engine bleed!

The APU starts its own combustion cycle via a small electric starter. It then happily runs by itself while powering a generator and giving off bleed air for pneumatics.

The main engines are too big for an eletric starter (will require a HUGE and ultra-heavy electric motor, requiring obnoxious quantities of electricity) therefore use a small pneumatic turbine. This pneumatic starter can be fed with bleed air from APU or an external hose. So:

1- Start APU (use batteries or external cart, but in you have the external cart it probably provides enough power and air not to need the APU...)

 

2 - use APU pneumatic power to start main engines

 

3 - shut down APU. (/or keep it ON during initial climb as backup power)

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The APU is not driven by the engines. The APU is a self-sustaining engine (NOT the thrust-providing kind), which delivers electricity, and bleed air, for use throughout the aircraft (Air Conditioning, Engine Start,...)

 


The main engines are too big for an eletric starter...

 

Just a fun little bit of trivia I learned earlier this week, not really related to the situation we're discussing here, but: The engines on the Avro RJ are actually started electrically. Granted, they're somewhat converted helicopter engines, and they're TINY when compared to engines used on wide body jets, but still interesting, I thought.

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Hi AF330,

 

every modern airliner has an additional jet engine inside - the APU. And this additional engine is what your picture shows, that is not one of the main engines under the wing! The APU gets started and runs independently of the main engines. It has its own generator too, so it can provide electrical power. And the APU produces compressed air (it is a jet engine after all, just very small). The APU bleed transports this air to the air conditioning packs, so they can operate on the ground even without ground connections! And you can start the main engines with it. Where the electrical power from the APU goes should be self evident.

 

The starter on the picture you ask about is simply the starter for the APU itself. The main engines have a similar layout, they are just much bigger.

 

Why? Safety and redundancy, but also think of the extremes:  Imagine an Airbus forced to land on an airport that does not provide ground power or air. You would be stuck without an APU providing the necessary electricity and compressed air to turn and start the main engines.

 

You were at the airport, surely you heared the whining noise of the APUs in the planes about to get pushed back??? That's an almost constant noise backdrop on a large airport!

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Yes, I agree with you.

 

But you need the APU to start. So there is an electric starter which will turn the APU's compressor/turbine.

When It will turn, the APU's compressor will give APU BLEED AIR. When the APU's turbine will turn, the generator will produce elec.

 

Do you agree?

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When It will turn...

When the APU runs, yes, it is indeed possible to use it as a bleed air source. Not when the starter runs. (Just making sure the "it" in your sentence is not confusing anybody.)

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You got it! APU turns fuel into electricity and bleed air!

- The APU is elecrtically started!

- Main engines are pneumatically started!

 

Jet aircraft without an APU (common, generally smaller bizjets) just have electrically started main engines (made exeption for a few ancient machines like the Canberra that ran the pneumatic starter with an explosive charge...)

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Hi AF,

 

You turn an engine to start it. After that it just runs. OK? Makes things less confusing to read.

 

Rephrased: You need the small electrical starter (battery power!) to turn the APU. When it runs, its compressor provides bleed air and the APUs own generator produces electrical power. Both are needed to turn and start the main engines. You can do this with ground power too, but as the APU has to be running until after takeoff in case something goes wrong with the generators, it runs anyway. The old airliners didn't have that option, a 707 needs ground power and air.

 

So I agree.

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Thanks,

 

So I come back to the A380:

 

The APU was ON. So APU's compressor/turbine turning but bleed air to engines (the big ones) not going.

 

So I would like to know when does compressed air go to the engines?

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When the pilot chooses to start said engine.

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AF330 in training? Yeah sure. In training for what? I'm a very patient person, for the most part, but, . . . . . . . . . .as multiple people have said,"here we go again", I get the feeling this person wants Astrophysics explained so as to work backwards to basic math. I have never been closer to a aircraft that had an APU than 50 feet. It is however very easy to figure out what, when, and why they do what they do and how it fits into the scheme of things. "Auxiliary" is kind of self explaining is it not? It is a ground power or battery started turbine used as a back up to provide electricity and compressed air on the ground and in emergency situations. Duh!!!! No I am not an expert but I did fly a simulator once! I do however admire the patience of everyone who keeps answering AF330. Apparently they have not seen the other posts or they wouldn't bother. I want to start my car, whats is this key for? Oh well, is there a face palm icon? Gee, it could not possibly be just to get attention could it? On the other hand, . . . . . . . . . I'm in training to fly a A330 myself, so, could someone kindly tell me what a A330 is and what it does and why?

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Here we go again...

Yep, episode #10

 

Isn't this the teen that keeps asking how things do this or that for only Airbus aircraft?

Yes

 

I do however admire the patience of everyone who keeps answering AF330.

As do I. At first I thought this guy was just trolling or doing this on purpose to get people ticked off. Frankly, I'm still not sure if this is the case or his interest is genuine.

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Just type: " stage en entreprise - 3e".....

----> Stage découverte.

You will be able to find the training we all have to do at this age.

 

I know what an APU is doing. But you are thinking that I am talking about the "big" engine.

 

Ok. WarpD: I am talking about APU's starter. Not the big engine's starter....

 

If you start the APU's starter, you will turn APU's compressor/turbine.

When does the APU's compressed air go to the engines'starter? When you want to start them.

 

You said that APU BLEED AIR valve should be OFF to supply air to the "big engine".

So when the pilots will press on APU (APU MASTER & APU), the starter will start, the electric pump and the isolation valve, the flap inlet will open and the comprssor/turbine will start turning and finally ignation will start.

So if APU BLEED AIR Valve is open, you will get A/C. If the APU GEN is ON, you will get Elec.

So now you want to start "big engines". What do you do now?

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Right, so your APU is now running in what you just described. Giving bleed air to the pneumatic system. The Main engine start valve is closed and the "big engines are stopped" The pilot opens the start valve and the air goes from the apu to the main engine starter spooling the HP system of the big engine for start!

 

Oh and NOT the APU bleed air thats off, just the Air conditioning valves.

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