Hey guys, how are you doing.

I was wondering here, how does it works the APU start when a GPU is connected? Because by what I understand, the electrical power of the airplane is provided by or GPU or APU, so what is the right way to do that? Is the GPU disonnected first and then the APU is started or is the APU started before the GPU be turned off?

I know there will be no need to start the APU if there is a GPU connected for start the engines, but and when it is in the gate, where you can start the engine just after a secure range away from the gate or whatever it is for security reasons?

The question apply as well for when the airplane is arriving at the gate.

Well, that's, I am sorry if it was a bit confusing and a mess, but I am working hard for get by english better... Oh, and if that question is already answered, I am sorry have made the same question, I searched for something that could asnwer my question but I was unable to find something which helped... Well, thanks.

The normal sequence before departure for switching from one to the other is to have the aircraft on GPU power, start the APU, switch to APU power, then disconnect the GPU. On arrival, the aircraft will normally arrive in the gate on engine power with the APU off. The GPU is connected while the engines are running, GPU power is switched on, and the engines are switched off.

All these power transfers are done with the aircraft powered up. In the cabin you might notice the lights flicker as power switches from the GPU to the APU.

shamrock137, big thanks your reply... that's helped a lot.

shamrock137 wrote:

The normal sequence before departure for switching from one to the other is to have the aircraft on GPU power, start the APU, switch to APU power, then disconnect the GPU. On arrival, the aircraft will normally arrive in the gate on engine power with the APU off. The GPU is connected while the engines are running, GPU power is switched on, and the engines are switched off.

All these power transfers are done with the aircraft powered up. In the cabin you might notice the lights flicker as power switches from the GPU to the APU.

Depends on the airline, we fire the APU just after landing at the latest, most often in the air even, prior to landing.

DiamondFlyer wrote:

shamrock137 wrote:

The normal sequence before departure for switching from one to the other is to have the aircraft on GPU power, start the APU, switch to APU power, then disconnect the GPU. On arrival, the aircraft will normally arrive in the gate on engine power with the APU off. The GPU is connected while the engines are running, GPU power is switched on, and the engines are switched off.

All these power transfers are done with the aircraft powered up. In the cabin you might notice the lights flicker as power switches from the GPU to the APU.

Depends on the airline, we fire the APU just after landing at the latest, most often in the air even, prior to landing.

Very true, depends on the available facilities at the airport as well.

This depends upon aircraft type. Two things can be supplied from an APU: electrical power and air for pneumatics and air conditioning. Most aircraft electrical systems have built in priority logic that selects electrical source but generally only one can power the aircraft. When a new source is selected, the prior source generally drops off line. Since most airliners use pneumatic air to start engines, the APU is the most portable and usable source. So even if external electrical power and ground air conditioning sources are used at the gate, the APU is generally started prior to push back to allow ground sources to be disconnected and the APU generator used until an engine is started and APU air for starting and air conditioning until after engine start. For short turns at the gate, or where external sources are not available, APU might be used continuously from after landing until after the next start. Fuel can be conserved by using ground sources for longer ground times rather than running the APU which consumes, typically, 300 pounds per hour.

I am really thankfull by your answers guys, it helps a lot.

The APU is required for engine start, even if a GPU is connected. The GPU doesn't provide any air pressure to start the engines.

The norm for most big airlines and major types is to start engines during pushback using APU bleed air.

Also, to disagree slightly with someone above. The APU is normally started after landing. Many airlines these days like to taxy in with one engine shutdown, and use the APU generator to make up for the engine generator being switched off. Even on all engines the APU is started before parking so that when on stand the engines can be shutdown immediately without waiting for a GPU to be started or connected (or found). This also allows the air conditioning to be run from the APU now that the engines are off.

greg85 wrote:

The APU is required for engine start, even if a GPU is connected. The GPU doesn't provide any air pressure to start the engines.

The norm for most big airlines and major types is to start engines during pushback using APU bleed air.

