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APU and Engine Bleeds

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I am a bit confused as to what the APU and engine bleeds do and when they should be switched on and switched off in the start up procedure and why. The manuals don't really explain the purpose of things.ThanksRenzo Nicoletti


Bill Miller

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Hi Renzo.This topic has come up on numerous occasions at this forum and been discussed extensively. I suggest you perform a search within this forum and I'm sure you'll find a wealth of information. But to get you started, Bleed is actually short for Bleed air, which is excess (for a lack of better term) air taken from the 5th and 9th stage compressors of the 737's engines. This air is channeled via a conduit system so that it can be used to provide air for the air conditioning system (packs) and for cabin pressurizaiton. It is also important for engine startup. Without bleed air the engines do not start. Your next question would be "but how can the engine bleed air be available for engine startup if they are supplied by running engines and the engines are obviously still not running?" That is where the APU comes in. As you might know the APU (auxilary power unit) is a rear mounted powerplant (engine) just like the ones underneath the aircraft's wings (only smaller) and it is used to power the aircraft on the ground when the wing-mounted engines aren't running. As a matter of fact in some aircraft such as the 777, the APU can be such a powerful engine in order to power the entire plane that it's thrust can be calculated. In other words, in some aircraft APUs can actually make the aircraft move forward! (never used for this purpose though!) But back on track It powers the buses with the electrical energy generated by it so you can have a working flight deck and passenger cabin. Among other things, it also generates bleed air, used by the airplane for airconditioning while on the ground and to startup the engines. So you must have the apu running to get your engines started. The correct way to do this, if I'm not mistaken, is to have APU bleed set to ON (which means that the valve in the conduit system is open and this the bleed air from the APU can get to the engines), engine bleeds to OFF and the isolaiton valve OPEN, and packs OFF (Can't remeber why packs need to be off though). After that you can proceed with the engine startup and finally disconnect the APU Bleed air from the system and start pressurizing the system from the engine bleeds. As long as the engines are on idle you can do that by first turning on the engine bleeds and then switching the APU bleed OFF. Next you set isolation valve to AUTO and packs as well. I cannot remeber the appropriate order of this but as I said you'll find that when you search the forums. Be sure to check out the links I listed as well. Hope I helped. Regards,Victorhttp://www.smartcockpit.com/http://www.b737.org.uk/airconditioning.htmhttp://www.b737.org.uk/pneumatics.htmhttp://www.b737.org.uk/pressurisation.htmhttp://www.precisionmanuals.com/images/forum/800driver.jpg


Cheers,
Victor M. Lima
 

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Guest Danfontan

Victor, you are 100% correct on everything you stated, but to clarify wwhy the packs have to be off:They must remain off in order to concentrate the volume of air being put out by the APU to start the engines. If you leave the packs on the manifolds will not presurize because the air is being used through the packs. Try it sometime by leaving the packs on and trying to start the engines. N2 will not build up enough speed because the volume of air necessary for the starter is being used by the turbine.Hope this helps.

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Thanks for the comprehensive response and the informative links. I will put my new knowledge to test this evening.RegardsRenzo Nicoletti


Bill Miller

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Guest frankathl

Hi Renzo,The bleed switches you refer to operate the APU and engine bleed air supply system upon which the following aircraft systems depend:

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Guest frankathl

Excellent post, Victor!I decided to post mine anyway, because I was just about finished when I saw yours :-), and because I have a question regarding Bleeds Off operation.BR,FrankPS You can leave the Engine Bleeds ON for engine startup. Dan has explained why the Packs need to be off when starting engines.

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Thanks for your informative reply Frank. Very interesting although I am still a little unclear as to the role of the isolation valve.Thanks again.Renzo Nicoletti


Bill Miller

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Guest Danfontan

>Thanks for your informative reply Frank. Very interesting>although I am still a little unclear as to the role of the>isolation valve.>>Thanks again.>>Renzo NicolettiI fly in an E-6B (same CFM-56 engines different accessories) which is very similar when it comes to pnuematic systems but the exact same theory. If you have a bleed air leak in flight you can isolate it by using the valve. Please remember bleed air is extremely hot and will start fires!If you have a leak somewhere between the left engine and the isolation valve you can close both the isolation valve and the engine bleed switch. Therefore the whole left manifold would remain un-presurized with no air leaking out to start a fire. In this scenario you would only be using the right pack for pressurization.Now here's a question for a real 737 pilot: Can the 737NG pressurize with only one pack at FL390? If not, then what is the limited ceiling of operations under these conditions?

