Hi davierosoft,I've actually got no confusion about this at all and have read the FCOM part 2 and looked at the schematics for pressurisation. In the past two pages there's quite a number of incorrect statements.Spin737 stated right back at the start that packs were for pressurisiation and that it's the engine bleeds that you need off if you need to increase thrust. The only thing that hadn't been spelt out in his summary was why you would ever do a packs off takeoff in an NG (as opposed to other aircraft like the 747). Based on my non-professional knowledge of the NG it seemed that you would only do this in a bleeds-off takeoff when you're not planning to use the APU - as stated in my post above. I can't think of any other reason you'd do this i the NG, and spin737's reply seems to have comfirmed this.I've found this a useful thread for clarifying an area I thought I fully understood but which I am now much more confident in.Your reply showed a lack of understanding of my question/clarification - perhaps read posts more carefully before telling people they're confused and need to rtfm.

If your knoledge about the pneumatic/airconditioning is so sufficient, the difference between bleed off and packs off may be obvious.And, as JET 1 said, with bleeds off the pack switch position is like a light switch during a black out... completely useless.So, from the first page we are talking about the same things. I don't think there is something that has not already answered.The aircraft can take off with or without packsIf they are used it is better for structural reasonsApu can be used to supply pressure and let you use the engine at full power.You can use packs off (only packs) you will recover the 99,9% of the engine powerYou can use bleed off (and packs are obviously off as they work with pneumatic pressure) and you will have the 100% of itThe difference from only pack off and bleed off is the remaining of the pneumatic system as shown on the FCOM, including potable water tank pressurization, hydraulic reservoir pressurization, and eventually all the leakages.The schematic is sufficient to show all these things.

See 6:50 in the video. This boy has fun!

If they are used it is better for structural reasons

Can you explain that sentence? If your packs are off the plane will not be pressurised, which will mean a smaller differential pressure with the outside atmosphere, which will mean less stress in your fuselage shell, which from a structural point of view is better.For structural reasons you should actually never pressurise your fuselage at all, but your passengers and crew might complain about that.

Can you explain that sentence? If your packs are off the plane will not be pressurised, which will mean a smaller differential pressure with the outside atmosphere, which will mean less stress in your fuselage shell, which from a structural point of view is better.For structural reasons you should actually never pressurise your fuselage at all, but your passengers and crew might complain about that.

Actually, I beg to differ.Take an unopened 2L of Coke and go stand on it. Then drain said 2L of Coke, leave the cap on, and stand on it...Which one is more structurally sound?

Edited January 13, 201214 yr by scandinavian13

Actually, I beg to differ.Take an unopened 2L of Coke and go stand on it. Then drain said 2L of Coke, leave the cap on, and stand on it...Which one is more structurally sound?

..what he said + passenger comfort reasons. Not an aircraft tech, but if my memory serves me right, aircraft fuselages are designed to be operated pressurized.Search for "pressure bump" and you'll find a lot of material on the comfort subject. Gots to do with outflow valve operation during liftoff and the rapid cabin pressure change it causes.

Hi fellas. I took it from our company Aircraft Manual:Bleeds On Takeoff: Right Air Conditioning Pack.............AUTO Isolation Valve ................................AUTO Left Air Conditioning Pack ..............AUTO No. 1 Engine Bleed Air Switch............. ON APU Bleed Air Switch .........................OFF No. 2 Engine Bleed Air Switch…...........ON Bleeds Off Takeoff: (APU bleed used for air conditioning): Right Air Conditioning Pack Switch.......AUTO Isolation Valve Switch ..........................CLOSED Left Air Conditioning Pack Switch ........AUTO No. 1 Engine Bleed Air Switch ...............OFF APU Bleed Switch.....................................ON No. 2 Engine Bleed Air Switch ...............OFF Wing Anti-Ice Switch...............................OFFThe WING ANTI-ICE switch must remain OFF until the engine bleed air switches are ON and the ISOLATION VALVE switch is in AUTO

Hi fellas. I took it from our company Aircraft Manual:Bleeds On Takeoff: Right Air Conditioning Pack.............AUTO Isolation Valve ................................AUTO Left Air Conditioning Pack ..............AUTO No. 1 Engine Bleed Air Switch............. ON APU Bleed Air Switch .........................OFF No. 2 Engine Bleed Air Switch…...........ON Bleeds Off Takeoff: (APU bleed used for air conditioning): Right Air Conditioning Pack Switch.......AUTO Isolation Valve Switch ..........................CLOSED Left Air Conditioning Pack Switch ........AUTO No. 1 Engine Bleed Air Switch ...............OFF APU Bleed Switch.....................................ON No. 2 Engine Bleed Air Switch ...............OFF Wing Anti-Ice Switch...............................OFFThe WING ANTI-ICE switch must remain OFF until the engine bleed air switches are ON and the ISOLATION VALVE switch is in AUTO

great stuff man, this helped for everybody thanks mate!

