Sign in to follow this  
Driver170

APU on the busses

Recommended Posts

You're at a hot country and During your Preliminary preflight flows you turn on the APU to cool the cabin down, ambient temp is +27 at Lisbon. GRD PWR has been established and on the AC transfer busses. This is my question would you put the APU on the busses or leave the GRD PWR for power and APU for air only?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

That is a very good question!

 

In my previous carrier- this issue came up on the very very hot days, we're talking 35º- sometimes the load on the APU would be so great that it would just shut itself off, if it were to be acting as a generator, and providing bleed air. The MEL we used was specific to the malfunction so you could still use the electrical portion, but not the air. 

 

Anyhow, tangent- to answer your question, depends on the carrier, how reliable the APU is- I'm guessing that like the 767, the 737 APU does just fine under most ambient conditions providing air and power- and is quite robust. In my example above, I'm talking about the Q400 which had a very weak APU that broke down often, and wasn't certified to be used in the air.

Share this post


Link to post
Share on other sites

I love my questions ;)

 

Its a piece of equipment i haven't thorughly studied and know where i could find this answer. could the APU still provide bleed air without being on the AC transfer busses?

Share this post


Link to post
Share on other sites

The APU on the NG's is a very robust and reliable unit, and has no problems providing electrical and full pneumatic power in high ambient temp conditions.

 

It's use in ETOPS operations attests to its reliability.

 

In your case I'd have the APU powering the airplane whenever it's on - it prevents that inevitable slip-up when you forget to transfer power from External AC prior to pushback!

 

It is of course possible to provide only pneumatic air without AC, although the generator is spinning anyways, why not use it?

 

As for my Dash-8 comrade, I've been fighting with that APS1000 unit since we've gotten the thing, although we limit the use so much now it's rarely an issue. Make sure you have that intake louver removed for +20*C operations.

 

The Avro RJ also uses the APS1000, which has been far more troublesome for us, as it's used all the time, and lacks the FADEC and associated troubleshooting help.

Share this post


Link to post
Share on other sites

Thanks patrick and i'll just put them online if APU is available

Share this post


Link to post
Share on other sites

I love my questions ;)

 

Its a piece of equipment i haven't thorughly studied and know where i could find this answer. could the APU still provide bleed air without being on the AC transfer busses?

If external electric power is available then that could be used to offload the APU leaving it dedicated to air supply.

 

The fact is electrical generation isn't free and if the APU gen is online then there is less load capacity to produce bleed air flow. If EGT gets near the limit bleed flow is restricted to prevent an exceedance. Running the generator would increase EGT and mean it would take less bleed air flow to reach the limit. On a hot day EGT is of course higher still which also reduces the amount of air flow available. What you should do depends on how near the limit EGT is.

Share this post


Link to post
Share on other sites

So leave the APU untill 6 mins before engine start then to avoid those troubles? So what you are saying is i can have the APU for bleed air and the GPU for the power to limit the load on the APU?

Share this post


Link to post
Share on other sites

Kevin's advice is correct, but in the NG's the APU is constructed a little different than the older style APUs we've been used to. The pneumatic air is actually supplied from a separate compressor that is driven within the APU. Loading of the compressor is controlled with IGVs via the FADEC. This makes for a very efficient and less temperature critical APU than those which tap the bleed air prior to combustion.

 

As a consequence, the NGs APU is much less susceptible to EGT spikes and temping out with both electrical and pneumatic power in use.

 

The only occasion one would utilize external AC with bleed only from the APU is if you required galley equipment running - warming up meals or something along those lines.

 

Notice how there us no EGT limit markings on the gauge? In theory, the FADEC will never let the APU temp out and auto-shutdown. It'll just keep unloading that compressor. Really, you don't even need the gauge.

 

The APU is very capable and when running both packs it's not even close to the full flow & pressure it's capable of producing. During engine starts the APU ramps up pneumatic pressure and flow right up to 40 psi and some obscene flow :).

 

This APU will have no problem providing electrical power and pneumatic air in the most demanding of conditions.

Share this post


Link to post
Share on other sites

Using a load compressor may be more efficient and controllable but the limiting factor remains EGT. A load compressor still takes shaft power to drive which will increase it. The NG APU is more powerful than classic 737 APUs so that extra capacity is probably the main reason it should cope, not the APU configuration.

 

Even a 737 classic APU shouldn't shut down under excessive load. The load control valve should throttle the flow to keep EGT in limits.

 

All this is academic when APU operation time is limited by local noise regs and fuel costs versus airport charges.

Share this post


Link to post
Share on other sites

And Kevin touched upon a good point there, some airports have strict times for APU startup and shutdown, you need to check the information on the aiport charts.

