What is the point of a bleeds on takeoff?
I was having a quiet moment and the thought stuck me - what is the point of a bleeds on takeoff?? Why isnt every takeoff a bleeds off? I am new on the 737 and wondering this. Any clever folk out there know the reason
Cheers |
At a guess, workload management and APU fuel burn...
ATB |
We use it to keep the passengers cool, unless additional weight is required, then its bleeds off.
Mutt |
I used to fly ATRs and every take-off on those were bleeds off.
I believe Airbus standard config for take-off are bleeds off too, but I may be wrong. . . ! |
Bleeds off provides extra engine performance and you can keep the APU running to cool the customers, if you so desire. Most of our take offs were packs off until 1000 ft.
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Bleeds on for pax comfort (on our a/c the APU is for ground use only).
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but I may be wrong Mutt |
More surge margin for tired engines.:}
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Most jets begin to pressurize the aircraft during the takeoff roll so that you don't get a spike after takeoff which is hard on passenger comfort, so the bleeds are left on when performance allows.
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Originally Posted by FliegerTiger
if all bleeds are off, no anti-icing is available
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I'll get me coat!
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And also on the 737 when you start to configure the air conditioning/pressurization panel for "normal operation" there is a marked jump on the cabin ROC when you select the right engine bleed on, and again one when you take the APU bleed out, and so on, until the system settles with both engine bleeds on. Not very comfortable for a pax with a head cold and sinuses hurting like hell...
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I would suspect that there is a good argument to be made that changing the configuration of the Air cond and press during the departure is an unnecessary distraction for most 737 destinations. I've only HAD to do it 3 times for performance. Last thing I want to do is change the system around on a complex SID out of a busy TMA. KISS Keep it simple stupid, works for me.
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Why take off with the bleeds off if you're able to keep them on? I don't understand. The norm is to take off with the bleeds on unless you're too heavy.
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I totally agree with Charterjake. Its very uncomfortable especially for passengers. But we just have to do it for extra payload.
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Surely if you need to go packs off due to weight and are concerned about passenger comfort just pop the apu on.
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Muppet,
Some aircraft are not permitted to use the APU during takeoff:) Mutt |
You don't necessarily suffer a loss of performance by having the packs on. I suspect that on many modern aircraft where the FADEC is running the show unless hot and/or high you get the same EPR regardless.
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A ‘packs-on’ take off removes the opportunity for the error; either not pressurising or not switching over from APU air.
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you could ask...
You could ask why do a reduced thrust take-off.. same thing. There are of course great reasons to do reduced thrust, as there are reasons for pax on.
Two primary reasons for bleeds ON take-off is that 1) you can and 2) for pax comfort. If you can't (due limitations) then pax comfort will take a backseat to performance and safety. |
You don't necessarily suffer a loss of performance by having the packs on Mutt |
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...... ):hmm: Seem to remember on the 73 it gives you about another ton on the MTOW for conditions on the day. |
8 to 10t sounds a little high for a bleeds off take-off. Are you sure that's the number?
Our a/c you gain 1000kg TOW with bleeds off and that's on an a/c around 100t MTOW. |
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.
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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. |
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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Hi people B737 lover again,
Passengers' comfort includes their baggage (luggage) as well, so bleeds off take off wins when it comes to both of them! |
Passenger comfort is one thing, but the ability to get sufficient fuel to get to the destination is a more important factor :):)
Mutt |
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.
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Passenger comfort is one thing, but the ability to get sufficient fuel to get to the destination is a more important factor |
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?
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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....
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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. |
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