Prepressurization on take off

[bio161]

Hi all there,

Reading the thread about "PACK ON / OFF take off" made me think about one thing of the bus.

On the A320 the pressurization system, during the take off roll, in AUTO mode pressurizes the cabin at a rate of 400feet/minute until 0.1PSI is reached to avoid a pressure surge at rotation.

At the same time, the RAM air inlet and outlet falp close during the take off roll (as well during landing) to avoid ingestion of FOD.

So, if we take off with PACKS OFF, how can this "prepressurization mode" work?

Thanks you all,
bio161

[fredgrav]

Bio161,
either SYS1 or SYS2 don't start pressurizing the a/c on takeoff roll, but shift in the CLIMB PHASE (keeping a normal CROC of 500fpm) upon reaching the Thrust Reduction Altitude, where PACKS should normally be turned ON one at a time, to avoid rapid pressure build-up and for pax comfort.

Hope that helps,
fredgrav

[BOAC]

bio - I really do not understand the question. Can you explain which part of the AB pressurisation system actually provides the air pressure for the cabin?

[bio161]

Hi fredgrav and thanx a lot for your reply.

But have a look again to FCOM 1.21.20 P5 in the section "pressurization"..There is the description of the Take off phase:

"To avoid a pressure surge at rotation, the controller (either SYS1 SYS2) prepressurizes the aircraft at a rate of 400feet/minute, until the delta P reaches 0.1PSI. At lift off, the controller initiates the climb phase."

So there is actually a pressurization during take off roll. So the question, unfortunately, remains open..

[bio161]

Hi BOAC,

the air usually comes from the 2 PACKS, whose air cycle machine is helped to cool down (beside the turbine) with the RAM air.

So my question is. If i have the packs off on take off, and the RAM air inlet/outlet closes as soon as i bring the thrust levers on TO/CLIMB position, how can the airbus prepressurize?

I hope it's a little more clear..

[BOAC]

the air usually comes from the 2 PACKS

- Have you not answered your own question?

[bio161]

Mmm..maybe i explained in a bad way myself.

If i have the 2 PACKS ON it's clear that i get the air from them to pressurize the cabin.

But, assuming we are performing a take off with PACKS OFF, then i am not anymore supposed to get the air from the 2 PACKS that i turned OFF for performance reasons.

So the mode PREPRESSURIZATION during the take off roll, which i described before, in my opinion, should NOT work.

Because i don't find any evidence of the function of this mode with PACKS OFF i was wondering if anybody could have an answer if it's automatically inoperative or if it works in an another way (which i don't think).

I hope it's a little more clear now.

[KBPsen]

It should be very obvious to anyone that there will be no pressurisation if the packs are off.

The pressurisation system can do whatever it wants, it can even jump around on one leg singing the ouverture to HMS Pinafore, it makes no difference. There will be no pressurisation.

[fredgrav]

Hi bio161,
you answered yourself by your first post here ... pre-pressurization (Packs OFF) is accomplished by using ram air and then keeping RAM air inlet and Outlet flap closed all troughtout the takeoff phase.

[bio161]

Hi fredgrav,

but is it a conclusion of yours or you found it written anywhere?

Because on the FCOM 1.21.10 P6 reports (as written before) during take off the RAM AIR inlet and outlet close both of them. So i'm wondering where the air should come in..

I think KBPsen is right and it just doesn't work..Otherwise it would be magic..

[KBPsen]

Forget about the ram air inlet and outlet flaps, they have nothing to do with pressurisation. Forget the notion that the pressurisation system pressurises the cabin, it does not. It is the air entering the cabin from the packs that pressurise, the pressurisation system merely controls how fast it exits the cabin again.

It really is very basic stuff.

[minimumminimum]

"Forget the notion that the pressurisation system pressurises the cabin, it does not.."

UAO as bio said then it's really magic! This really made my day!!!

[Field In Sight]

The outflow valve closes on take-off and seals the aircraft. The aircraft now contains sea level air pressure.

As the aircraft climbs, the outside pressure reduces. This therefore increases the differential pressure. This happens whether the packs are supplying air or not.

FIS.

[Trent 972]

Cannot speak of the 320, but on the 330 and 380 -
Before take-off, all outflow valves open, to ensure there is no differential pressure between the air inside the aircraft and the external air.
Take-off Phase, when all doors are closed, the oleos are compressed and (at least 1/(3) engine/s) at take-off power, the outflow valves close, There is no 'pre-pressurisation' with the packs Off. After 'Lift Off' the cabin rate of climb is determined by the cabin leak rate. (200fpm on the 380)
After thrust reduction and when the 1st pack is turned on, the outflow valves will open to accomplish the programmed cabin pressure control.

This stuff should be known before you ever get to strap an aircraft to your bum.

edit: Field in Sight, you beat me to it as I typed, though I would change your "sea level air pressure" to ambient air pressure.

[ampclamp]

How does it work? It does not. The OFV just stops whatever air that is in the cabin from getting out (to a degree anyway)

As for the ram air in / out doors...
I actually know someone who thought that the ram air in actually provided the cabin air pressure.

[fredgrav]

Because on the FCOM 1.21.10 P6 reports (as written before) during take off the RAM AIR inlet and outlet close both of them. So i'm wondering where the air should come in...

Bio161,
RAM air enters the aircraft before takeoff and definitely both the RAM air inlet and outlet valves (as well as the outflow) stay closed fot takeoff. Though the valves (and packs) have remained open while the pre-daperture checks and taxi-out phase, so that a pressure slightly higher than outside has built up inside the cabin: when the valves automatically close for takeoff (TO phase), this slighlty higher pressure remains trapped into the a/c, allowing the so called "cabin pre-pressurization". This way of building up pressure prevents a surge from occurring upon the selection of PACKS, then increasing the cabin comfort.

fredgrav

[BOAC]

I actually know someone who thought that the ram air in actually provided the cabin air pressure.

- I think we may have found one............. Fredgrav - make sure you read up on pressurisation before you take to the bus.................. Start with what does the 'ram air door' do?

[bio161]

Sorry fredgrav,

but where do you read what you wrote?

[fredgrav]

I actually know someone who thought that the ram air in actually provided the cabin air pressure.

BOAC,
never thought it sir, and least of all, said it ... but the thing I know for sure is that every kind of source bringing air into the cabin and not allowed to exit, does increase the pressure in it ... and, that's called Pressurization. If the RAM air allows air to freely enter the cabin, and at a certain point is not allowed to exit, it also contributes to a pressure build-up, obviously weaker and slower (the so called pre-pressurization).

Bio161,
as far as I know the "rule" applies for every aircraft on earth, and not just the bus. To minimize the hoop stress into the fuselage, only a small differential pressure is allowed fot takeoffs and no residual differential pressure's allowed for landings. That's what I studied ...

[BOAC]

If the RAM air allows air to freely enter the cabin

- yes, but does it? Read it up and come back with an answer.