Can anyone please explain how the isolation valve should be set (openautoclosed) in normal operations in relation to packs and bleeds. If I've missed this in the above posts then apologies. :(

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

"Pack" is not an acronym, it's an abrieviation of package, as in "air conditioning package". You're quite right though that without bleed air the pack won't run.Kevin Hall

We must use a can of coke a bit longer with a thing called wing attached under it.Now, on take off phase the wing (where main landing gears are attached) will suffers of some vibration due to runway surface.This vibration is partially transfered to the structure. If the can is pressurized (positive pressure) the can will resist and will not flex. Without it the cabin will have a banana effect normally visible on -4/8/900 planes when on ground.I must search pictures about it.Remember that if fuselage deflect when is empty and parked, when it is doing rotation (with stabilizer and elevator forcing down and wings up) the stress will be increased.The pressure controllers will close the outflow valve automatically at the beginning of the take off phase, exactly when the throttles are advanced.The controller will add 0.1 PSID of pressure in the cabin and then, after lift off, it will start its cabin climb profile.Same thing on landing, with the difference (if altitude is set correctly) that the pressure will be .15 and not .10 PSID. This because on landing the stress is bigger.

Can anyone please explain how the isolation valve should be set (openautoclosed) in normal operations in relation to packs and bleeds. If I've missed this in the above posts then apologies. :(

That was not mentioned previously in that topic, but there are others where the logic was explained.Basically the isolation valve isolates left and right pneumatic duct preventing a dual bleed conflict (2 sources for the same users)It has a switch with 3 position, 2 are obvious, and the auto function will automatically open or close the valve depending on bleeds/packs switch position.Basically the valve is closed only when all ENGINE bleeds and packs are ON (auto or high for packs).It will automatically open if one of them is switched off.ENG1 BLEED (ON)ENG2 BLEED (ON)LH PACK (AUTO OR HIGH)RH PACK (AUTO OR HIGH)Isolation valve closed.Either of the above offISOL closes.WARNING It is linked to the pack/bleed switch position, it will not check if the bleeds or packs are operating.Company checklist sometimes forces pilots to move the switch to the open or closed position also if in the AUTO position the logic commands the same thing, it can be for safety reason I can immagine.

That was not mentioned previously in that topic, but there are others where the logic was explained.Basically the isolation valve isolates left and right pneumatic duct preventing a dual bleed conflict (2 sources for the same users)It has a switch with 3 position, 2 are obvious, and the auto function will automatically open or close the valve depending on bleeds/packs switch position.Basically the valve is closed only when all ENGINE bleeds and packs are ON (auto or high for packs).It will automatically open if one of them is switched off.ENG1 BLEED (ON)ENG2 BLEED (ON)LH PACK (AUTO OR HIGH)RH PACK (AUTO OR HIGH)Isolation valve closed.Either of the above offISOL closes.WARNING It is linked to the pack/bleed switch position, it will not check if the bleeds or packs are operating.Company checklist sometimes forces pilots to move the switch to the open or closed position also if in the AUTO position the logic commands the same thing, it can be for safety reason I can immagine.

Thanks Andrea, so for normal operations (no failures or emergencies) the valve would be set to auto except for start-up when it is open?

As I told, the valve is closed only when all switces are ON.Now, when you start the engines you have to turn off packs, so, the valve will automatically open.This is why I told about the checklist that forces sometimes to manually move the valve in open position when, in AUTO, the valve is already opened.I always leave the valve in AUTO as the logic is normally correct.I found few pictures in my "old" archive about the "banana" effect on the lower aft fuselage when the aircraft weight forces the skin to flex just after the landing gears.Here is on the grey part of the fuselage (left in picture)Here it can be seen in the grey part, after the cargo door.And, years after years, probably with few take off without packs, ard landings and more, in that area it is possible to find cracks, and they must be repaired as you can see here:

Edited January 14, 201214 yr by davierosoft

That was not mentioned previously in that topic, but there are others where the logic was explained.Basically the isolation valve isolates left and right pneumatic duct preventing a dual bleed conflict (2 sources for the same users)It has a switch with 3 position, 2 are obvious, and the auto function will automatically open or close the valve depending on bleeds/packs switch position.Basically the valve is closed only when all ENGINE bleeds and packs are ON (auto or high for packs).It will automatically open if one of them is switched off.ENG1 BLEED (ON)ENG2 BLEED (ON)LH PACK (AUTO OR HIGH)RH PACK (AUTO OR HIGH)Isolation valve closed.Either of the above offISOL closes.WARNING It is linked to the pack/bleed switch position, it will not check if the bleeds or packs are operating.Company checklist sometimes forces pilots to move the switch to the open or closed position also if in the AUTO position the logic commands the same thing, it can be for safety reason I can immagine.

Maybe a little typo Andrea ?Fred.

Guys, as long as you are here,theoretical situation:I am performance lmited, thus packs off, but conditions require usage of antiice (wing and engine).my dispatcher says I can do that.How do I set up the air system?

Edited January 14, 201214 yr by Fabo