You can do either, but why? If you want to takeoff with the ENG Bleeds OFF, it will give you more thrust. However, you will typically use the APU bleed for pressurization/air conditioning.

Could use, but not typically. The time/altitude from takeoff to thrust reduction, during which the packs are inoperative, is negligible from pressurization point of view.Using the APU on every takeoff would come with a maintenance/fuel penalty.There could be other MEL requirements that dictate it must be used though, such as a non-functional engine driven generator.

Just to clear the major doubts:

Hey , thanks all,i thoughts engine bleeds are pumping pressure for vital systems like APU AND ENGINES, and the packs are controlling on the pressurization and air conditioning, am i right here?

Engines and APU are the sources where pressurized air is bleeded into ducts inside the plane to supply pressure to some users.The users are basically antiice and packs.The packs convert hot pressurized air from APU or ENGINE into fresh/warm air to be blown into the cabin.A valve controls how many air is exhausted overboard when the packs are on.If the air blown in is more than the air going out the aircraft will pressurize, if the air exhausted is more the aircraft will depressurize. The valve(s) control the amount of air to let passenger to stay alive and comfortable and always with clean and conditioned air.So.To pressurize the plane, packs must be on (at least one), and there must be avaiable at least one source (bleed)The turbine engine (turbofan is a turbine engine) use air for its work, it will compress, heat, and exhaust the air in a cycle to give thrust.If you take from it some air, the resulting thrust will be obviously less.Packs off or bleed off can be used to restore the missing thrust, BUT IT IS A CHOICE related to some factors like the runway, OAT, runway elevation and weight.If you need all the thrust from your engines, you can do a bleed off procedure take off, but you can use the third pressure source (APU) to pressurize the aircraft.This method will help you by pressurizing and still having full thrust.There is no "wrong" or "correct" thing about what is on or off, it is related on the situation.On most of normal conditions, you will take off with packs on and bleeds on, all feeded by engines.

The program called TOPCAT will assist you with determing if you can take off with the packs on or off based on weight, runway length and weather.

Could use, but not typically. The time/altitude from takeoff to thrust reduction, during which the packs are inoperative, is negligible from pressurization point of view.Using the APU on every takeoff would come with a maintenance/fuel penalty.There could be other MEL requirements that dictate it must be used though, such as a non-functional engine driven generator.

I don't think we are speaking about every take off.This procedure applies only when the conditions requires full thrust, more thrust that packs on can give.In this situation the APU is te best solution.The pre-pressurization that starts as soon as you advance the throttles will prevent some fuselage stress.

great job guys, really beautiful answers, enjoyed to read them, nice one!

Just to clear the major doubts:Engines and APU are the sources where pressurized air is bleeded into ducts inside the plane to supply pressure to some users.The users are basically antiice and packs.The packs convert hot pressurized air from APU or ENGINE into fresh/warm air to be blown into the cabin.A valve controls how many air is exhausted overboard when the packs are on.If the air blown in is more than the air going out the aircraft will pressurize, if the air exhausted is more the aircraft will depressurize. The valve(s) control the amount of air to let passenger to stay alive and comfortable and always with clean and conditioned air.So.To pressurize the plane, packs must be on (at least one), and there must be avaiable at least one source (bleed)The turbine engine (turbofan is a turbine engine) use air for its work, it will compress, heat, and exhaust the air in a cycle to give thrust.If you take from it some air, the resulting thrust will be obviously less.Packs off or bleed off can be used to restore the missing thrust, BUT IT IS A CHOICE related to some factors like the runway, OAT, runway elevation and weight.If you need all the thrust from your engines, you can do a bleed off procedure take off, but you can use the third pressure source (APU) to pressurize the aircraft.This method will help you by pressurizing and still having full thrust.There is no "wrong" or "correct" thing about what is on or off, it is related on the situation.On most of normal conditions, you will take off with packs on and bleeds on, all feeded by engines.

Okay, I think I can follow that but here's a question. Under what conditions would one set the the packs to "High"? Also, when should the recirc fans be set to "Off" or "Auto". I've been wondering about that.

For example in smoke in the cabin situation. For more information, consult your QRH.

Edited January 12, 201214 yr by Fabo

Ryan, out of curiosity. Is this simulated? Obviously the procedure is, but is performance changed if bleed air devices are used. ie Packs, de-ice

The engine bleeds and packs are but not sure on the de-ice.George

Edited January 12, 201214 yr by falcon999

Auto is clearly understandable, the packs can work in normal or high flow condition.Auto is the normal position for all switches with the AUTO position (as the aircraft will do the choice for you)High flow will be automatically triggered for example if you're in flight and a pack fails, the other one, in AUTO, will automatically starts to operate at high flow.Just to say numbers (Memory, so they maybe incorrect) The normal mode flow for each pack is around 50-60 PPM (pound per minutes), in High mode with engines, over 100 with APU.To have all this values automatically chosen by the system, just put the switch to AUTO always.Use HIGH only in hot summer days, on ground, when you want to stay fresh. Pay attention that this may cause a pack trip off,RECIRC fan(s) is (are) blowers that creates an airflow that reuse the already conditioned air, and re-mix it with new air coming from the packs.They must be on to improve passengers air recirculation.As Fabo told you, if you have problems like smoke, the first thing you must do is to not expand the problem to the entire cabin.The recirculation fan(s) uses cabin air to recicle it and mix it with new air from the packs. If smoke is present it will be also remixed and then again blowed but in the entire cabin.

The engine bleeds and packs are but not sure on the de-ice.George

Wing AI will trip off on take off, not sure about eng cowls.I'm sure that in flight idle control will change with cowl ai, but not sure if them remove a bit of resulting thrust.

To the OP - packs off actually gives you MORE thrust. It sounds like you're thinking the packs are a vital part of the engine's thrust-developing capability or something - not true, the bleed air load they place on the engine actually reduces thrust by a substantial amount. If the plane is heavy or the temperature and/or altitude is high, the packs usually get turned off for takeoff so that the engines develop more thrust.

Just to add a little to that !A no engine bleeds takeoff would be done with both engine bleeds turned off and the APU bleed turned on to provide bleed air to the left pack.The APU would obviously be running.The isolation valve would be closed.Fred.

Just to add a little to that !A no engine bleeds takeoff would be done with both engine bleeds turned off and the APU bleed turned on to provide bleed air to the left pack.The APU would obviously be running.The isolation valve would be closed.Fred.

My friend at US Airways regularly (Summer months) performs no APU Bleeds-off takeoffs. Of course that's in the Classic 737

Edited January 13, 201214 yr by ZachLW

Packs = Pressurization and Air Conditioning Kit...They don't run with no bleed air...hence, bleeds off, Packs off.

Edited January 13, 201214 yr by StewCal

I liked the video, even though it was a scarebus. Captain very cool customer.

Fred, is this different from the normal operation on ground (both packs, ISOL open or auto) for some higher reason?

@Peter !Yes, it is a bit different.Instead of me trying to explain it I will use some pics.You use a letter " C " type scan to set the switches for a No Engines Bleed takeoff or landing.( A no engines bleed landing could be used if extra go-around thrust is needed )After takeoff you do the same switch scan but in reverse order.It is recommended by Boeing that before you turn back on the No 1 engine bleed, that the Cabin Rate of Climb is stable !This makes sure the APU bleed valve is fully closed before you open the engine bleed valve.I hope the Pics are clear and easy to follow.Fred.