A320 Prepressurisation

[krisfar]

Hi guys

The A320 prepressurises on take-off to avoid a pressure surge during rotation.

Does anyone know why this happens? Is it due to a positive pressure on the outflow valve on rotation?

Thanks! :)

[davecfm56]

I think most A/C do this. From the boeing FCOM VOL1;

'The cabin begins to pressurize on the ground at higher power settings. The controller modulates the outflow valve toward close, slightly pressurizing the cabin. This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the passengers and crew, and also gives the system better response to ground effect pressure changes during takeoff.'

So you thought right. :)

[toby320]

Hi, is simple for passengers comfort, avoiding excesive pres. changes.

toby.

[FlightDetent]

But what kind of pressure surge would happen at liftoff if the hull would not be pre-pressurized? How does the pre-pressurization work with packs-off?

FD (the-unreal) admits to be lazy now to open FCOM1...

[Gerd53]

Without packs you cannot pressurise an aircraft. Hence you also will have no prepressurisation.

[javelin]

Quite -

Our lot do packs off all the time which is against Airbus philosophy, doesn't let the pre pressurisation occur and causes big surges on the 330 when the packs go back on at 1500'. The rapid pressure increase causes ear popping etc :ugh:

Would be interesting to hear from an Airbus chap about fatigue, pack life etc.

[waren9]

Our lot are just in the process of going to packs on for T/O. Our way around the "pressure bump" was to take one pack on after thrust reduction and the other when clean.

Apparently the perennial argument is the cost of pack cycles and pack life versus the cost of fuel and engine life.

The ever changing equation of the price of fuel, and how you pay for engine time/power and maintaining the packs will determine which way the operator will want to go.

Some engineer/bean counter types might be able to elaborate further.

[ampclamp]

It reduces the pressure surge on rotation (change of aircraft attitude , ground effect etc).
As for affecting pack life, I'd say it would be negligile.Air bearings etc.There is a risk of loading up valves with the surge of pressure at climb power though.
Had a 320 pilot cycle pack s/w's once at altitude and blew every presure blowout panel out of its mounts.Whoops!
Not using packs during t/o imho is just a real "tight anus" way of saving a few pence on fuel.

[IFLY_INDIGO]

In A320, before landing also cabin has the diff pressure of +0.1psi which equates to appro -200ft below the landing elevation. on touchdown pressurization switches mode from DES to GND and the outflow valves starts moving toward fully open position with V/S max 500fpm... what is the logic behind the pressurisation below the landing elevation????

[mcdhu]

I used to fly an ancient turboprop whose pressurisation was totally manual.

At rotation/liftoff the cabin VSI would always show a descent of 3-500fpm before showing the same ROC as the ac by about 2-300' agl at which point we would inch the 'spill valves' closed to introduce supercharger air (Nb not compressor air).

I guess the pre-pressurisation is to avoid this sudden cabin descent on t/o.

Cheers all
mcdhu

[EW73]

Hi Javelin...

If you get to around 1500' before you select the packs on, by then the outflow valve will have been trying to pressurize, and will have fully closed...thus the ear-popping surge when you turn the packs on.
What we used to do in that situation, was to initially select the outflow valve to manual, drive it about half open, then reselect it to auto, and immediately select one of the packs on.
The outflow valve will again drive towards closed, but with a pack operating, it will settle at a position that establishes the correct ROC for the cabin...when that has occurred, select the second pack on, which will only cause a minor blip on the cabin ROC indicator.
Then all is OK
Cheers

[rudderrudderrat]

Hi IFLY_INDIGO,
what is the logic behind the pressurisation below the landing elevation????
Passenger comfort.

If the aircraft was depressurised before landing, you'd feel the aircraft's ROD (800ft/min) on your ears. Most passengers can cope with about -300 ft/min cabin altitude without feeling too much discomfort.
(It gives them time to equalise the pressure differential on their Eustachian tubes by yawning, swallowing etc.) If the +ve pressure differential on their ear drums builds too rapidly, the Eustachian tube will squash and equalisation is more difficult to achieve, causing greater discomfort.

+500 ft per min is easy to accommodate because the Eustachian tube is stretched open instead.

[IFLY_INDIGO]

Hi IFLY_INDIGO,

Passenger comfort.

If the aircraft was depressurised before landing, you'd feel the aircraft's ROD (800ft/min) on your ears. Most passengers can cope with about -300 ft/min cabin altitude without feeling too much discomfort.
(It gives them time to equalise the pressure differential on their Eustachian tubes by yawning, swallowing etc.) If the +ve pressure differential on their ear drums builds too rapidly, the Eustachian tube will squash and equalisation is more difficult to achieve, causing greater discomfort.

+500 ft per min is easy to accommodate because the Eustachian tube is stretched open instead.


BUT why would we feel aircraft's ROD? like CPC maintains cabin altitude 200ft the landing elevation, it can also maintain landing elevation itself. I guess answer lies in the eustachian tube.
probably airbus wants to ensure that when CPC depressurises the aircraft after touchdown by opening the outflow valve fully, there would be higher pressure in the middle ear and lower pressure outside, IF AT ALL, so that eustachian tube is stretched open like you said and painful situation of tube squashed is prevented..

[Microburst2002]

Once the cabin has reached "landing elevation" there should be no cabin V/S at all. For instance, if you start descend and then you have to hold for a long time at low altitude, and land after 40 minutes, the cabin will have been at landing elevation for over 15 minutes.

What is the difference between landing with cabin at landing elevation and landing with cabin at landing elevation minus 150 ft?

Differential Pressure, I guess. I would say that is the reason for it. With the cabin lower than the airfield elevation there is some positive differential pressure, which may be desirable for some reason.

Question is: what can that reason be?

[rudderrudderrat]

Question is: what can that reason be?
Accuracy of the equipment sensors & for passenger comfort.

We want to avoid pressure surges / rapid cabin ROD (as explained above). Maximum permissible differential pressure for take off and landing is usually 0.125 psi (for minimum delay during emergency evacuation). So the pressurisation controller works within those parameters.