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Guest neeraj.pendse

Very informative thread overall! Thanks guys!- Neeraj.

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Hi all. Thanks for the incentive Frank (as usual giving better answers ;) ). And thank you Dan for clearing the packs issue. To answer your quesiton I believe I read in the Continental manual somewhere that one pack or bleed source is enough to pressurize the NG. I just don't know if that goes up to FL390. Might not be able to maintain the max pressure psi difference. Will look into it tonight. Regards,VictorEDIT: because I just found an answer ;)Ok, this comes from a real 737 driver:"Cabin altitude can be maintained with only one pack operating. Two pack operation from a single engine bleed air source is not recommended due to excessivebleed requirements."Since this statement does not specify a ceiling for one pack or Tupac (shakur) (sorry couldn't resist) operation, I'm gonna go ahead and consider that one pack should suffice for the 737's service ceiling...Regarding Franks prior question: are you sure you read that somewhere Frank? In my mind it doesn;t make a whole lot of sense. Maybe you can do that with the right pack to off... In the 777 they do something similar. When conditions demand it, they takeoff with packs OFF as oposed to Bleeds off. The explanation given to me was that since almost all of the bleed air is fed to the packs, you can still use some engine bleed for its other intended purposes during takeoff and not compromise your thrust output since there's little bleed requirement.This has raised a question though. When they perform bleeds off takeoff in the 737, who provides bleed air for hydraulic reservoir pressurization and engine anti-ice(should it be needed)? the apu as well? If so, since the isol valve is closed, then who provides bleed for anti-ice on the right engine?http://www.precisionmanuals.com/images/forum/800driver.jpg


Cheers,
Victor M. Lima
 

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Guest frankathl

Hi Dan,"Can the 737NG pressurize with only one pack at FL390? If not, then what is the limited ceiling of operations under these conditions?"Yes, according to the 737 Flight Manual:"Cabin altitude can be maintained at or below 8,000 feet when the aircraft is at maximum certified ceiling with only one system operating."Note:here, "system" = "pack"The NG series has a max operating altitude of 41,000 ft.BR,Frank

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Guest frankathl

Hi Victor,Yes, it will be the APU providing bleed air to the systems with engine bleeds off. I'm afraid I don't know the answer to your question on wing anti-ice, but I'm sure it'll come!"Regarding Franks prior question: are you sure you read that somewhere Frank?"The question in question(:-)) was: "According to the manual, you can leave the Right Engine Bleed Air Switch ON when performing a Bleeds Off takeoff....Can you explain why?" Yep :-) See your manual, Section 3, page 84.BR,Frank

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Guest thscholz

>The question in question(:-)) was: "According to the manual,>you can leave the Right Engine Bleed Air Switch ON when>performing a Bleeds Off takeoff....Can you explain why?" >>Yep :-) See your manual, Section 3, page 84.>>BR,>FrankFrank,as I understand it No. 2 Engine Bleed is left ON if you PLAN to do a bleeds off takeoff. You are refering to the 'After Start Checklist'.For the takeoff you switch No. 2 Engine Bleed to OFF as well (See 'Before Takeoff Checklist' Section 3, page 106).CheersThomas

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Guest Danfontan

>This has raised a question though. When they perform bleeds>off takeoff in the 737, who provides bleed air for hydraulic>reservoir pressurization and engine anti-ice(should it be>needed)? the apu as well? If so, since the isol valve is>closed, then who provides bleed for anti-ice on the right>engine?Anti-ice blled air is drawn directly from the engine after the pressure regulating valve (fed by the fith or ninth stage blled air) and the bleed valve which is controlled from the cockpit. Therefore you can shut off the bleed air valve for the engine, the isolation valve or have 0 psi throughout the duct and still have anti-ice bleed air.The diagram attached will show what I am talking about.http://forums.avsim.net/user_files/121971.jpg

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