Also, to disagree slightly with someone above. The APU is normally started after landing. Many airlines these days like to taxy in with one engine shutdown, and use the APU generator to make up for the engine generator being switched off. Even on all engines the APU is started before parking so that when on stand the engines can be shutdown immediately without waiting for a GPU to be started or connected (or found). This also allows the air conditioning to be run from the APU now that the engines are off.

You can't say the APU is started normally after landing. It really depends on the airline and their procedures and the type of aircraft.
In my airline it depends on the airport if we have to start the APU after landing or not. It depends on the airport because it is required to have the ground crew ready with the external power and the possibility to plug in the external power already before the jetty has to be moved. At most stations we can taxi-in without APU.
On the Airbus we can also taxi on one engine (or two engines on the A340) without the APU, so the one engine taxi procedure does not require the APU to be started either.

Wait? You taxy single engine with no APU? Not the A320??

greg85 wrote:

Wait? You taxy single engine with no APU? Not the A320??

Well, I had to check the A320 FCOM, my memories have faded a bit on that type. You're right, on the A320 the APU needs to be started for one engine taxi. But on the A330 you can taxi on one engine without APU.
On the A320 the requirement to start the APU seems to be maninly due to electrical transients (which are really nasty as far as I remember). On the A330 we have the NBPT (No Break Power Transfer) which equalises the frequencies of different electrical sources before connecting/disconnecting, which allows smooth electrical switchings.

I knew about the 4 engine airbus', but I'm still surprised about taxying on 1 generator on the 330. It's quite rare in aviation to have no redundancy. But the A320 is well known for its electrical transients.

greg85 wrote:

The APU is required for engine start, even if a GPU is connected. The GPU doesn't provide any air pressure to start the engines.

The norm for most big airlines and major types is to start engines during pushback using APU bleed air.

Also, to disagree slightly with someone above. The APU is normally started after landing. Many airlines these days like to taxy in with one engine shutdown, and use the APU generator to make up for the engine generator being switched off. Even on all engines the APU is started before parking so that when on stand the engines can be shutdown immediately without waiting for a GPU to be started or connected (or found). This also allows the air conditioning to be run from the APU now that the engines are off.

It really depends on the airline, and airport equipment. In my expierence if GPU power and PC air are availiable, the APU is never started after landing. The aircraft arrives at the gate and the engines are left running until the GPU is connected.

I worked at Stockholm ARN for British Airways.
The airport has a rule that APU will not be used on arrival.
British Airways has a rule that No one will approach the aircraft until the engines are off and the beacon switched off. If the APU is inop, there is a procedure to connect GPU, but only when APU inop. On the A319 the GPU connection is inside the danger zone in front of the engine. So BA made one rule for all and it is strictly policed. Couple up GPU with the engines running, and the pilot will raise an incident report.
The airport rule comes from the days when most aircraft at ARN were MD80. We try to get it changed, but just break it every day.

At my airline, it is SOP to always start the APU approaching the gate. We have no requirement to start the APU before shutting down an engine for RETI on the 777.

Out here.....The Apu is shut down at the gate, as some Airports require mandatory use of the GPU & Ground AC cart for Electrical & Pneumatics/Air conditioning......Environmental noise pollution reasons

Just to add...most aircraft types.....The GPU provides electrical power......The APU provides Electrical power & Pneumaic power upto a certain Altitude.

GPU can be used to start an APU or the APU can be started by the Aircraft battery.GPU cannot provide pneumatics where as APU can.
The APU has an electric starter hence needs electrical supply to start [GPU/Battery].

Tristarsteve wrote:

I worked at Stockholm ARN for British Airways.
The airport has a rule that APU will not be used on arrival.
British Airways has a rule that No one will approach the aircraft until the engines are off and the beacon switched off. If the APU is inop, there is a procedure to connect GPU, but only when APU inop. On the A319 the GPU connection is inside the danger zone in front of the engine. So BA made one rule for all and it is strictly policed. Couple up GPU with the engines running, and the pilot will raise an incident report.
The airport rule comes from the days when most aircraft at ARN were MD80. We try to get it changed, but just break it every day.