 

I always assumed that if you needed APU bleed air, but had a GPU then use the GPU for power since you're not putting any load on the APU, other then it running and bleed air, so would use less fuel

Share this post


Link to post
Share on other sites

As someone has already pointed out location of the airport can play a part on weather you can start the apu prior to engine start.

In many hot countries ground cooling air can be supplied to the aircraft through air con connections depending on the aircraft type along with gpu's for the electrics.

 

Also these days it depends on the operators, they may stipulate on how long before departure the crew can start the apu.as quite large fuel savings can be made over time and on the size of the fleet.

Share this post


Link to post
Share on other sites

That HVAC unit attached to the jetway is much much less expensive to operate than any APU, even a mobile unit is preferable.

Share this post


Link to post
Share on other sites
Note: Whenever the APU is operating and AC electrical power

is on the airplane busses, operate at least one fuel boost

pump to supply fuel under pressure to the APU to extend

the service life of the APU fuel control unit.

Share this post


Link to post
Share on other sites

That HVAC unit attached to the jetway is much much less expensive to operate than any APU, even a mobile unit is preferable.

 

Not quite. Those things are good for a temp range of about 40-80. Outside of that, they really don't work too well (at least the ones at IAD didn't, and they were brand new in 2008 when I worked there).

 

Their main purpose is to keep the cabin temps reasonable, which means less time is needed to adjust the temperature when people are about to get on. For some aircraft, an APU wasn't available to cool things off (the SLAAB), so the gate air was all they had, and we'd leave it connected all the way up to immediately before engine turn.

 

Otherwise, it was gate air to keep it "in range" of comfortable (though certainly not comfortable to the average human), and then the APU was brought online just before boarding to get it in the normal comfortable range.

 

This, of course, speaks to the US. I know Europe is far more draconian with APU limits and so on. Then again, we usually have a bit more land buffer between our airports and cities since we have more land to use (in general).

 

 

Note: Whenever the APU is operating and AC electrical power
is on the airplane busses, operate at least one fuel boost
pump to supply fuel under pressure to the APU to extend
the service life of the APU fuel control unit.

 

Not sure how this relates to the discussion. This refers to the fuel control unit for the APU, and not the topic of APU on for air, and the question of on/off for power.

Share this post


Link to post
Share on other sites

Not sure how this relates to the discussion. This refers to the fuel control unit for the APU, and not the topic of APU on for air, and the question of on/off for power.

 

Ahh true sorry! I thought it was ground power that was supplying the power. I just seen AC

Share this post


Link to post
Share on other sites

Out of interest, what temps where you seeing for FWD and AFT with only the AC? I was parked in Madrid yesterday to test this out, with GPU and AC connected, and it didn't rise above 21 and that was with air temp of 33. I've never found I've had to use the GPU as well as AC for air. Unless I'm reading the temperature gauge wrong, or not waiting enough

Share this post


Link to post
Share on other sites

 

 


Those things are good for a temp range of about 40-80.

 

That might work in Virginia but not anyplace in Texas LOL.  I passed through DFW one summer night about 11 pm Local and it was still 97 outside. Even the terminals were uncomfortable and the jetways were like ovens still cooling off from the days sun .The aircraft was warm but tolerable (it was not on APU) probably because it had just come in from the stratosphere. Not just the heat (think Phoenix) but the humidity (think Houston) adds to conditioning loads. Regardless, that ground unit is going to be less expensive that anything you stick on an airplane... some rule of nature I think.

Share this post


Link to post
Share on other sites

That HVAC unit attached to the jetway is much much less expensive to operate than any APU, even a mobile unit is preferable.

 

It's been my experience that external PCA units, either portable or on the jetway, are hit-or-miss at best.  If you do get a unit that happens to put out cold air the hose attached to the aircraft will have three or four kinks in it restricting the airflow. 

Share this post


Link to post
Share on other sites

Am I right in thinking that the GPU and AC units come at a rent cost from the airport? So is it not a toss up between cost of fuel VS cost of the units? Or is that not the case and they are owned by the airlines anyway? Maybe that's dependent on airport...if you have a base there or not, as it where.

Share this post


Link to post
Share on other sites

That might work in Virginia but not anyplace in Texas LOL.  I passed through DFW one summer night about 11 pm Local and it was still 97 outside. Even the terminals were uncomfortable and the jetways were like ovens still cooling off from the days sun .The aircraft was warm but tolerable (it was not on APU) probably because it had just come in from the stratosphere. Not just the heat (think Phoenix) but the humidity (think Houston) adds to conditioning loads. Regardless, that ground unit is going to be less expensive that anything you stick on an airplane... some rule of nature I think.

 

That's kinda what I was saying, though.

