Prepressurization on take off
 

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? :confused:

Thanks you all,
bio161

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

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?

Hi fredgrav and thanx a lot for your reply. :ok:

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.. :confused:

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.. :)

- Have you not answered your own question?

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. :ok:

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. ;)

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.

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.

Hi fredgrav,

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

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.. :)

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.

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

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

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.

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.

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.

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

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

Sorry fredgrav,

but where do you read what you wrote? :confused:

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 ...

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

fredgrav,
i don't know who told you that but what you wrote before (predepartues check taxi out phase period ecc ecc..) it does not apply to airbus 320 series at all..
don't pretend to extend your knowledge to the all the aircrafts..each one has its own way of operating.. :=
rgrds,
minimumminimumm

Hi Minimumminimum,
what should I be pretending ?! ... I don't know how every aircraft on earth manages its valves, but simply said that, and I'm kinda sure of it, during the pre-daparture checks and taxi-out on the bus, the outflow valves, the Pack inlet and outlet stay open or partially open, allowing ram air to go in. Just that ...

Best,
fred

- you are wasting our time!

fregrav,
have a look again to your books. it seems you are a little bit confusing between packs ops and ram air ops :ok:

bio,
see my posts above, never mentioned Ram Air OPS but always talked of Ram Air. Ram air = a fluid moving freely around the a/c or entering in it. Be sure that if I DID NOT know what I'm talking about, I wouldn't have written in this thread ... then I can either be right or wrong but just thought that my explanation was the only one to be possible in trying to figure out how the bus gets pre-pressurized on takeoff.

p.s. mentioned a couple of time the RAM air inlet, but wrongly and inadvertently instead of Pack Inlet Scoop.

As you are so sure then i get back my words ;)

No it is not. With packs off there is no pre-pressurisation. The pressurisation system might still enter the pre-pressurisation mode and drive the outflow valve towards closed but there will be no pressurisation. As the aircraft climbs the cabin altitude will then increase at a rate determined by how leaky the particular aircraft is.

No it does not. Unless you have opened the emergency ram air inlet in which case there will be no pressurisation.

There will not be a slightly higher pressure inside the cabin. With packs off the ventilation system will cause it to be slightly lower than ambient.

But you did.

KBPsen,
you're right. Was so convinced as if pack inlet and outlet valves did actually allow ram air to freely enter, that finally wrote a nonsense.
But definitely don't take this as I did not really know what I was talking about, I simply made a mistake.

Bio,
thought back to what I wrote and just got to be in the wrong. Please excuse me! ;)

Best,
fredgrav

I used to operate 747 Classics for many years, all of which were 'packs-off' takeoffs...

I'm still trying to get around this 'pressure bump' you guys are talking about...
All 3 packs off until around 200', then 1 pack on, then 2nd pack on when stabilized in the climb, followed by the 3rd, again, after.....
Then one pack off during the cruise.

I don't know what all this 'pressure-bump' is about, it was never a problem, worked just fine, and was ultra-reliable...

EW73

Hi EW73,
you're right. I don't know the 747CL, but as far as I know the 747-400 has a 1 PACK ON / 2 PACKS OFF TO ... though the conf. may change from plane to plane.

No doubt about it. I was talking about the "pressure surge" that might happen upon the selection of 2 or 3 packs at the same time; this thing has all to deal with comfort and is not supposed to be a problem for a large cabin like that of the 747. The less the cabin size, the faster it gets pressurized. Don't want to keep runnin' off-topic ... :)

Best Regards,
fredgrav

- and that is EXACTLY why we don't do that:ugh:

BOAC,
there should be a misunderstanding in here, sir !
Initially I was talking of the RAM air that enters via the Pack Inlet Scoop, and finally admitted to be in the wrong, cause I actually was ... but:

I really never questioned that ! :ok:

EW, The pressure bump I am led to believe is that experienced when an aircraft rotates, changing the attitude and therefore the airflow around the OFV.
If the OFV is closed and fuse is pressurised the above cannot happen.
I dont have any experience of the 74 as above regarding selecting packs one at a time time to prevent a pressure surge.I think this maybe be semantics but a slightly different phenomena to a pressure bump eliminated by pre pressurising whilst on the ground.ie the 737 pre press to -387' from memory to prevent the rotation bump.

