How do planes pressurize the cabin when leaving from high altitude (>8000’)?
Posted by PlaneDogFood@reddit | aviation | View on Reddit | 52 comments
Currently leaving Cusco airport (11,000’) and flying to Lima. I understand how they bleed air in when going up in altitude, but when flying from high altitude and going to sea level, the plane doesn’t go back down to standard cabin pressurization altitudes until landing.
lovelivelet@reddit
As the aircraft descends the outflow valve opens a little more to allow the air to leave the cabin at 300-500 feet per minute, the cabin completely de-pressurises before touch down.
ShaemusOdonnelly@reddit
Nope. Opening the outflow valves would cause an increase in cabin altitude, not a decrease. The cabin doesn't really depressurize in flight in the traditional sense - the internal pressure actually increases during descent. What actually happens is that the Extertal pressure increases faster, causing the differential pressure to drop to 0 before engine shutdown.
lovelivelet@reddit
Please let me know how the aircraft reduces the Delta PSI if not opening the outflow valve? Also i'd like to point out the Delta PSI is 0.1 before landing and on touch down 0 and not at engine shut down.
ShaemusOdonnelly@reddit
The delta PSI is reduced by the increasing pressure on the outside in the descent. "Delta PSI" means "Difference in Pressure". If the outside pressure increases, nothing needs to be done to the inside pressure to fully cancel delta PSI. In reality, since cabin altitude is not 0 in most cases, the cabin pressure actually increases too btw. The outflow valves will only open significantly after engine shutdown to fully equalize pressure before the doors are opened.
Break
Even if all this is true, delta psi could be reduced without touching the outflow valves by changing the input flow into the cabin. If an airplane experiences all engines failure and therefore loses its bleed air supply, delta PSI will creep to 0 despite the outflow valves being fully shut. That's why you need oxygen masks for that procedure.
lovelivelet@reddit
While some of what you are saying after the break is true, it's the RPCU that fully equalises before door opening. The aircraft cabin is just at the outside ok touch down.
If you need the FCOM references please let me know
ShaemusOdonnelly@reddit
That certainly isn't true about every Airbus. Mine keeps slight positive pressure between engine start & shutdown, so at the ground cabin altitude is slightly below aircraft altitude.
No matter how accurate my comment is, your initial answer "The outflow valves open to let air out at 300-500 fpm" is completely wrong. You actually need to keep more air in (or increase pack supply) to get cabin altitude down, and you need to close the outflow valves for that.
lovelivelet@reddit
May I suggest you to look at the outflow valve in cruise and in descent. Look at the position of the valve.
May I also suggest reading up on CPC 1+2 at FL390 with Delta P at 8.6 cabin at 8000 ft what does the QRH ask you to do?
After switching to MAN V/S control? Are you opening the outflow valve or closing it? When you use the MAN V/S?
lovelivelet@reddit
Operation: Toggle the MAN V/S CTL switch:
UP: Opens the outflow valve, increasing cabin altitude (cabin climbs).
DN (Down): Closes the outflow valve, decreasing cabin altitude (cabin descends).
lovelivelet@reddit
I haven't written this This Airbus writing - from the FCOM
ShaemusOdonnelly@reddit
Have you looked at your original comment? You said the outflow valve OPENS for descent, and then postet a page from the FCOM that says you need to CLOSE the outflow valve to decrease cabin altitude.
lovelivelet@reddit
Where does it say you need to close the outflow valve to decrease the cabin altitude?
ShaemusOdonnelly@reddit
Look at the procedure. Turning the MAN V/S CTL to "UP" increases cabin V/S by opening the outflow valve, while "DN" reduces cabin V/S. Now, you need to understand that "increase" in this case means "more positive" and "decrease" means "less positive". Therefore, if you want to achieve a negative rate of climb (as you would in a descent), you need to "reduce" cabin V/S from 0 to a negative value.
It isn't explicitly statet in your FCOM snippet, but that's the only physically sound thing to happen. Descent means negative V/S, which means increasing pressure, wich requires more air to enter the cabin than to leave it, which means the outflow valves need to close slightly if we assume constant pack flow.
Now there is one thing where I assume you could have gotten your idea from. In some cab press faults, my FCOM states "FOR DEPRESS IN FINAL APPR: MAN V/S CTL....FULL UP" which is confusing, because it is basically the opposite of what I just said. But here it makes sense: Near the ground, we actually want the cabin altitude to exactly match the aircraft altitude, and the easiest way to do it is to bring cabin altitude up to aircraft altitude by opening the valves. If we did this at 30.000 feet at TOD though, we'd be in a world of tremendous shit.
lovelivelet@reddit
Good we are talking You now agree and as stated in the FCOM For descent we need to open the outflow valve There is a chart which says how much cabin altitude at what FL and no one would foolishly open the outflow valve to full open. This can only be controlled by measured opening of the outflow valve At FL200 and again stated in the FCOM the cabin is at 0 Ofcourse this is all under manual control of the motor On touch down the OUTFLOW valve to open fully either in automatic mode or manual mode ( physically done by the crew ) and commanded Fully open.
So In the beginning of this post I did say outflow valve opens slightly more than usual With cabin V/s decreasing 300-500 fpm
need to "reduce" cabin V/S from 0 to a negative value. This is exactly what I said Reduce cabin at a rate of 300-500.fpm
ShaemusOdonnelly@reddit
I am at the end of my wits with that argument. Nothing in my FCOM states that you need to open the outflow valve for descent. And the snippet of your FCOM doesn't state that either, it says to close the outflow valve to reduce cabin V/S (which is what you would want in a descent).
lovelivelet@reddit
Try it out in the next Simulator session We can resume our conversation thereafter
lovelivelet@reddit
lovelivelet@reddit
You certainly have not had a look at CPC 1+2 you need to open the outflow valve for descent and not close it.
