APU on the busses

[Driver170 42]

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?

[bjratchf 80]

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.

[Driver170 42]

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?

[Houghton11 12]

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.

[Driver170 42]

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

[kevinh 409]

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.

[Driver170 42]

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?

[Houghton11 12]

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.

[kevinh 409]

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.

[codechris 9]

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

[mazelda 124]

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.

[downscc 1,327]

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

[Driver170 42]

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.

[scandinavian13 5,192]

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.

[Driver170 42]

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