However, I know of one case where a 320 was having bleed trouble ( nothing new back then, late 80's early 90's) and the crew deselected both bleeds and reslected on.Blew out all the blow out panels but did surely get bleeds flowing:uhoh: That was a surge!

fred, it is good place to learn.Ram air in and out puts cooler outside air across heat exchangers/precoolers to cool bleed air for use in air con / press. systems.

Thanks amp,
you got it, there ain't any place more efficient than this to learn from others ... ;)

Pressurisation Bumps
 

Notwithstanding some of the positively surreal posts on this thread, here's the explanation of pressurisation bumps...

Many/most simpler old-school pressurisation systems connect the squat switch into the pressurisation system - often as a direct connection to the outflow valves, independently of the controller. That very simply satisfies the requirement to dump cabin pressure when you land. But the inverse is also true, that during the takeoff roll, wheels on the ground, the outflow valves are held open. As soon as the aircraft lifts off the squat switch opens, and transfers control of the outflow valves to the pressurisation controller. This handover of control can mean that the outflow valves are quite quickly driven closed as the controller hunts for the right setting, causing a noticeable "bump" in the ears of the passengers, especially if the controller is not quite set up correctly.

To answer the 747 comment - the size of the bump depends on cabin size - the smaller the cabin, the bigger the bump.

There are two pretty basic solutions. One is to arrange that there is no significant inflow when the outflow valves take over - that's the "no packs" solution, and it works - but the bleed air must be introduced to the cabin carefully later. The second, which is more popular in modern designs, is to give control of the outflow valves to the pressurisation controller when you are still on the ground. The controller can drive the outflow valves towards the closed position to modestly pre-pressurise the cabin. It can do this without bumping because it KNOWS that you are on the ground and can therefore self-calibrate the delta pressure between ambient and target cabin pressure.

But it has nothing to do with attitude, ram air, or airflow.

"But it has nothing to do with attitude, or airflow".Agree on ram air that is nonsense.
Well, I better get back to Mr Boeing and correct their 737 trainers on the other 2.
we do agree that pre pressurisation is used to alleviate the bump.

Not sure of the airbus, 767/757 pre-pressuration can be easily seen by moving throttles to full and watching the outflow valve moving. One good reason to pull all circuit breakers when working near the valve - can move at anytime.

What you really need is a good old-fashioned Flight Engineer to put you pilot-type chaps (not all, but many) at rest with your perceptions of air conditioning and pressurisation theory.

Ram Air was mentioned... and that's where the initial mistake was made. Ram Air is used within the primary stages of 'Heat Exchange' within the Air Cycle Machines and nothing to do with air entering the cabin.

BOAC appears to be the only chap here to have a grasp of the situation. Over the years, the method in which the pressurisation of aircraft is achieved, hasn't changed that much. Yeah, more modern systems have 'tweaked' the theory in order to refine passenger comfort.

A modern system will start to close the outflow valve(s) during the takeoff roll, or, whenever the Pressure Controller has been selected to AUTO. This is in order that there will be a much smoother transition to pressurised flight when those tricky little squat switches handover full control the the Pressure Controller. The Pressure Controller will sense how many packs are ON/OFF, and, it will also know the position of the Outflow Valve(s) and that it modulate the Outflow Valve(s) with respect to the rate of change of cabin pressure.

Simples...

TCF

Hi,

Field in Sight (post13) answered the original question exactly.

I agree with TCF. In addition the A320 series has a Ram Air System which will provide cabin ventilation - but not pressurisation.

rudderrudderrat; it's my bet that you're an ex TriStar man!