A320: Manual control of cabin

[Speedwinner]

Dear collegues,

i just had a simulator event where we discussed the manual operation of the outflow valve. To be honest, i had some difficulties to understand the table provided in the ECAM status:

It says that below FL200 cab alt 0. So i was assuming we slowly let the valve open during descend. The other crew member said we had to close the valve again. So can somebody explain again to me the principle of this one? Or any nice documents which explain that?

Thanks!!!

SW

[pineteam]

Hello,

If both CPC have failed then you have to operate the outflow valve manually.
So basically that table tells you that when you are below FL200, you can pressurize the cabin altitude down to zero. So you check the Cab Press page and adjust the vertical speed using the toggle switch on the overhead panel to about -300 feet per min ( for pax comfort) until the cabin altitude reaches zero. You are actually closing the outflow valve as you as pressurizing the cabin during descent. ( less air is escaping through the outflow valve). When it reaches zero, you have to adjust the outflow valve manually to maintain the cabin pressure at zero. Basically you adjust it until Vertical speed = Zero. You don’t want to overpressure the cabin. Then during final approach you have to make sure you open the outflow valve to the full up position to avoid residual pressure.
Also as written in the FCOM, it may takes up to 10 seconds in manual mode before you notice a change of outflow valve position. So you push the switch then wait 10 seconds to check the output and adjust accordingly.
Hope that helps. =)

[vilas]

It says that below FL200 cab alt 0. So i was assuming we slowly let the valve open during descend. Thinking in terms of opening and closing the outflow valve can be confusing. It is easier to think in terms of raising or lowering the cabin altitude and it is done by simply raising the switch or lowering the switch. To lower cabin altitude from 3000 to zero lower the switch. Rest is as pineteam explained.

[pineteam]

One trick a friend told me to avoid to get confused is if you look at the outflow valve on the SD page: When it goes down, the cabin altitude goes down. When it goes up the cabin altitude goes....... up! xD

[Bus Driver Man]

It says that below FL200 cab alt 0. So i was assuming we slowly let the valve open during descend. The other crew member said we had to close the valve again. So can somebody explain again to me the principle of this one? Or any nice documents which explain that?


If you open the valve, the cabin altitude will eventually match your current altitude.
You must be confusing it with cabin differential pressure. That will be 0 with an open outflow valve.

Keep it simple:
If the cabin altitude must go up: toggle the man v/s ctl to UP.
If the cabin altitude must go down: toggle the man v/s ctl to DN.

If you have to get rid of any residual cabin pressure (e.g. evacuation after manual cab alt control): man v/s ctl full UP.

[Samten]

This part of the procedure (bringing the cabin alt to 0) always sounded wrong to me. Unfortunately, I currently do not fly A320, so I cannot check how the cabin altimeter works. In my aircraft (which is similar to A-320) it shows altitude in the cabin, based on pressure 1013hpa (29.92). So if you land on the airfield with RW elevation = 3000', with QNH=1013Hpa and open doors, you will see Cabin Alt = 3000'. If QNH is lower or higher than standard, then Cabin Alt will be higher or lower (based on 27.3' per 1Hpa). May I ask somebody, who is flying A-320 to check, if it is the same for Airbus?
If the principle is the same for A320, then think twice when you bring your Cabin Alt to 0, in preparation for landing on high altitude airfield - you will probably get Very uncomfortable Cabin V/S, when you fully open your outflow valve on final.

[Citation2]

Well it seems that you will not reach a cabin Alt of 0 . It states to apply a cabin rate of 300 feet /min so if your cabin was at 8000 ft it’s going to take 8000/300 = 26,7 minutes

Generally speaking the TOD is more or less at 20 minutes from landing that means your cabin will be approximately at 2000 feet during approach.

what do do you think ?

[Samten]

to Citation2:
Hmm, I see your point... Never thought about it. Thanks!
You may be right in certain scenarios. However, if you are approaching a busy airport (long transitions, holding patterns), then you will probably have enough time to reach the target Cabin Alt. Also, even for a short STAR, the pilot will probably increase Cabin V/S above 300 fpm, if unable to reach the target.

[Citation2]

The question is whether or not the TOD should be readjusted to meet a cabin alt of 0 when below FL200 or keep the TOD of the profile and let the cabin rate descent progressively. For the latter, a cabin alt of zero , « the target » will not be achieved.

Also , when will the cabin altitude meet the aircraft altitude ? If at 4000 feet the cabin alt is still at 4000 feet there is no point of making the cabin descent further to zero and then make the cabin alt rise with the « full up » procedure.

