View Full Version : What is the point of a bleeds on takeoff?
capster
22nd Dec 2007, 08:05
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
Cough
22nd Dec 2007, 08:11
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. . . !
HotDog
22nd Dec 2007, 09:36
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.
speedrestriction
22nd Dec 2007, 09:40
Bleeds on for pax comfort (on our a/c the APU is for ground use only).
but I may be wrong
The airbus like most boeings, embraers or douglas aircraft can be operated with PACKS ON or OFF as per the airlines chosen policy.
Mutt
lomapaseo
22nd Dec 2007, 12:14
More surge margin for tired engines.:}
tbaylx
22nd Dec 2007, 12:43
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.
dolly737
22nd Dec 2007, 14:41
if all bleeds are off, no anti-icing is available
On the 737s, cowl anti-ice is available with the Engine Bleed Air Valve closed!
FliegerTiger
22nd Dec 2007, 14:53
I'll get me coat!
CharterJake
22nd Dec 2007, 15:36
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...
richarjm
22nd Dec 2007, 15:43
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.
huckleberry58
22nd Dec 2007, 16:03
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.
B737 lover
22nd Dec 2007, 18:28
I totally agree with Charterjake. Its very uncomfortable especially for passengers. But we just have to do it for extra payload.
muppet
22nd Dec 2007, 18:37
Surely if you need to go packs off due to weight and are concerned about passenger comfort just pop the apu on.
Muppet,
Some aircraft are not permitted to use the APU during takeoff:)
Mutt
H Peacock
22nd Dec 2007, 19:57
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.
safetypee
22nd Dec 2007, 20:21
A ‘packs-on’ take off removes the opportunity for the error; either not pressurising or not switching over from APU air.
yoohoo748
22nd Dec 2007, 20:35
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
Tell that to Mr Boeing... we lose 8-10,000 kgs due to packs on!
Mutt
Oxidant
23rd Dec 2007, 07:47
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.
typhoid
23rd Dec 2007, 08:47
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.
Kennytheking
23rd Dec 2007, 10:43
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.
Capn Bloggs
23rd Dec 2007, 11:33
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.
Canuckbirdstrike
23rd Dec 2007, 15:45
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.
B737 lover
23rd Dec 2007, 21:32
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
actus reus
25th Dec 2007, 01:49
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.
Capn Bloggs
25th Dec 2007, 07:15
Passenger comfort is one thing, but the ability to get sufficient fuel to get to the destination is a more important factor
Obviously.:cool:
yoohoo748
25th Dec 2007, 19:33
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?
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....
barit1
27th Dec 2007, 19:39
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.