Bizarre British Airways takeoff procedure - why?

[VHOJT 172]

Hi there allDo any experts on this forum know why British Airways would use something called an "APU-to-Pack" take-off configuration as standard procedure, whereby: They perform T/O and climbout until acceleration with Packs 1 and 3 off (and isolated), and the APU "feeding" pack 2? Wouldn't this waste a lot of fuel considering they have 57 747-400s flyign around all day? Why would they do it?I don't suppose anyone has any ideas?CheersRudy

[Sylle 48]

Hi Rudy,I'm not a 744 expert at all :-) but I have an idea as this seems similar to what our company does with the 737s.Not using the bleed air of the engines for the packs probably allows them to increase the assumed/flex temperature setting for take-off hereby reducing the wear of the engines.The increased cost of the fuel for the APU is only small compared to the cost saving on maintenance.It could also help a little bit on performance limited runways.Cheers,Sylvain

[VHOJT 172]

Hi Sylvain (I shouldn't have put "expert" in there, I meant everyone really!)That sounds like a likely reason! Maybe the procedure is not so bizarre after all!Thanks for your helpCheersRudy

[c402_cvs 0]

You know the next question is, is this modeled in fs2009? ie If I do not switch on the bleeds from the motors will I get increased performance? I would guess the answer is no due to fs9 limitations but we can hope ;) FRED

[opherben 0]

1. The imrpoved engine condition results from its operation at lower RPM and temperature for producing certain nominal thrust. Not thrust increase. 2. BA does not usually operate B744's from short strips.3. When FS2009 comes out, you'll see from running Pack 2 on APU, reduced maintenance costs you could cash-in on the spot :-) >You know the next question is, is this modeled in fs2009? ie>If I do not switch on the bleeds from the motors will I get>increased performance? I would guess the answer is no due to>fs9 limitations but we can hope ;) >>FRED

[c402_cvs 0]

Ok great. I know jack about jets and just assumed like the BE20 when you open the icevanes you loose torgue or in this case power.FRED

[Qavion 19]

"You know the next question is, is this modeled in fs2009? ie If I do not switch on the bleeds from the motors will I get increased performance? I would guess the answer is no due to fs9 limitations but we can hope."Unfortunately, if you switch off bleeds from the motors, you lose pneumatics for Leading Edge Flap retraction, you lose 1&4 Hydraulic demand pump operation for assistance in retracting flaps and gear, loss of leading edge anti-ice, no aspiration for TAT probes, no pressurisation for hydraulic reservoirs, etc, etc. If you have 4 Air-Driven Hydraulic Pumps, you lose all demand pumps.Maybe you won't crash because of this, but it's certainly increasing the risk... Cheers.Q>

[Guest neerajpendse]

Maybe they keep the bleeds ON, but only not feed the packs from the engines ... that would keep everything active but save the duct pressure so engines do not have to work hard and may either produce more power or prolong their own lives.

[de Lattre WGA 0]

Hello,Yes, this is what they are doing bleed on and packs feeded by APU.Cyrille de LattreAsus P4P800/PIV 3 Ghz/2 Giga DDRAMATI X800 GT 256 MegWin XP SP2 / FS9.1 PSS Beta tester

[cowpatz 243]

I am not sure why they would adopt this as a standard procedure because there would not be any great saving. What would be saved by having the packs off would be at the expense of the APU (to some degree). Usually the APU on takeoff is for hot area operations (44 degrees for our airline). The APU door being open also increases drag and reduces payload (400 kg). A lot of airlines just use a packs off procedure. Just before line the packs are progressively turned off. After thrust reduction, and prior to 3000ft the first pack is turned on and then the remaining 2 after that.This allows more thrust to be produced which can either be used to increase payload or to get a greater TO thrust derate and therefore reduce engine wear and fuel consumption. The bleeds on the 747 are not turned off unless conducting maintenance, Despatch requirement or as called for in a abnormal checklist procedure.

[Qavion 19]

"Yes, this is what they are doing bleed on and packs feeded by APU."Only one pack is fed by the APU, the Bleed Isolation Valves are closed.I believe one pack is kept on to give the cabin some kind of pressurization for takeoff. A steep pressure rise is created on rotation which is uncomfortable for the pax, so the cabin pressure is increased to an altitude 50' below the airport.Cheers.Q>

[Guest pallehj]

GentlemenConsider this, a slight improvement in the comfort for the passengers who pays for the flight.CheersPalle

[cowpatz 243]

Actually Palle you are looking at about a total time of 2 to 3 min without packs. Unless in very hot climates it is not noticeable. I have done may packs off TO's (as crew and passsenger) and have not felt any discomfort whatsoever. I had heard of the pressure bump on TO but this seems to be something that was designed for but in reality is a non event.

[markadeane 272]

Not 744 related strictly but I've had the priviledge of two Dash 8 300Q jumpseat rides in the past week. The bleed air is controlled by a variable dial (ie not just 'on' or 'off'), on the first one the crew flicked it right on after clean-up and the effect was an most abrupt popping of the ears, not uncomfortable as such but definitely noticeable. On the second sector the captain (PNF) eased it back on over a period of 20 seconds. The result? Unnoticable.Steve mentions the gradual reactivation of the 744 packs, ie one prior to 3,000 feet and the other two afterwards. One could assume that is at least in part to ensure comfort is maintained?