Speedbrakes related to eng bleed fault

[miloase]

Hello to all, just checking my MEL ATA 35 "engine bleed air supply system", I check that the last condition to dispatch the aircraft is to have all speed brakes operative, I want to understand that is because just in case to loose the other bleed it may take us to perform an emergency descend, (not taking in count apu bleed)....but Im not 100% sure that this is the only reason about the speedbrakes operative in this MEL ITEM.

Any other opinion?

Cheers,

[Piltdown Man]

I'll have a guess - Does the engine run at a higher flight idle setting if dispatched with this defect? If that is the case then the airbrakes will be required to achieve the required ROD.

[miloase]

I dont see that anywhere in the FCOM, as far as I know the idle increases when there is an extra demand of air by any other system, eg anti ice.

[InSoMnIaC]

Isnt iy ATA 36? My guess is that it has something to do with reverse thrust unavailability during Rejected TO

[seventhreedriver]

There are 2 conditions for the bleed air fault. Flight level limitation (FL315) or no FL limitation. The only difference in the conditions is the requirement for speed brakes at the no FL limitation case. Although it is not really apparent in any documentations, my best guess would be this is due to the ability to perform an emergency descent within the regulatory time allowance.

[3bars]

Surely with the bleeds off, there's more thrust?!

[Ollie Onion]

It is to do with the time taken to perform an Emergency Descent if the other bleed system fails. There are to MEL's, one says that if Speedbrakes operative you have NO flight level limitation due to the higher rate of descents acheivable with the speedbrake. If the speedbrakes are not operative then you are limited to FL315 to ensure you can get down to 10,000 in the same amount of time.

[IFixPlanes]

Airbus answered the question about the conjunction of FL-speedbrake-bleed/pack with the ability to descent in a given time to FL100 if the second bleed/pack is lost. IIRC: At the A319 it depends on the type of FWC if the aircraft is able to descent in the given time without speedbrake or not. The MEL reflects that.

[Cough]

IFP - How does the FWC standard affect the descent rate?

(curious to learn...)

[IFixPlanes]

I think that AI wrote that newer FWC standards give slightly different emergency descent procedures when CAB PR EXCESS CAB ALT ECAM warning is triggered.

[miloase]

Thanks to all, then is what I thought, but not written anywhere, at least in my MEL....

Regarding the A319 maybe the limitation is a little big higher since the speedbrakes even with Autopilot engaged can deploy full deflexion.

Regards,

[Westnest]

Surely with the bleeds off, there's more thrust?!

In fact this is very true between 0-10,000 ASL. Having the APU Bleed instead of ENG Bleed on will surely cause a better fuel index, so more thrust with less fuel. The magic of oxygen.