It states on a separate note :
V/S CTL FULL UPWhen on intermediate approach (below airfield pressure altitude +2 500 ft), adjust ΔP = 0. »

So in one hand you target a cabin alt of zero and then target a Delta P of zero?

1 st case ) you descended earlier and took into consideration the 300 ft/ min cabin rate so you started your descent let’s say 40 min before ETA. The target cabin alt of zero was met when below FL200.
For an airfield of 3000 feet elevation , if before approach the cabin was at zero I will probably enter a hold and make the cabin rise progressively to meet aircraft altitude ( and that is to meet the delta P of zero small note)
When applying the « full up » and rising the cabin from zero to aircraft altitude and releasing the delta P in few seconds will certainly be beyond the acceptable limits of comfort.

2 nd case) you descent normally as per the profile and as you target a cabin alt of zero , the target is never met before approach. That is to say that the aircraft altitude will meet the cabin altitude somewhere before approach hence you have a delta P of zero because you are at 4000 feet and cabin alt of 4000 feet.
when applying the « full up » procedure little to no residual pressure will be released. You then meet the requirement of having a delta of zero ( small note in FCOM).

In one hand a cabin alt of zero requirement along with a second requirement of Delta P of zero seems contradictory.

Would you target a cabin alt of zero ( 1st requirement)or a delta P of zero ( second requirement)?

I would rather go for case 2 .

More inputs will be appreciated.

[pineteam]

I think what it’s really important is to not exceed the maximum delta pressure and to try to limit the vertical speed of the cabin during the descent for passenger confort. They say 300 feet that would be ideal but if required I will go down to 1000 feet/min. That’s usually a max ROD used by non pressurized aircraft and still confortable for the passengers.
Then finally not to forget to open the outflow valve at some point during approach to avoid residual pressure. If there is still a big delta pressure it’s not really an issue as for a rising cabin altitude, the fast change of pressure is not an issue for the passengers and crew.

[vilas]

I think the STATUS is misunderstood. It is in two parts. The first part is the cruise part. It gives the cabin altitude you would maintain for actual aircraft altitude. In that if the aircraft cruise altitude was below FL200 the cabin altitude to be maintained is zero which is done by closing the outflow valve as much as it takes. You cannot close more than fully anyway. And if cabin needs to climb or descend then do it at the given rate. The second part is about when on approach, landing and after landing. Here initially on intermediate approach keep the ∆P zero by manipulating the outflow valve and on final approach you completely depressurize the aircraft by putting V/S control full up, that's it. After landing recheck ∆p is zero before opening the doors. The approach part could've been written in reverse order.

[Speedwinner]

thanks guys!

i think my misunderstanding is: why do I have to close the outflow valve during descend? I thought we toggle it open to get the same ambient pressure when we descend. I think I’m asking for the basics of the pressure cabin!

[sonicbum]

thanks guys!

i think my misunderstanding is: why do I have to close the outflow valve during descend? I thought we toggle it open to get the same ambient pressure when we descend. I think I’m asking for the basics of the pressure cabin!

Hi,

try and have a look at the LIM-AIR chapter where there is a graph on aircraft altitude vs cabin altitude and it will most likely help you to understand the logic behind opening/closing of the outflow valve in order to control the pressurization.

[mcdhu]

With years of manually controlling cabin pressurisation in old turboprops, the following might help:

The A320 family manual control of the pressurisation is time consuming so KEEP IT SIMPLE.
Imagine that the outflow toggle switch is connected directly to the cabin VS. ie toggle up, cabin VSI needle will move up (not necessarily climb) - toggle down, the needle will move downwards (not necessarily to a descent)
Yes, for cruise at or below FL200, you can select a cabin alt of sea level, but why would you?
In order to reduce your workload on the approach, why not select a cabin alt of your platform altitude for the expected approach (unless of course you are landing at a high up airport).
if you maintain a cabin alt of, for example, 3000', once the ac is at 3000', you can open the Outflow Valve fully to depressurise the ac and then the cabin will descend with the ac at a comfortable 700 fpm; this will leave the PM free to carry out his primary monitoring duties.
Once on the ground, you should see 0 diff but you could also crack a FD window to make absolutely sure of that.
i hope this helps.
mcdhu

[gearlever]

thanks guys!

i think my misunderstanding is: why do I have to close the outflow valve during descend? I thought we toggle it open to get the same ambient pressure when we descend. I think I’m asking for the basics of the pressure cabin!
During descent the eng pwr is at idle, low bleed pressure.....