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This question would go to guys flying for the airline/coporate, and such.

How often do you turn on/off the packs during a flight? I was reading an Airbus A320 manual the other day, and in it, it said that packs go on and off several times during start up... before engine start, if your using FLEX thrust for take-off.

And what is the purpose of the packs anyway?


And what is the purpose of the packs anyway?


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The packs are the ones paying for the seats and causing profit for the airline!

We don't usually turn the packs off while inflt except for a non-normal situations. Normally there are 2 packs running while only one is really required to pressurize the aircraft. Simply put, packs are air-conditioning units which cool bleed air from the engines to maintain a desired internal temperature and pressurize the aircraft.

They are turned off either manually or automatically during engine start. You don't want to extract bleed air from the APU to run the packs and start the engines at the same time. Most APU's can't handle that and the engines require a lot of air to start.
K - I'll bite.... what's a "pack"??

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It's a term used to refer to the air cycle machine(ACM)in turbine powered aircraft. Normally, an aircraft has 2 of them for redundancy. The "pack" is a "package" of environmental equipment used to control heat/cooling and also pressurization bleed air to the aircraft. When we "extract" air from the compressor sections of a jet engine we are "bleeding" it to support other systems such as anti-ice and environmental systems. This air, when extracted, is very normally very hot. This is good for anti-icing, but not good for environmental systems.

Simply put, bleed air from one or more high stage compressor sections (relatively hot air) of the engine is routed through the ACM(s) in order to cool it via an expansion turbine and heat exchangers. This provides air-conditioning to the cabin along with pressurization. If more heat (referred to as "trim air") for the cabin is needed, the air by-passes the expansion turbine in the ACM and warmer air enters the cabin. Usually, a combination of the above gives the desired cabin temperature. ACM's are a lot like your car's radiator.

On the ground you can see the "packs" (which can also be operated using APU bleed air) operating by the inlet and exhaust doors of one or more pack units on the center belly of large aircraft. You also might see excess condensed water dripping from the pack(s) during hot humid conditions as the air is being cooled.

You may be dispatched with only 1 pack operating but will probably have some altitude restrictions.
We don't really turn off our packs for takeoff. We have Bleeds and High-Pressure bleeds which draw air from the engines to feed our ACMs, or packs. For takeoff, we operate with the Bleeds and HPs off to give the engine better power, since it isn't drawing air any longer to feed the ACMs or pressurization. However at about 500' AGL, in our climb flow we turn both the Bleeds and HPs on.

For landing, in case of go-around, we turn off the HPs, but leave the bleeds on.

Regarding the packs, we can dispatch with one pack inoperative, provided maintenance comes out and drops an air scoop located on the bottom of the aircraft where an air cart plugs in. We are then restricted to 210 knots IAS max, and cannot fly in icing conditions.
I was riding in the back of a Delta 767 on a hot day in SLC on our way to Anchorage and the captain came over on the PA saying they were going to turn off the pressurization system (packs) to get more power on takeoff.

In the RJ we take off with the APU running the packs, then shortly after takeoff we transfer to the engine bleeds and shut the APU down.
On the MD-11 we have 3 packs, and the default setting in the FMS is for a packs off takeoff. If we do not alter that, the packs and bleeds automatically turn off when we advance the power for takeoff (the bleeds stay on if anti-ice is selected) and the packs then come on at 1500' AGL unless climb power is not selected (it is normally automatic with profile mode selection at 1500'), in which case the packs come on at 4000' AGL.
Hey R2F,

Have you picked up the Turbine Pilot's Flight Manual yet? It's got some pretty good descriptions (albeit basic) or turbine engines and systems. Somewhere in the book there are diagrams for the pressurization systems, and it shows how the bleed air flows through the heat exchangers and through the whole system.

Looks exactly as described above, but for those visual learners out there, it might help.
Hey R2F,

Have you picked up the Turbine Pilot's Flight Manual yet?

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On my list of "Books To Buy".

Right now, I'm all about Gliem's "Commercial Pilot FAA Written Exam" and the FAA "Airplane Flying Handbook".