 

When the temp is 40-80 at IAD, the ground unit will cope pretty well with that (exceptions made for high humidity). Despite that, the ground unit is preferable for its lower cost until just prior to departure time when the APU is all but required to get reasonable temperatures in there. As long as that ground cart is pushing drier air, and the air is moving, it'll take a decent amount of time before it feels stuffy, even if it's not as cool.

Share this post


Link to post
Share on other sites

 

 


Am I right in thinking that the GPU and AC units come at a rent cost from the airport?

 

Good question.  I assumed that the air and power is provided as a utility that is included in the cost of the gate... never seen meters at the gate.  The airline leases the gate from the airport, which includes the "real property" such as room/air/light and the airline provides people and stuff like computers and maybe vehicles(?).  As a privately owned aircraft, I pay the FBO for the use of facitites through fuel cost but it's extra if I want a ground cart to power my on-board A/C (we have JB Air unit with an electric compressor instead of engine mounted) so we never rent the cart, but I get "free" fresh cookies and a pilot's lounge with recliners, TV and computer terminals.  I would be surprised if the utilities (air/power) were charged to airlines on a pay as you go basis; which means you are paying for it through the gate lease or landing fee used or not.

Share this post


Link to post
Share on other sites

Am I right in thinking that the GPU and AC units come at a rent cost from the airport? So is it not a toss up between cost of fuel VS cost of the units? Or is that not the case and they are owned by the airlines anyway? Maybe that's dependent on airport...if you have a base there or not, as it where.

 

 

Good question.  I assumed that the air and power is provided as a utility that is included in the cost of the gate... never seen meters at the gate.  The airline leases the gate from the airport, which includes the "real property" such as room/air/light and the airline provides people and stuff like computers and maybe vehicles(?).  As a privately owned aircraft, I pay the FBO for the use of facitites through fuel cost but it's extra if I want a ground cart to power my on-board A/C (we have JB Air unit with an electric compressor instead of engine mounted) so we never rent the cart, but I get "free" fresh cookies and a pilot's lounge with recliners, TV and computer terminals.  I would be surprised if the utilities (air/power) were charged to airlines on a pay as you go basis; which means you are paying for it through the gate lease or landing fee used or not.

 

Depends on the airport. I know that the 'A' gates at IAD were built by ACA/IDE, so modifications to the structure and ramp were more discretionary. When UAX took over the ramp area, that area had gone to MWAA control. MWAA were the ones who added the ground air (the GPU hardpoints were there from ACA), and I remember that if anything ever broke, it had to be routed to them for repair.

 

All that said, I think it was all included in the costs for the gate agreement, as Dan has alluded to.

Share this post


Link to post
Share on other sites

That sounds about right then, so unless you need more bleed air, which I've heard is possible in places such as Dubai, then there is little reason to use the APU apart from push-back. Also, has been stated, check any charts for European Airports and most will have APU restrictions due to noise, same as a lot have request for only using idle reverse on landing. I don't see these advisories for US airports usually.

Share this post


Link to post
Share on other sites

 

 


I don't see these advisories for US airports usually.

 

Not usually, though we do have our outliers like SNA and SMO (not sure what the restrictions are out on the west coast, but CA does seem to have a problem with allowing developers to toss up homes next to airports). Not that Leesburg didn't allow the same thing at JYO. You can even see the roughed in streets from where they were going to build before the county stepped in and said "hey now...we know that the land just north of the field isn't ours to zone but knock it off before you shut your own airport down..."

https://www.google.com/maps/@39.0922064,-77.5611022,2474m/data=!3m1!1e3?hl=en

 

So far, we've gotten away with only a "Runway 17 is the calm wind runway" addition to the AWOS broadcast.

Share this post


Link to post
Share on other sites

Not quite. Those things are good for a temp range of about 40-80. Outside of that, they really don't work too well (at least the ones at IAD didn't, and they were brand new in 2008 when I worked there).

 

Their main purpose is to keep the cabin temps reasonable, which means less time is needed to adjust the temperature when people are about to get on. For some aircraft, an APU wasn't available to cool things off (the SLAAB), so the gate air was all they had, and we'd leave it connected all the way up to immediately before engine turn.

 

Otherwise, it was gate air to keep it "in range" of comfortable (though certainly not comfortable to the average human), and then the APU was brought online just before boarding to get it in the normal comfortable range.

 

This, of course, speaks to the US. I know Europe is far more draconian with APU limits and so on. Then again, we usually have a bit more land buffer between our airports and cities since we have more land to use (in general).

 

 

Not sure how this relates to the discussion. This refers to the fuel control unit for the APU, and not the topic of APU on for air, and the question of on/off for power.

 

 

Aaaaaannnnnd.... Mesa kids used to steal the air hoses attached to our saabs while we were inside grabbing releases/coffee. Don't worry, I'm not bitter!  

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this