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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?


Ed Wilson

Mindstar Aviation
My Playland - I69

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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!


LORBY-SI

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