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Guest frankathl

BLEEDS OFF Takeoff

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Guest thscholz

>Kris wrote:>>Just to outline one big consideration when doing bleeds off>take offs, you MUST remember to have the Bleeds off when the>APU bleed is ON. The problem is that engine bleed air when>engines are above idle can back pressure the APU and cause>major damage to it. Effectively the back pressure tries to>turn the APU backward and tends to melt things! This is why on>the B737 we don't tinker with Boeing procedure when it comes>to air systems. >Gentlemen, you're starting to confuse me:Kris (B737-300 F/O) says that you have to turn engine bleeds off when you turn APU bleed on.Brad Marsh (B737NG Captain) in his Geezer Guide for the PMDG says 'APU Bleed ON (Engine bleeds are always left ON and only ever turned off as part of an emergency checklist)'.:-hmmm Are you guys sure that you know what you're doing?:-lol Edit: On second thought...if you leave engine bleeds on like Brad says, it will damage the APU as Kris says, so you get the emergency that Brad mentions and you have to turn the engine bleeds of as Kris says.:-hah CheersThomas

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Guest jbB737

>Brad Marsh (B737NG Captain) in his Geezer Guide for the PMDG>says 'APU Bleed ON (Engine bleeds are always left ON and only>ever turned off as part of an emergency checklist)'.>Well, I hate to say it... but Brad would be wrong. Read my posts above regarding max power, bleeds off takeoffs. Gotta turn off the engine bleeds; we do it all the time, no emergency required.JohnB-737 FO

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Thomas,I can agree it looks confusing at first. What I think Brad is refering to though is the situation when you are on grund with engines in idle. In that case you can have all three bleed air valves ON. But as have been discussed here and earlier. You want to close the engine bleed valves if you rev the engines up above idle. There is an additional check valve in the APU bleed line to prevent bleed air from going backwards into the APU. But it seems noone, not even the manufacturer, trusts this poor valve to handle any greater differential pressures.Cheers,

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Guest thscholz

>There is an additional check valve>in the APU bleed line to prevent bleed air from going>backwards into the APU. But it seems noone, not even the>manufacturer, trusts this poor valve to handle any greater>differential pressures.That's what I was asking myself: Why not simply put a valve?Thank you John and Mats.And...I'm sure that pilots know what they are doing.:-beerchug CheersThomas

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hehe... I'll tell my friend (772 captain) to try that next time they want to put him on hold there. The star we get coming up from Brazil is the Mexico One arrival, commonly called Mexico One "Surprise" Arrival. It's also not that complicated but the surprise part comes from controllers who tend to make it more interesting by making you do a 90 degree left turn way before the actual fix where you are supposed to according to the charts and because of that turn you end having to capture the loc at a truly obscene angle. I think they have no regard for how you plan to reach the IAF at the appropriate altitude as well, because we ended up high and hot.There was also another unfortunate incident last year where a controller vectored an MD-11 to the IAF right thru a CB. The story goes that the PF was a rather green FO and for some reason instead of rejecting the order did as commanded. The result was the aircraft going all the way down to 400 AGL and all sorts of GPWS callouts being made. I thought those controller guys had WX radar as well ;)anyway, we're way off topic now. Cheers, Victorhttp://www.precisionmanuals.com/images/forum/800driver.jpg

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Brad was most likely saying this so that the "L-drivers" wouldn't touch the switches unless they really knew what they were doing ;-)Bleed off is a special procedure. If manuals/tutorials included every special procedure imaginable, the beginner would not be able to cope with the amount of information presented.BTW, am I correct in assuming that one wing will not be anti-iced during these type of takeoffs?TIACheers.Ian

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Ian,If my memory serves me right the EAI bleeds are taken before the bleed air valve. So EAI is possible with BLEED VLVs closed.The APU is not powerful enough to provide air for WAI. But with open ISOL VLV one engine can support both left and right WAI.As for WAI during take-off. The WAI VLVs will close once the thrust levers are advanced, and WAI will trip on air mode on RMLG so really no problem there. BTW. What does "L-drivers" stand for? :-hmmm TGIF,

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>BTW. What does "L-drivers" stand for?Learner ;-) (aka newbie)Cheers.Ian.

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Guest Zapper

Gents,Seems I may have not been clear enough.My apologies for misleading anyone.In the tutorial I stated that Bleeds are switched Off only as part of a QRH drill for ENG Fire etc.. was not thinking of a BO T/O. My oversight. We use BO takeoff reasonably often and it is quite a normal procedure.If this ever happens again please e-mail me poste haste.Again I apologise for any misunderstanding.Oopsa!

