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View Full Version : bleeds/packs...warning newbie questions!!


timbotch
28th Apr 2002, 01:11
ok 3 very newbie questions, no need for really exhaustive answers, just a future pilot and current ramper wondering :),
1) packs are used to supply air for air conditioning, correct?
2) are bleeds for engine start?
3) at the airline I work for, when departing from KSNA, noise abatement procedures apply. This calls for a bleeds off takeoff. Why?:D

thanks for your patience, timbotch

fieldlanding
28th Apr 2002, 01:16
Bleeds can be used for engine start, they also can feed the air conditioning packs.

With the blleds ON power is in effect taken away from the engine since air is being removed for other uses and therfore to achieve the same thrust you have to apply more power and thus create more noise, not good in a noise sensitive area!!

Also if you are hot and high you may have to do a bleeds off take off to achieve the required power from the engines.

Checkboard
29th Apr 2002, 10:17
The front half of a jet engine is just a big air compressor. Bleed valves take some of that air, and it is then piped on to various systems:

The hot compressed air is piped to a set of airconditioners, called "packs", so the packs don't "supply air for air conditioning", they cool down the air (and reduce its pressure) so that it can be piped into the cabin for people to breath. The engines (or APU) supply the air, via the bleeds.

Apart from that, bleed air is mostly used for de-icing the leading edge of the wing, and around the front of the engine in flight, by piping it along the inside edge of the wing, letting the hot air spray onto the inside of the surface, heating it up, before it is vented overboard.

Typically engines are started with either an electric starter motor (for the smaller ones), or by using compressed air. For those using compressed air, the air is used to spin up a rotor, a shaft from that rotor runs into the engine where it is geared to the internal compressor and starts it turning. The compressed air comes from either a ground compressor, the APU, or an other operating engine (using its bleed air). So the bleeds aren't used to start their own engine, they are used to provide air to start another engine.

For noise abatement take-offs you want to get as high as possible as quickly as possible, as the furthur away from the ground you are the less noise people can hear. For this reason you can close the bleed valves, as using bleed air from an engine robs it of some power, which of course also means the airconditioners don't work during the take-off. As this is only for a few minutes or so, that's OK - but once airbourne you have to turn the bleeds back on, or the cabin won't be pressurised for the climb.

Having said that, you wouldn't perform a bleeds off take-off just to satisfy noise abatement procedures! You really only perform a bleeds off take-off to meet a climb or short runway requirement.

Hope that makes sense!

mono
29th Apr 2002, 11:14
Carefull Checkboard,

SOP for all A319/20/21 is for a bleeds off T/O.

SKYYACHT
29th Apr 2002, 15:58
Errrr....Mono, but I would use caution there. My company operates A319/320/A321, and we would only do an APU Bleed ON (Or Packs Off if APU Bleed was placarded inop) take off if Aircraft performance (Or lack of) dictates otherwise. I accept that many operator use the Airbus Industrie proscribed method, but not all.

Hope that helps.

thermostat
3rd May 2002, 02:17
Checkboard, Great answer, very well explained.

Mono, FCOM 3.03.11 P1 Before Takeoff-- Airbus SOP states "-Pack 1 and 2..........As Reqd. Consider selecting packs off or APU bleed ON either to improve performance or to reduce maintenance costs by reducing takeoff EGT. etc. etc.
N.B. This is Airbus SOP, not a particular airline's one.
Regards.

Young Paul
3rd May 2002, 09:49
... and when you get full flex even with packs on, it doesn't seem greatly worthwhile losing the engine bleed.