What is the point of a bleeds on takeoff?
Joined: May 2004
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From: Godzone
The reason most companies do "bleeds off" take offs is very simple...money!
By going bleeds off you get a slightly better flex temp & therefore less engine wear.
(Well thats the bean counters view, before you go down the road of pack valve failures etc...... )
Seem to remember on the 73 it gives you about another ton on the MTOW for conditions on the day.
By going bleeds off you get a slightly better flex temp & therefore less engine wear.
(Well thats the bean counters view, before you go down the road of pack valve failures etc...... )
Seem to remember on the 73 it gives you about another ton on the MTOW for conditions on the day.
Joined: Jun 2002
Posts: 336
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From: Dubai
I'm with mutt on this. I ran the numbers on the A343 laptop one dayfor fun. Made about an 8 ton difference to a sea level airfield. This is no big deal since, more often than not, we seem to be limited by landing weight, but it does translate into roughly 6 deg of flex for reduced thrust take off.
Last edited by Kennytheking; 23rd December 2007 at 10:44. Reason: spelling
Joined: Mar 2002
Aviation Qualifications: ATPL
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From: Seat 1A
What's the point? Passenger comfort! Make the ride nice for the people that are paying the company it's revenue. Give htem a nice, cool, no-ear-bump ride and they might come back.
In the little Boeing Mad Dog, there is only an advantage going packs off on short runways (about 4% RTOW advantage). There is no advantage on a long runway because the packs shut down automatically if an engine fails when airborne.
In the little Boeing Mad Dog, there is only an advantage going packs off on short runways (about 4% RTOW advantage). There is no advantage on a long runway because the packs shut down automatically if an engine fails when airborne.
Joined: Dec 2000
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From: Canada
Remember the reduction in engine wear from flex is not a linear function. So I would agree that if doing a packs off takeoff resulted in FLEX vs. full thrust it is beneficial, but if we are already achieving significant FLEX without packs off then selecting them off may provide very little benefit at all.
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A 'Packs Off' Take-Off actually puts the engine in a MORE critical bleed configuration than a 'Packs On' T/O. The engine in a Packs Off take- off has a reduced compressor stall margin due to the absence of 'bleed offload' via the packs. Various 'power by the hour' engine contracts call for periodic High-Power engine runs (usually 'ground runs') for performance retention guarantee. Generally, you can show compliance by a similarly limiting take-off configuration if you wish to avoid the maintenance costs / down-time of the high-power ground run.
Joined: Dec 2001
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From: Canada
excessive
Not sure what you are flying MUTT... could be. Mr Boeing calls for a Penalty from 1700 to 2700 dependant on temp and alt for the 76. Really not much of a hit. Packs on when we can (90%) and off when we have to. What you flying that takes such a hit?
Top Dog
Joined: May 2000
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From: Close to FACT
744F. Couple of tons gained with packs (bleeds) off take-off. I remember doing HRE-LUX 384T t/o weight, flaps 20, about 30-32 celcius.... (HRE is roughly 4500ft ASL, 14K ft rwy). Used every inch....
Joined: Feb 2005
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From: flyover country USA
My memory of the 747-200 AFM is that baseline rated TO performance is based on APU running, and that an APU OFF takeoff incurs a few hundred Kg TOGW penalty.
I was told by a Boeing flight ops guy that the APU actually generated a small bit of thrust, and that Boeing treated this as a 4.001 engine takeoff (as opposed to 4.000 engines). Don't know if this is still true.
I was told by a Boeing flight ops guy that the APU actually generated a small bit of thrust, and that Boeing treated this as a 4.001 engine takeoff (as opposed to 4.000 engines). Don't know if this is still true.