Hey TriStarSteve... why would an airport authority care if the APU were being used on arrival? That kinda sounds like a local authority trying to dictate the operating procedures of an airline! Better yet, how would they know?

FWIW, at my airline on the CRJ9, we have the GPU powering things, start the APU when needed, disconnect GPU, push, once both engines running, shut down APU.
On arrival, conditions depending, we start the apu when exiting the taxiway onto the ramp (apron) and typically leave it running until passengers are off the plane. Engines are shut down straight away once we set the parking break. SOmetimes the APU is left on during the whole turn. (My airline is making a big deal about apu cycles.)

Alias1024 wrote:

thefactorypilot wrote:

That kinda sounds like a local authority trying to dictate the operating procedures of an airline! Better yet, how would they know?

FWIW, at my airline on the CRJ9, we have the GPU powering things, start the APU when needed, disconnect GPU, push, once both engines running, shut down APU.
On arrival, conditions depending, we start the apu when exiting the taxiway onto the ramp (apron) and typically leave it running until passengers are off the plane. Engines are shut down straight away once we set the parking break. SOmetimes the APU is left on during the whole turn. (My airline is making a big deal about apu cycles.)

A great example of an airport authority dictating operating procedures is the fire marshall in DEN refusing to allow refueling of CRJ-200s with the APU running. The fire marshall has decided that the APU exhaust is too close to where the fuel truck must park.

I am curious about the procedures at your airline for APU usage though. When environmental conditions aren't extreme why not leave the APU down after landing until just a few minutes before pushback? The recirculation fan on the CRJ-900 does a decent job of keeping the cabin comfortable in moderate weather. Shut down the left engine some time during taxi in and the ground crew can get in to the cargo bins as soon as you park, and GPU power should be available within a minute. Beats running the APU for half an hour or adding another cycle (my airline has said it's $30 every time we push the start button on the APU).

My mentor, a TWA captain for 40 years, said it was a $10 bill every time you stood on the brakes in a 727...

Anyhow, I guess it could be interpreted as not needing the APU if it is say, 60 degrees F outside... But we are Delta Connection, and the rampies we typically utilize are less than interested. Therefore it is often many minutes before they get the bridge power hooked up. Company wants us to be able to cut off the main engines as soon as we hit the chocks, hence having the APU available for loading. I suspect it is more about the electrical requirements than environ.
Additional, A) Delta owns us as a full subsidiary, and B) Daddy Delta made a big push starting a year ago for us to run the APU WHENEVER cabin comfort was in question. We have a pretty small temperature window of when they say the APU might not be necessary. Otherwise they want it running.
Lastly, we don't get bridge comfort air hookups at most places... only our hubs, and even then only a portion of the time. Otherwise we have to use ships environ supplies.
That being said, we still have some captains who make it seem like the apu fuel bill comes out of their paychecks and will only run it for engine start. I never understood it.

One of the reasons for having the APU running at taxi in is the possibility of engine tailpipe fire at shutdown. Having the APU online at this point ensures there's electrical and pneumatic power available to dry-crank the engine to put the fire out.

I know, however, that procedures vary from type to type and airline to airline. I have heard they every APU start cycle on a medium jet can cost on order of 100$ in maintenance overheads - so some airlines prefer no APU on taxi in.

DiamondFlyer wrote:

Depends on the airline, we fire the APU just after landing at the latest, most often in the air even, prior to landing.

Out of curiosity - why would you do that so early? We normally start ours just before taxi in to the parking spot - to have it online just by the time we're ready to shut the engines.

thepinkmachine wrote:

Out of curiosity - why would you do that so early? We normally start ours just before taxi in to the parking spot - to have it online just by the time we're ready to shut the engines.

Cabin airflow, the particular airplane is known for not having much airflow when the power is set to flight idle. The only chance to keep the cabin cool, is to get the APU on to move some air. And you get a master caution if you have the flaps out, anti-ice on and packs not on the APU for their bleed source...