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Guest Zapper

Thinking about going fly fishing in a few hours and double posted.This post should fix that .. but won't catch me any fish. :-hahAway to the water!Rgds,

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Guest canyonblue737

> >According to the flight manual, to perform a BLEEDS OFF>Takeoff, the air conditioning panel should be set up as>follows:>>1-APU ON>2-Right Air Cond Pack Switch: AUTO>3-Isolation Valve Switch: CLOSED>4-Left Air Cond Pack Switch:AUTO>5-No.1 Engine Bleed Air Swtich: OFF>6-APU Bleed Air Switch: ON>7-No.2 Engine Bleed Air Switch: OFF>>My problem is with step 3. Since the APU is the air source, if>the Isolation Valve is CLOSED, will this not deprive the right>duct of bleed air pressure and cause the right air con pack to>shut down?>>Thanks,>Frankthe procedure is correct. during takeoff the right pack will not have air so the aircraft will pressurize only from the left pack being supplied air from the APU. when reconfiguring after clean up you follow a reverse "C" pattern. first you put the right engine bleed on. this pressurizes the right pack. then you turn the apu bleed off. this depressurizes the left pack, but you are still pressurizing off the right pack. then you turn the left engine bleed on which gets the left pack going again. then finally you place the iso switch back to auto.in a next gen airplane (not the 3/4/5 series where an APU can only run one pack) you could in theory use the APU to power both packs during takeoff with the iso switch in open. however there is extreme danger for damaging the APU due to back pressure if you forgot to close the ISO switch post-takeoff prior to starting the engine bleeds again. the procedure as written maintains excellent pressurization and air conditioning while at the same time mitigating the chances of pilot error inducing significant pressurization shifts or APU damage.

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Thanks for that informative post! I have a quesiton that adds a little further to the topic however. Suppose you're on the ground prior to engine star and you'll be performing a normal (i.e engine bleeds on) takeoff. Suppose the aircon/press panel was left in the following setup by the previous crew:R Eng Bleed OFFL Eng Bleed OFFAPU Bleed ONISOL VALVE AUTO(APU is on and supplying power to a/c systems)Now we proceed to engine start. Packs off, valve open etc. After starting the engines (selecting ground on the ignition knob and then supplying the fuel feed) what's the next step pressurization-wise?Perform the same procedure described above for bleeds off takeoff? Can I first turn off the APU bleed and only then turn on the engine bleeds or will this depressurize the aircraft since no bleed air from any source is being supplied momentarily? Finnally, will any of the engines do this APU back-pressure thing or only the right one wih the valve open? My guess is yes since the real guys tell us to turn off the apu bleed valve before turning on the left one; right? confusing? I thought so. I make an effort to write the most confusing posts you guys see around here :)Cheers, Victorhttp://www.precisionmanuals.com/images/forum/800driver.jpg

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Guest canyonblue737

>Thanks for that informative post! I have a quesiton that adds>a little further to the topic however. Suppose you're on the>ground prior to engine star and you'll be performing a normal>(i.e engine bleeds on) takeoff. Suppose the aircon/press panel>was left in the following setup by the previous crew:>R Eng Bleed OFF>L Eng Bleed OFF>APU Bleed ON>ISOL VALVE AUTO>(APU is on and supplying power to a/c systems)>>Now we proceed to engine start. Packs off, valve open etc.>After starting the engines (selecting ground on the ignition>knob and then supplying the fuel feed) what's the next step>pressurization-wise?>Perform the same procedure described above for bleeds off>takeoff? Can I first turn off the APU bleed and only then turn>on the engine bleeds or will this depressurize the aircraft>since no bleed air from any source is being supplied>momentarily? Finnally, will any of the engines do this APU>back-pressure thing or only the right one wih the valve open?>My guess is yes since the real guys tell us to turn off the>apu bleed valve before turning on the left one; right?>confusing? I thought so. I make an effort to write the most>confusing posts you guys see around here :)>>>Cheers, Victor>http://www.precisionmanuals.com/images/forum/800driver.jpgAs long as the aircraft doesn't have more than idle thrust there is no danger of APU back pressure so in that situation I would simply put the packs on, put the engine bleeds on and then turn the APU off. it is a much bigger issue in flight because obviously there is the potential for a huge amount of back pressure if ever the apu bleed and engine bleed where supplying air to the same pack at the same time. it is very unrealistic to assume you would have the previous crew leave it the way you describe but lets go to a realistic scenario during engine start...packs off, iso auto, engine bleeds on, apu bleed on... then post start... packs on, apu bleed off (prior to taxi, ie. when you first push the thrust levers forward.) that is the normal scenario and for a brief moment the left pack, left bleed, and apu are all "on" there is no risk.

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Guest rogueman1585

I fly GA aircraft out of KSNA and to see those Aloha 73Gs take off in about 4000 to 4500 feet with all that weight is something else.David

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