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Guest frankathl

BLEEDS ON Takeoff Weight Penalty

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Guest frankathl

Why does a weight correction have to be added to the actual gross weight of the aircraft for a BLEEDS ON takeoff? In the case of the -700 it is +4000 pounds(worst case scenario). Is it that the use of engine bleed air reduces the power available for takeoff?Thanks,Frank

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Guest Hawkeyeted

Yes. When bleed is on, you have effectively reduced the pressure inside the engine, thus the amount of available thrust. Most other airframes take the penalty in the form of runway length, vice weight, but either will do.

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Guest hlm65

Hi Frank,yes, tapping air from engine compressors will "spoil" available thrust. This is the reason why in hot/high airports with heavy TO weights, it's imperative to make a bleed OFF takeoff (and use APU bleed instead).

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Guest frankathl

Thanks Ted of the Hawk Eyes! :-)Makes sense!BR,Frank

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Guest frankathl

Yep, got it, Enrico! Suspected as much, but thanks for the good explanation.Best wishes,Frank

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Guest BlackDog

A question, does the PMDG model for this loss of engine effiency when the bleeds are on?

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Yet another question: Is it normal procedure to keep the APU on and use its bleed air for such takeoffs or should we just turn off the packs and keep the passangers hot for a couple of minutes? Oh, this raises a last quesiton: what do we turn off? the bleed switches or the packs?

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Guest frankathl

This implies that the Takeoff N1 calculated by the FMC or taken from the performance tables is for a BLEEDS OFF takeoff. Correct?BR,Frank

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Yes, normally you'll use the APU to drive the packs. The NG's APU is quite powerful so you should be able to operate both packs. On the classics you'd only operate one pack. Usually the one that supplies the flight deck ;-)Bleed air isn't just important for the packs, but for correct operation of the hydraulic system (the reservoir has to be pressurized by bleed air to maintain positive flow). On other aircraft, where hydraulic pumps can be operated pneumatically (B767 or B747), it would be wise to have the APU running just as a redundancy should you lose the primary pumps.Regards,Mark

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Guest hlm65

Hi,it's normal procedure to have the APU running to performa Bleeds OFF takeoff.The checklist regarding this procedure is as follows:1-APU ON2-Right Air Cond Pack Switch: AUTO3-Isolation Valve Switch: CLOSED4-Left Air Cond Pack Switch:AUTO5-N.1 Engine Bleed Air Swtich: OFF6-APU Bleed Air Switch: ON7-N.2 Engine Bleed Air Switch: OFF

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Guest hlm65

Hi Frank,it's not immediately clear if FMC calculates a Bleed Off takefoo.From the Flight Manual it's clear that the Accuload calculates a Bleeds Off takeoff and the performance charts seem to say the same. They say to add "xxx" lbs for bleeds ON manual calculations.Regards

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Guest frankathl

Thanks, Enrico! I should have remembered the ACCULOAD data.By the way, what's a "takefoo"....is it when you take off backwards?...Hey, just kidding! :-)BR,Frank

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Guest hlm65

Hi Frank !for a backwards takeoff you should read "ffoekat"...or even "anti-landing" (that's not an autoland !)...still kidding !I'm not sure if the FMC calculates the bleed on or off N1 numbers. I didn't find a mention of it in the Flight Manual (but I could have skipped it somewhere...). There should be a way to tell the FMC what kind of TO are you planning.But I'm almost sure it calculates N1 values for bleed off takeoff, that is with the maximum available thrust in the current conditions.Wow, if there were a "simming" version of the ACCULOAD, we could have our flights "perfectly" planned...

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Guest frankathl

"Wow, if there were a "simming" version of the ACCULOAD, we could have our flights "perfectly" planned..."Mats Johansson is working on this project, even as we speak!You are Mats, aren't you? Mats?.....Mats?.....There, told you, Enrico! He's just too busy even to answer. :-)Frank

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On the classic, the the FMC calculates N1 based on if you select bleeds on or off on the overhead!So if you leave turn them off, the N1 will actually change!Not sure 'bout the NG, but I _suppose_ it will be similar.Regards,Mark

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I'm here Frank.... Frank?Yes, I am still working on it. I have gathered a group of real-world dispatchers and a couple of pilots as well who will help me with the more intricate problems. Unfortunately my other commitments have been taking a heavy toll on the time I have available for the development of the VIrtual DIspatcher. But I see light in the tunnel. Don't mistrust. ;-)Thanks,

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Guest frankathl

Much thanks for letting us know things are coming along, Mats! I'm sure many people are excited to hear that your project is alive and well."Don't mistrust. ;-)"On the contrary, Mats, I feel a special attachment to your virtual dispatcher, given that it was first revealed here on Avsim in a thread I started(now, how's that for trying to associate myself with greatness? Grin!)!"But I see light in the tunnel"...which, hopefully, isn't the 15:30 from Toronto!:-)Best wishes,Frank

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Guest Rajah

mmm, interesting. One problem. I don't quit understand the terms-Bleed air- and -Packs- in this context. I know it has to do with the aircondotion, but I don't get to the root.Frans Dekker / TFF074

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A jet engine compresses air coming in the front of the engine with rows and rows of small compressor blades. Fuel is then added to this compressed air and the fuel-air mixture is ignited to produce thrust. However, before the fuel/fire is added, a small quantity of air is tapped off (extracted) from the engine. This air is at high pressure (and is quite hot from being compressed). This air is known as "bleed air". This air is delivered to a bleed air distribution system (a network of pipes and valves). Some of this air is used to turn airconditioning compressors/turbines which compress, cool and expand the air. These compressor/turbines are known as "Packs". In this way, the air from the engines is used in the cabin for pressurization and cooling/heating.Hope this helps.Cheers.Ian.

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Guest Hawkeyeted

Enrico,I don't think the FMC calculates for Bleeds on either. However, this is the only airframe if've seen where you take a weight penalty, vice a runway lenght penalty, for bleed air.And, as Mark said, running the APU back-up for hydraulic systems is good measure too. As he said, the hydraulic reserviors have a head in them that are pressurized pneumatically. If you were to loose an engine (or two!) you could have the APU driving the hydraulic res, and at least have some measure of control of the aircraft.

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"If you were to loose an engine (or two!) you could have the APU driving the hydraulic res, and at least have some measure of control of the aircraft."Don't forget that the 737 has manual reversion anyway (the pilots can still move the controls if hydraulics fail).Cheers.Ian.

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As I already said, the classic FMC calculates N1 for a bleed on takeoff if the bleed switches are on and for a bleeds off takeoff if the switches are turned off.Regyrding the APU: you don't really run it as a backup as a full hydraulic loss is considered too unlikely. But as a bleeds off takeoff is not a non normal situation, ie COULD happen on many takeoffs, it would damaga the hydraulic pumps if they would run without their reservoirs being pressurized.I believe some 737 operators will require their crews for running the APU during when LVP operations are in effect (low vis) as on the classic, if you lose a generator, you'll lose a lot of systems on the affected side. The APU can then power the offline systems.Regards,Mark

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Guest Rajah

Ian,Thanks for your explanation. It's very clear now!Frans Dekker / TFF074

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