DiamondFlyer wrote:

Cabin airflow, the particular airplane is known for not having much airflow when the power is set to flight idle. The only chance to keep the cabin cool, is to get the APU on to move some air. And you get a master caution if you have the flaps out, anti-ice on and packs not on the APU for their bleed source...

Seems legit...

What a/c type is that? IIRC on the EMB145 we sometimes started the APU before landing and open the APU bleed to run the packs. This was to prevent unpleasant pressure surge in the cabin during de-rotation, or maybe reverser deployment. It was more of a personal technique of some CA's, rather than an official procedures.

thepinkmachine wrote:

DiamondFlyer wrote:

Cabin airflow, the particular airplane is known for not having much airflow when the power is set to flight idle. The only chance to keep the cabin cool, is to get the APU on to move some air. And you get a master caution if you have the flaps out, anti-ice on and packs not on the APU for their bleed source...

Seems legit...

What a/c type is that? IIRC on the EMB145 we sometimes started the APU before landing and open the APU bleed to run the packs. This was to prevent unpleasant pressure surge in the cabin during de-rotation, or maybe reverser deployment. It was more of a personal technique of some CA's, rather than an official procedures.

CRJ-200, the thing wasn't built real well, from a air conditioning standpoint.

The biggest thing about operating the APU is not the fuel it consumes but the hours - it's damn expensive overhauling an APU. At SAA where I worked, they were very conscious of this and forced us to use ground power most of the time. The flight crew would start the APU when they arrived to do their preflight. They also didn't want any power surges when setting up the flight computers.

Well as long as we are talking about APU usage and passenger comfort, one well known airline operating of out of Georgia likes to wait until 10 minutes before push back to start the APU and turn on the packs. Clueless is the best way to describe this operation. When you put anywhere between 100 and 150 pax on a airplane and the OAT is anything near 15C the darn airplane turns into a sauna quickly.

greg85 wrote:

I knew about the 4 engine airbus', but I'm still surprised about taxying on 1 generator on the 330. It's quite rare in aviation to have no redundancy. But the A320 is well known for its electrical transients.

Quite normal to taxi in with one engine shut down and no APU running, we do it all the time. Having only one AC source means practically nothing safety-wise when you're not airborne.

As mentioned above, the GPU is only for supplying electrical power to the aircraft. When the APU is not serviceable (ie. inoperative). Pre-conditioned air (heat or cooling) can be supplied by an external hose if required. Engine start without an APU requires a pneumatic start cart (basically an APU contained on a cart with a high pressure hose). The hose is connected into the external pneumatic connection and the air is used to start one engine along with GPU power at the gate. At this point the GPU and air start cart is disconnected. Once the aircraft is clear of the gate (pushed back), they start the second, third, and/or fourth engines using supply bleed air from the start engine(s). This procedure is called a cross-bleed start (because you're using bleed air from across the wing I guess).

JAGflyer wrote:

As mentioned above, the GPU is only for supplying electrical power to the aircraft. When the APU is not serviceable (ie. inoperative). Pre-conditioned air (heat or cooling) can be supplied by an external hose if required. Engine start without an APU requires a pneumatic start cart (basically an APU contained on a cart with a high pressure hose). The hose is connected into the external pneumatic connection and the air is used to start one engine along with GPU power at the gate. At this point the GPU and air start cart is disconnected. Once the aircraft is clear of the gate (pushed back), they start the second, third, and/or fourth engines using supply bleed air from the start engine(s). This procedure is called a cross-bleed start (because you're using bleed air from across the wing I guess).

If I understand correctly, does this mean the GPU can't run the air conditioning?

If so, what's the explanation for this?

MPadhi wrote:

JAGflyer wrote:

As mentioned above, the GPU is only for supplying electrical power to the aircraft. When the APU is not serviceable (ie. inoperative). Pre-conditioned air (heat or cooling) can be supplied by an external hose if required. Engine start without an APU requires a pneumatic start cart (basically an APU contained on a cart with a high pressure hose). The hose is connected into the external pneumatic connection and the air is used to start one engine along with GPU power at the gate. At this point the GPU and air start cart is disconnected. Once the aircraft is clear of the gate (pushed back), they start the second, third, and/or fourth engines using supply bleed air from the start engine(s). This procedure is called a cross-bleed start (because you're using bleed air from across the wing I guess).

