FCOM Pressurisation
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Joined: Feb 2024
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From: Pune
FCOM Pressurisation
Greetings everyone,
I have a question which I feel only this website can answer.
I was going through the A320 FCOM and was in the pressurisation chapter.
As I was reading about its pre-pressurisation function, the FCOM went on to say that the system on the take off roll pressurises the cabin at a rate of -400 fpm.
Is there someone who could help me out to understand why that happens. If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
As always, thank you.
I have a question which I feel only this website can answer.
I was going through the A320 FCOM and was in the pressurisation chapter.
As I was reading about its pre-pressurisation function, the FCOM went on to say that the system on the take off roll pressurises the cabin at a rate of -400 fpm.
Is there someone who could help me out to understand why that happens. If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
As always, thank you.
Joined: Apr 2004
Aviation Qualifications: LAME
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From: Dorset UK
Our Mk 1 Airbus, (A300), did this. I think it is to get the pressurisation system working before lift off so there isn't a sudden change in cabin pressure when airborne. After take off the system will control the cabin rate of climb to around 500fpm as the aircraft climbs at 2000 or 3000 fpm.
If you abandon take off the outflow valves will open to ensure there is no differential to prevent doors from being opened.
Hope this helps.
If you abandon take off the outflow valves will open to ensure there is no differential to prevent doors from being opened.
Hope this helps.
Joined: Sep 2016
Posts: 937
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From: USA
I have no insight into Airbus'es design process on this system, but just reasoning from first principles here: The pressurization control system, like any other closed loop control system, is constantly undoing tiny disturbances to meet its target. Like balancing a ruler on your fingertip. And, when doing that, it helps to gently place it on your fingertip, in as still and upright of a state as possible... instead of suddenly throwing it on there and yelling "go!" And when the pressurization control system goes from wide open on the ground (not controlled) to controlling, it's kind of like the sudden yelling "go." There's no easy starting point.
If they tried to make it go straight for the +500 fpm after liftoff, the outflow valve does not know where to immediately go from wide open, so the only way to find the sweet spot would be to slowly close until the computer sees the +500. In the meanwhile, the cabin rate is gradually coming down from +3000 to +500, and the passengers are uncomfortable.
So, instead, while still on the roll it closes until it sees -400, and now it has a good already-stable starting point from which to gradually control to +500, (probably some time late in the roll or around the time of liftoff) and resume the climb from there, already under control.
If they tried to make it go straight for the +500 fpm after liftoff, the outflow valve does not know where to immediately go from wide open, so the only way to find the sweet spot would be to slowly close until the computer sees the +500. In the meanwhile, the cabin rate is gradually coming down from +3000 to +500, and the passengers are uncomfortable.
So, instead, while still on the roll it closes until it sees -400, and now it has a good already-stable starting point from which to gradually control to +500, (probably some time late in the roll or around the time of liftoff) and resume the climb from there, already under control.
Joined: Mar 2012
Posts: 2,605
Likes: 154
From: Having a margarita on the beach
Greetings everyone,
I have a question which I feel only this website can answer.
I was going through the A320 FCOM and was in the pressurisation chapter.
As I was reading about its pre-pressurisation function, the FCOM went on to say that the system on the take off roll pressurises the cabin at a rate of -400 fpm.
Is there someone who could help me out to understand why that happens. If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
As always, thank you.
I have a question which I feel only this website can answer.
I was going through the A320 FCOM and was in the pressurisation chapter.
As I was reading about its pre-pressurisation function, the FCOM went on to say that the system on the take off roll pressurises the cabin at a rate of -400 fpm.
Is there someone who could help me out to understand why that happens. If the system maintains cabin rate of -400 fpm, the pressure differential would increase and make the job of the pressurisation system harder and make it more uncomfortable for the passengers.
As always, thank you.
TAKEOFF (TO)
To avoid a pressure surge at rotation, the controller pre-pressurizes the aircraft at a rate of 400 ft/min, until the ΔP reaches 0.1 PSI.
At liftoff, the controller initiates the climb phase.
As the vast majority of operators use Packs OFF takeoffs this pressure surge is avoided by selecting Packs ON after takeoff with a 10 seconds delay between each other.
Joined: Mar 2012
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From: Having a margarita on the beach
Packs Off takeoffs is an option which is quite routinely adopted as standard in Europe, for example.
Joined: Jan 2004
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From: Here and there
Joined: Nov 2010
Posts: 1,167
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From: USA
Haven't flown in the EU for a while, but it most definitely wasn't done back then. Now in the USA, NOBODY does it here.
Joined: Mar 2012
Posts: 2,605
Likes: 154
From: Having a margarita on the beach
I would be very surprised if any EU legacy carrier + Easy & Ryan use packs off for T/O as a regular SOP. I would be astounded if the vast majority of operators do that.
Haven't flown in the EU for a while, but it most definitely wasn't done back then. Now in the USA, NOBODY does it here.
Haven't flown in the EU for a while, but it most definitely wasn't done back then. Now in the USA, NOBODY does it here.
Joined: Nov 2010
Posts: 1,167
Likes: 180
From: USA
I would be very surprised if any EU legacy carrier + Easy & Ryan use packs off for T/O as a regular SOP. I would be astounded if the vast majority of operators do that.
Haven't flown in the EU for a while, but it most definitely wasn't done back then. Now in the USA, NOBODY does it here.
Haven't flown in the EU for a while, but it most definitely wasn't done back then. Now in the USA, NOBODY does it here.
