BLEEDS ON Takeoff Weight Penalty

[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

[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.

[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).

[Guest frankathl]

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

[Guest frankathl]

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

[Guest BlackDog]

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

[victorlima01 2]

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?

[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

[marcom 139]

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

[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

[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

[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

[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...

[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

[marcom 139]

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