Packs on APU or Packs OFF

[cyrus74]

Hi! My Question : Can anybody tell me, bottomline, where I have more Power for the Tako Off : Packs on APU or Packs Off ? (on a A320)

thank u
:-)

[Dream Land]

Well if power is your concern, both methods are accomplishing exactly the same thing.

[cyrus74]

great! thats what i hope to expect - if i understood the system right! thanks for the fast answer anyhow !!!! :-)

So, what are Pros & Conts of each Method?

I would, with my knowlegde now, resume, that Packs on APU is the best way, since i have the Power for the Take off, and the PAX do get the Airconditioning, right? Only thing is, it costs some fuel for the APU. ... right?

[Hotel Mode]

Only thing is, it costs some fuel for the APU. ... right?

It costs the fuel use of APU from after engine start to a couple of mins after airborne, all to provide air conditioning to 2-3 mins between entering runway and airborne. At 130 kg an hour thats going to be a good 50kg per sector which would cost around $40-50. Thats quite a bit of profit margin for not much gain. Most airlines unless they are in a consistently hot environment will opt for just packs off. Thats excluding the increased APU maintenance cost.

[Border Reiver]

Slight risk if the APU is used after the fuselage has been deiced. It is possible for the APU to be extinguished at rotate as the fluid moves backwards into the APU intake. Engine EGTs will increase as the packs transfer to engine bleed, ie APU bleed closes as no APU air is produced.

[BOAC]

Not if it is done correctly, Border. I take it you don't know what you are talking about?

[rudderrudderrat]

Hi BOAC,

If Border is talking about A320 - then that's exactly what would happen.
I take it you must be talking about some design which pre-dates computer logic.

[BOAC]

In that case I offer my apologies. On real aeroplanes the engine bleeds are selected off for that and other reasons and we do not rely on ?logic? circuits.

That sounds like yet another design fault. Anything which could reduce critical take-off performance with a simple APU failure just has to be crass. I was under the impression that AB took off packs or bleeds off?

[skianyn vannin]

Normal sop where I work is to turn packs off for takeoff (bleeds stay on) and then turn packs on after thrust reduction to CLB thrust. The packs are turned off when entering the runway.

The main reason for this is to obtain a higher flex temp, it normally makes about 1 or 2 degrees of difference.

[capt. solipsist]

cyrus,

the main reason why you would NOT use APU for aircon during takeoff is that the computed thrust for takeoff, once set, is maintained.

if APU shuts down during the takeoff roll, the aircon bleed source automatically reverts to engine bleed.

to satisfy this unscheduled bleed demand while maintaining the original computed takeoff thrust , the FADEC increases the fuel flow, thus increasing the possibility of very high EGT during the takeof rol.

this is in addition to the possibility of engine exhaust ingestion into the aircon system thru the APUl :ok:

[rudderrudderrat]

Hi capt. solipsist,

That's strange - because my copy of FCOM says:

FCOM 3.03.11 Page 1
BEFORE TAKEOFF
Pack 1 and 2 … As RQRD
Consider selecting packs OFF, or APU bleed ON.
This will improve performance when using TOGA thrust.
In case of a FLEX takeoff, selecting packs OFF or APU bleed ON will reduce takeoff EGT, and thus reduce maintenance costs.

[parabellum]

rudderrat - the way I read your FCOM it isn't taking into account the possibility of an APU failure during take off? I think Capt. Solipsist's summary, which does allow for APU failure, makes good sense, but I have never flown an AB!

[ray cosmic]

On the 744F you'd have a 135kg penalty on your MTOW, which you would not have if you'd leave the APU off and perform a packs-off take-off. This is due to the open APU door.. Not a big number, but it's a number..

[Old Smokey]

I was about to say something similar, ray cosmic, 100 Kg penalty for the B777. That A320 APU Air has to be coming from somewhere!

Regards,

Old Smokey

[Border Reiver]

Capt solipsist and parabellum reflect what happened in previous operator, with a V2500 A1 which had no EGT margin left at the best of times the EGT increase as the APU failed did get your attention. As I said it is a slight chance and as my current operator is packs off when we can't lift the load packs on, an almost forgotten 11 year old bit of history for me.

[guiones]

From ancient Airbus times there is a concern of FOD by the APU Intake during rotation due to the closeness of the tail to the rwy.

[Slasher]

Present mob is packs on TO with APU off unless performance
limited, and even then its packs on with APU bleed. The norm
with almost all the cheap and nasty low-cost outfits I know of
are packs and APU off irrespective of anything.

[capt. solipsist]

rudderrudderat,

FCTM NO-040 P10/12:

"..The APU bleed may be used with packs ON, thus maintaining engine performance level and passenger comfort. In case of APU auto-shutdown during take-off, the engine thrust is frozen till the thrust is manually reduced. The packs revert to engine bleed which causes an increase in EGT to keep N1/EPR." (emphasis mine)

[rudderrudderrat]

capt. solipsist,

That's understood - but where does it say:
this is in addition to the possibility of engine exhaust ingestion into the aircon system thru the APU (emphasis mine).

[capt. solipsist]

rudderrudderrat,

the same reason why we turn OFF APU Bleed just after engine start, or turn ON APU Bleed just before shutdown (after PARK BRK On):

Turn APU Bleed pb-sw off just after engine start to avoid ingestion of engine exhaust gases. (PRO-NOR-SOP-09), and;

Select APU Bleed ON just before engine shutdown to prevent engine exhaust fumes from entering the air conditioning. (PRO-NOR-SOP-25)

:ok:

[rudderrudderrat]

capt. solipsist,
the same reason why we turn OFF APU Bleed just after engine start, or turn ON APU Bleed just before shutdown (after PARK BRK On):
That's to stop engine exhaust being ingested by the APU due to some air flow in that general direction at low speed / stopped. During the take off roll and flight - the APU inlet is in clean air - else they would have had to redesign it.

[cactusbusdrvr]

APU failure during takeoff roll is a very remote possibility and should not play any part in the decision to go packs off or APU bleed on.

Having flown A1 powered Airbi and needing that extra thrust we always go APU bleed on when needed. Actually we use it more on the 321.

Normal preference is FLEX, then TOGA, then APU bleed on. Packs off is only used when APU is on MEL. There is also a certain level of passenger discomfort with an unpressurized T/O, which is what a packs off condition gives you. Now I will admit that there have been a few done when the F/O has inadvertently shut the APU down on the afterstart flow before checking the numbers.