If I understand correctly, does this mean the GPU can't run the air conditioning?

If so, what's the explanation for this?

Short answer: The GPU gives electrical power only, but the packs don't run on electricity. They run on high pressure air. To run the packs you need something to pressurize the bleed air manifold, be it engine bleeds, APU bleed or high pressure external air from a start cart.

Long answer:

Type specific I bet, but for the 330/350: The GPU gives electrical power only, so no air conditioning. The air start cart(s) pressurizes the bleed air manifold via the high pressure ground inlet(s). If we then turn the packs on they will run just as if the manifold was pressurized by the engines or the APU.

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

Start cart air is "high pressure air". "Normal" external air conditioning from is "low pressure air" and is piped from the low pressure ground inlets directly into the mixer unit without going through the packs.

Starlionblue wrote:

MPadhi wrote:

JAGflyer wrote:

As mentioned above, the GPU is only for supplying electrical power to the aircraft. When the APU is not serviceable (ie. inoperative). Pre-conditioned air (heat or cooling) can be supplied by an external hose if required. Engine start without an APU requires a pneumatic start cart (basically an APU contained on a cart with a high pressure hose). The hose is connected into the external pneumatic connection and the air is used to start one engine along with GPU power at the gate. At this point the GPU and air start cart is disconnected. Once the aircraft is clear of the gate (pushed back), they start the second, third, and/or fourth engines using supply bleed air from the start engine(s). This procedure is called a cross-bleed start (because you're using bleed air from across the wing I guess).

If I understand correctly, does this mean the GPU can't run the air conditioning?

If so, what's the explanation for this?

Short answer: The GPU gives electrical power only, but the packs don't run on electricity. They run on high pressure air. To run the packs you need something to pressurize the bleed air manifold, be it engine bleeds, APU bleed or high pressure external air from a start cart.

Long answer:

Type specific I bet, but for the 330/350: The GPU gives electrical power only, so no air conditioning. The air start cart(s) pressurizes the bleed air manifold via the high pressure ground inlet(s). If we then turn the packs on they will run just as if the manifold was pressurized by the engines or the APU.

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

Start cart air is "high pressure air". "Normal" external air conditioning from is "low pressure air" and is piped from the low pressure ground inlets directly into the mixer unit without going through the packs.

Does anyone who work the 787 know if gpu power can run the "packs" since there is no bleed air?

Starlionblue wrote:

MPadhi wrote:

JAGflyer wrote:

As mentioned above, the GPU is only for supplying electrical power to the aircraft. When the APU is not serviceable (ie. inoperative). Pre-conditioned air (heat or cooling) can be supplied by an external hose if required. Engine start without an APU requires a pneumatic start cart (basically an APU contained on a cart with a high pressure hose). The hose is connected into the external pneumatic connection and the air is used to start one engine along with GPU power at the gate. At this point the GPU and air start cart is disconnected. Once the aircraft is clear of the gate (pushed back), they start the second, third, and/or fourth engines using supply bleed air from the start engine(s). This procedure is called a cross-bleed start (because you're using bleed air from across the wing I guess).

If I understand correctly, does this mean the GPU can't run the air conditioning?

If so, what's the explanation for this?

Short answer: The GPU gives electrical power only, but the packs don't run on electricity. They run on high pressure air. To run the packs you need something to pressurize the bleed air manifold, be it engine bleeds, APU bleed or high pressure external air from a start cart.

Long answer:

Type specific I bet, but for the 330/350: The GPU gives electrical power only, so no air conditioning. The air start cart(s) pressurizes the bleed air manifold via the high pressure ground inlet(s). If we then turn the packs on they will run just as if the manifold was pressurized by the engines or the APU.

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

Start cart air is "high pressure air". "Normal" external air conditioning from is "low pressure air" and is piped from the low pressure ground inlets directly into the mixer unit without going through the packs.

