What is the point of a bleeds on takeoff?

[capster]

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]

At a guess, workload management and APU fuel burn...

ATB

[mutt]

We use it to keep the passengers cool, unless additional weight is required, then its bleeds off.

Mutt

[5150]

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]

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]

Bleeds on for pax comfort (on our a/c the APU is for ground use only).

[mutt]

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]

More surge margin for tired engines.

[tbaylx]

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]

Originally Posted by FliegerTiger

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]

I'll get me coat!

[CharterJake]

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]

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]

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]

I totally agree with Charterjake. Its very uncomfortable especially for passengers. But we just have to do it for extra payload.

[muppet]

Surely if you need to go packs off due to weight and are concerned about passenger comfort just pop the apu on.

[mutt]

Muppet,

Some aircraft are not permitted to use the APU during takeoff

Mutt

[H Peacock]

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]

A ‘packs-on’ take off removes the opportunity for the error; either not pressurising or not switching over from APU air.

[yoohoo748]

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.