Pack off vs Bleed off

[flapsupdown]

For improved Take Off performance - Bleed off and Pack Off - are these two the same concepts?

And for take off, does this mean the cabin is not pressurized?? or just that there is no air conditioned air??


Thanks!

[Pub User]

Yes they are the same concept. The cabin is normally pressurized (and conditioned) by the APU in such circumstances.

[Lord Spandex Masher]

Sooooo, packs on then!

[FlyingStone]

Well, in 737 the bleeds off takeoff is a more correct term, since engine bleeds are off if we are performing an "A/C OFF" takeoff, while both packs are on AUTO, but only the left one is being supplied with APU bleed air.

Cabin is pressurized during both normal and bleeds off takeoff and the air supplied to the cabin is conditioned, provided the left pack is operating normally.

[Capt Claret]

In the Douglas/Boeing 717, I don't recall seeing performance charts/options to take off with bleeds off.

We do have the option of selctng the packs off. This is toggled via the FMS/MCDU, and causes the packs to shutdown with the application of take-off power (gee it's quiet) and the packs automatically come back online at 3000 AAL.

[WhySoTough]

I am a newbie myself, but will post what I understand, and hopefully someone can correct me as well please. (A320)

I usually hear and read packs off(not bleed off)
This way bleed is just not supplying packs but still wing anti ive, potable water tank pressurization etc.

Also I remember reading somewhere that packs off isn't allowed in very high elevation to risk forgetting to turn them on or something which will cause aircraft to be unpressurized. So yes air conditioning and pressurization.

[framer]

Boy the 717 sounds like a good system compared to the 737.
A fair few flights have had the masks drop due to crew forgetting to reconfigure the bleeds or configuring them incorrectly.

[FlyingStone]

Originally Posted by WhySoTough

Also I remember reading somewhere that packs off isn't allowed in very high elevation to risk forgetting to turn them on or something which will cause aircraft to be unpressurized. So yes air conditioning and pressurization.

The higher the airport elevation, the greater chance the aircraft can't takeoff with engine bleeds on, since the performance limited takeoff mass (PLTOM) is lower due to higher density altitude and one way to increase PLTOM is to perform a bleeds off takeoff.

[Capt Claret]

Boy the 717 sounds like a good system compared to the 737.
A fair few flights have had the masks drop due to crew forgetting to reconfigure the bleeds or configuring them incorrectly.

You're not wrong there framer. I think I've had two jump seat rides on a 737 flight deck over the years (my sole 737 experience) once a -200 series with Ansett and the other an NG with Qantas.

I was amazed when on the NG, ther crew made a step climb and the F/O had to reach up to the overhead panel and reset the pressurisation system! . The 717 is all driven through the FMS, and for most ops, simply monitored with no pilot input except telling the FMS what aerodromes one is going from and to.

[flapsupdown]

Thank you for all the responses so far =)

[Clandestino]

For improved Take Off performance - Bleed off and Pack Off - are these two the same concepts?

Similar but not quite the same. It is type dependent. If more performance is required, A320 takes off with bleeds on, packs off while Q400 uses bleeds off. I'll be darned if I understand why, I just fly it as told.

[Blinkz]

Bleeds off and Packs off are NOT the same thing. Its important to know the difference. For the A320:

Bleeds off takeoff generally means that the Packs are being supplied by the APU bleed and the engines will have maximum power for take off (or more flex depending on why you are doing bleed off) The cabin will pressurise as normal. There will be a point (usually when flaps are retracted that the APU bleed is switched off and the engines take over supplying the packs and the APU is switched off.

A packs off take off means that the packs will be OFF, not supplied by either the engines nor APU. The engines will still have the same power as a bleeds off takeoff however the cabin will NOT be pressurising. A very important fact to remember if you don't want to do another Helios. Again at a point, usually once flaps are retracted you will then switch the packs back on (and usually one at a time so that you don't over pressure the system and get a fault!)

[Capt Claret]

The 717 APU Bleed Air is not certified for airborne use, hence for us, the only option is packs off.

[FE Hoppy]

Ejet:

Pilot sets take-off data on the thrust Rating System page of the MCDU. The bleed options are ECS(environmental control system) on/off and AI (Anti-ice) Off/ENG/ALL.

The aircraft configures the engine beed valves, pack valves and anti-ice valves automatically.

If you've asked the engines to not supply air for take off, they won't until either 500' all eng operating or thrust reduction after an engine failure.

If the APU is on it will provide bleed air for pressurisation until the engines take over at the above stated alt except if you have requested anti ice ALL. In this case the engines will provide the bleed air for anti ice and the pack valves will remain closed as per the above logic. (APU cannot provide wing anti ice as the demand is too high).

simples

[Intruder]

The 744 requires 3 bleeds operating normally to dispatch, so it's either "Packs Off" or "APU to Pack 2" for takeoff when less bleed air use is required for takeoff.

[safelife]

One of the reasons why there is no "bleeds off" take off on the Airbus is that the bleed is used for purposes other than the packs as well.

Actually it is one of the things you have to learn early on this time: APU bleed on, packs off on the ground: to have the coffee machine working! No bleed, no coffee, no joy.