Do the packs have some sort of filter, or is the bleed air from the APU clean and breathable?

MPadhi wrote:

Starlionblue wrote:

MPadhi wrote:

If I understand correctly, does this mean the GPU can't run the air conditioning?

If so, what's the explanation for this?

Short answer: The GPU gives electrical power only, but the packs don't run on electricity. They run on high pressure air. To run the packs you need something to pressurize the bleed air manifold, be it engine bleeds, APU bleed or high pressure external air from a start cart.

Long answer:

Type specific I bet, but for the 330/350: The GPU gives electrical power only, so no air conditioning. The air start cart(s) pressurizes the bleed air manifold via the high pressure ground inlet(s). If we then turn the packs on they will run just as if the manifold was pressurized by the engines or the APU.

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

Start cart air is "high pressure air". "Normal" external air conditioning from is "low pressure air" and is piped from the low pressure ground inlets directly into the mixer unit without going through the packs.

Do the packs have some sort of filter, or is the bleed air from the APU clean and breathable?

AFAIK there is no filter to clean out impurities from the bleed air manifold.

The APU bleed air is as clean as the outside air. Again, "type specific warning", but your typical APU is a gas generator driving a pneumatic pump and an electrical generator, so the air pressurizing the bleed air manifold is just ambient air.

Starlionblue wrote:

MPadhi wrote:

Starlionblue wrote:

Short answer: The GPU gives electrical power only, but the packs don't run on electricity. They run on high pressure air. To run the packs you need something to pressurize the bleed air manifold, be it engine bleeds, APU bleed or high pressure external air from a start cart.

Long answer:

Type specific I bet, but for the 330/350: The GPU gives electrical power only, so no air conditioning. The air start cart(s) pressurizes the bleed air manifold via the high pressure ground inlet(s). If we then turn the packs on they will run just as if the manifold was pressurized by the engines or the APU.

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

Start cart air is "high pressure air". "Normal" external air conditioning from is "low pressure air" and is piped from the low pressure ground inlets directly into the mixer unit without going through the packs.

Do the packs have some sort of filter, or is the bleed air from the APU clean and breathable?

AFAIK there is no filter to clean out impurities from the bleed air manifold.

The APU bleed air is as clean as the outside air. Again, "type specific warning", but your typical APU is a gas generator driving a pneumatic pump and an electrical generator, so the air pressurizing the bleed air manifold is just ambient air.

Sorry to drag this OOT a little bit, but is that the reason you got nasty smell whenever the plane is on APU?

benbeny wrote:

Sorry to drag this OOT a little bit, but is that the reason you got nasty smell whenever the plane is on APU?

Which type aircraft are you referring to...in regards to this smell when on APU.

HAWK21M wrote:

benbeny wrote:

Sorry to drag this OOT a little bit, but is that the reason you got nasty smell whenever the plane is on APU?

Which type aircraft are you referring to...in regards to this smell when on APU.

A320. I always smell that nasty smell whenever I board the A320, but never on 737 classics or NG.

benbeny wrote:

Starlionblue wrote:

MPadhi wrote:

Do the packs have some sort of filter, or is the bleed air from the APU clean and breathable?

AFAIK there is no filter to clean out impurities from the bleed air manifold.

The APU bleed air is as clean as the outside air. Again, "type specific warning", but your typical APU is a gas generator driving a pneumatic pump and an electrical generator, so the air pressurizing the bleed air manifold is just ambient air.

Sorry to drag this OOT a little bit, but is that the reason you got nasty smell whenever the plane is on APU?

Type specific.

In our airport APU is started before the push-back and after landing the aircraft. We disconnect the GPU before commencing the push-back and aircraft PACS are supplied by the APU bleed. During the push-back one engine is started(on A320 left engine and on A330 right engine and I do not know about aircrafts with more than two engines) and bleed supply to PACS(pressurizing and air-conditioning) are shifted to the started engine. When the aircraft is landed and when reached the bay,the APU is started and engines are turned off. We connect the GPU after sometime and turn off the APU.

APU is not intended to run through all the flight time. For A 320's.
APU can supply bleed air for both PACS up to 15,000ft.
APU can supply bleed air for one PAC up to 20,000ft.
APU can started from battery power up to 25,000ft.
APU can supply electrical power up to 39,000ft and it can be started from aircraft power up to 39,000ft.

One thing I want to get cleared!!
In four engine aircrafts which engine we start first? Is it the engine which has the EDP powering the hydraulic system for parking brakes? If there is a specific sequence please explain a bit about it.

Thank you very much!!!

skywalker92 wrote:

In our airport APU is started before the push-back and after landing the aircraft. We disconnect the GPU before commencing the push-back and aircraft PACS are supplied by the APU bleed. During the push-back one engine is started(on A320 left engine and on A330 right engine and I do not know about aircrafts with more than two engines) and bleed supply to PACS(pressurizing and air-conditioning) are shifted to the started engine. When the aircraft is landed and when reached the bay,the APU is started and engines are turned off. We connect the GPU after sometime and turn off the APU.

We always start the left engine first on the 330. Operator specific?

On the 777 we start both at the same time on the -200 and -300; the -300ER's huge engines demand too much air so it's one at a time, but in whichever order the pilot feels like on the day.

skyhawkmatthew wrote:

On the 777 we start both at the same time on the -200 and -300; the -300ER's huge engines demand too much air so it's one at a time, but in whichever order the pilot feels like on the day.

You can start both -115B on the 77W / 77L / 77F at the same time. It is just most airlines don't to not put additional stress on the APU.

Starlionblue wrote:

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

One more question to those who know.

AFAIK:
The packs do their job, combining two ducts from bleed air, when the engines are on. One is the intake from cold duct of the engine, the other goes via pipes and valves to heat transfer in the engines, where it gets warm. Then it's up to packs (and the steering computers) to mix those two to get whatever is set on the panel, let's say 24C (75F).

Is it (more or less) the same with APU? Heat exchange, cold and hot air, mix to reach appropriate temperatures?

How about hose from truck?

Just wonder, if the heat transfer units are inside the engines. It seemed to me natural to have these as part of engine equipment, but it seems I could go wrong somewhere.

Cheers,
Adam

gloom wrote:

Starlionblue wrote:

There's a big caveat. We must first ensure the air from the start cart is not contaminated by oil and such. Running the packs with contaminated air is bad feng shui.

One more question to those who know.

AFAIK:
The packs do their job, combining two ducts from bleed air, when the engines are on. One is the intake from cold duct of the engine, the other goes via pipes and valves to heat transfer in the engines, where it gets warm. Then it's up to packs (and the steering computers) to mix those two to get whatever is set on the panel, let's say 24C (75F).

Is it (more or less) the same with APU? Heat exchange, cold and hot air, mix to reach appropriate temperatures?

How about hose from truck?

Just wonder, if the heat transfer units are inside the engines. It seemed to me natural to have these as part of engine equipment, but it seems I could go wrong somewhere.

Cheers,
Adam

The following should hold generally true for most large airliners.

Your statement about the packs is not entirely correct. The packs don't mix different engine airs. The mixing of bleed air from different engine stages happens in the engine before the (total) engine bleed air goes to the bleed air manifold. The APU, on the other hand, does not need to do such mixing to regulate air temperature since it runs at a constant speed.

There are three sources of bleed air, a) the APU, b) the engines or c) a start cart. Any of these three sources can pressurize the bleed air manifold. The packs then take air from said manifold. The packs don't care where the bleed air came from.

Low pressure external air bypasses the packs, going directly to the mixer unit. Note, this is not the same as high pressure air from a start cart. Low pressure air is fed in by the big yellow flexible ducts you often see trailing from the belly.

(While the engines are starting, the packs shut down because bleed air is needed for the starters.)

The packs regulate temperature, pressure and humidity for air to the cabin. However temperature fine tuning is done downstream by the trim valves which take hot air from upstream of the mixer unit to regulate individual zones.

First, thanks for very informative post. I could almost see the pipes and hoses

So, basically the manifold gets air from any engine or APU, at a more-or-less fixed temperature. Packs take care of getting it from manifold and adjusting for temperature, pressure, moisture etc. and put that air into mixer (which can be fed by low pressure external pipe as well). Air from mixer is then fed into sections, with maybe small adjustments made in that final pipes as well. Am I getting the picture right?

My final question is, except for the first level (engine), the adjustments made along the packs and pipes seem pretty small - max a couple degrees I guess. Is that final adjustment done by mixing the hot/cold air again, or electrical? I think electrical would be simplier, but could draw quite a lot of power, so want to make sure if I'm right here.

Cheers, Adam

gloom wrote:

First, thanks for very informative post. I could almost see the pipes and hoses

So, basically the manifold gets air from any engine or APU, at a more-or-less fixed temperature. Packs take care of getting it from manifold and adjusting for temperature, pressure, moisture etc. and put that air into mixer (which can be fed by low pressure external pipe as well). Air from mixer is then fed into sections, with maybe small adjustments made in that final pipes as well. Am I getting the picture right?

My final question is, except for the first level (engine), the adjustments made along the packs and pipes seem pretty small - max a couple degrees I guess. Is that final adjustment done by mixing the hot/cold air again, or electrical? I think electrical would be simplier, but could draw quite a lot of power, so want to make sure if I'm right here.

Cheers, Adam

Correct on the first paragraph. And there are a ton of pipes.

The packs make massive adjustments. Bleed air is around 40 PSI and over 200 degrees C. Obviously if you fed this to the cabin you'd have some highly compressed and well-cooked passengers. The air that comes out of the packs, on the other hand, is quite cool. Normally it is a bit cooler than is comfortable, which is why hot trim air is fed into it. At its coldest stage in the pack the air can be minus 20-30 degrees, which is why you need an ice separator.

Here's a typical bleed air system.


Here's a typical pack.

Ahhh, the pictures

But really, seeing the picture with pressure/temp, and the pack inside view, now it all makes perfect sense.

Excellent explanation, very, veery appreciated, and a big Thank you.

Have a Happy New Year. Same goes to all the readers.

Cheers, Adam

Czesc Adam

Pack is not just a mixer - it is an Air Cycle Machine. It compresses the (pre conditoned) bleed air, removes the energy by heat exchanger and cools the or by expanding it on a turbine. Works more or less like a refrigerator:) This way it can deliver very cool air, less then 0 C.

Also, the pack "does not care" if the bleed air comes from the APU, or Engine compressor.

Final temperature adjustment is done by mixing cold, conditioned air with a bit of in-conditoned hot bleed air (usually called trim air)

thepinkmachine wrote:

Also, the pack "does not care" if the bleed air comes from the APU, or Engine compressor.

It was initially what sparked me to go a bit on the line in the topic.

It's easy to imagine an inlet from the engine with cold air. And hot one's easy too. Hot's piece of cake in APU. But how to take a cold one with only APU running? It has none, no fan there.

That's how the pack picture helps. It does cool air very much like refrigerator, or typical air conditioning, the only difference is the size and additional effects, like deicing inlet. From that picture, and pressure/temp scheme, one can nicely derive all the physics behind.

The only question remaining is the zoning (I've seen a couple of birds with different temps in front, middle and back zone of the plane), but that's easy trick to do either by multiplying mixers, or adjusting on manifolds, so I can answer myself that

Czesc,
Adam

We always start the left engine first on the 330. Operator specific?

yes you are correct Starlionblue. It should be the other way around . A320 right engine and A330 left engine.

On the 777 we start both at the same time on the -200 and -300; the -300ER's huge engines demand too much air so it's one at a time, but in whichever order the pilot feels like on the day.

thank you very much skyhawkmatthew

I have another question(sorry for deviating from the topic)
1) How we turned off the APU? I mean the correct sequence.
2) How the APU is fed with fuel and is there a dedicated pump? How the fuel is supplied to the APU for starting it. I have turned on the APU with out turning on any fuel pumps.