Do you fly the Airbus or do you maintain the Airbus?
lovelivelet@reddit
lovelivelet@reddit
Please let me know if my understanding of the outflow valve is wrong.
lovelivelet@reddit
Do you fly the Airbus?
ShaemusOdonnelly@reddit
Yep.
armspawn@reddit
Modern jets are positively pressurized by air conditioning “packs”, which function by borrowing bleed air from the engines and cooling it down. The packs push air into the cabin, which is held in by automated pressure relief valves. The valves make sure the cabin altitude is safe for the people inside, but also not so high that it risks damaging the structure of the plane. Remember that as you go up in altitude air pressure decreases, so the pressurization system maintains a higher air pressure (lower cabin altitude) inside the jet compared to the outside.
As we descend, the pressure controllers gradually adjust the inside pressure to match the outside pressure at the intended landing altitude. In my airplane I have to set the landing altitude manually.
So taking off from Cusco at 11,000’ my jet starts to pressurize during takeoff roll. By the time it climbs to cruise, say 35,000’ it’ll be pressurized to 8.6 psi differential, or somewhere around 6,000’ cabin altitude. Before descent I set the altitude to match Lima (113’) and the jet gradually increases the interior pressure (decreases cabin altitude) until it matches Lima’s pressure on landing. Actually a little lower to keep positive pressure until the doors open: “The cabin altitude is descended to approximately 100 feet below the selected landing altitude so that the cabin is pressurized on landing. At touchdown, the controller opens the outflow valve to depressurize the airplane at a controlled rate. The pressurization system then automatically conducts a self-test of the other (backup) automatic system.”
Great_Specialist_267@reddit
787’s don’t use bleed air (they have separate electrically driven cabin air compressors). That improves engine efficiency…
armspawn@reddit
I should have said “normal not fancy-pants modern jets…”
paulHarkonen@reddit
"I don't trust any plane younger than I am. They just haven't proven themselves yet".
Neither-Way-4889@reddit
I got my license in an airplane older than my parents
triggerfish1@reddit
And reduces oil fumes in the cabin.
Conor_J_Sweeney@reddit
And the efficiency improvement isn’t as much as they hoped for, but the lower risk of fumes events is looking more attractive by the day.
Great_Specialist_267@reddit
“Oil fumes in the cabin” produced two fatalities a bit over a week ago (not on the plane with the fumes however)…
Light_Is_Power@reddit
“Cabin crew: the doors may be opened”
shhbedtime@reddit
Your question has been answered so I won't again, but thankyou for asking this today. I was studying the bleed system of a new type today and it mentioned the changes to the system for airports over 9500' and I thought "gee I wonder what airports are that high. And now I know, Cusco apparently.
Great_Specialist_267@reddit
Standard cabin pressurisation at cruise is about 8,000 ft in most modern airliners. The pressure is ramped up as soon as the pressurisation system is triggered by gear retraction. High altitude airports are problematic because you get a sudden loss of pressurisation as the wheels hit the ground on landing if the cockpit crew hasn’t made manual adjustments in flight.
ShaemusOdonnelly@reddit
You don't really need to make manual adjustments if the plane has a proper FMS. Landing elevation should be an automated input into the pressurisation system when the crew selects a destination airport. There are edge cases (airports above 10.000 feet might cause alerts because of high cabin altitude/oxy mask deployments) but that depends on the design of the plane.
SkyHighExpress@reddit
A lot of people don’t understand the question or the pressurisation schedule. The schedule is simple. When you take off as you are above the normal cabin altitude of 6000-8000, the cabin would remain at the departure airport alt for approx 30 mins(to cater for an emergency return to the departure airport) and accept a lower differential psi. At that it switches to the destination altitude. So if that is above 6-8000’ , the cabin would adjust to that. If below then cabin alt would lower in accordance with the pressurisation schedule which would maximise cabin differential psi to achieve the lower cabin alt possible
praetor450@reddit
The pressure itself won’t be keep at the depart airport, what some airplanes keep is the elevation value that the pressurization system used as a starting point.
If you initiate a descend during the time period it keeps the value, then it begins the schedule to bring the airplane back to depart elevation altitude, since it was designed with the assumption you are returning to departure field.
The 737 does something. If you don’t reach the set cruising altitude and then initiate a descent, it will set a schedule for the departe airport not for the landing field you have set.
The cabin itself does not stay at the pressure of departure, because if it did then the pressure differential would exceed the structural limit of the airplane. Especially if departing from sea level, not sure about those places like in the OPs question.
SkyHighExpress@reddit
I’m answering the question of the op. The aircraft has departed from a high elevation airfield. That is what the op is asking about, not sea level airports. The diff is lower because the cabin is higher than normal. You are correct but talking about lower altitude airports
praetor450@reddit
Yes I get that, but I don’t know of any airplane that will keep the cabin altitude from departure for 30 mins. That’s what I was referring to.
It might keep the set value but not the actual pressure.
Natural_Stop_3939@reddit
This makes me wonder if there are ever inadvertent mask deployments when planes depressurize at Daocheng Yading (14,472 ft)
praetor450@reddit
For something like that the airplane would have had some “hot and high” package added to be able to land and take off from such airports. As part of that I’m sure that the cabin pressure monitor system that auto deploys the mask above certain cabin altitude (usually set at 14,000 feet) is modified to not deploy for those airports.
Or it could be the crew has a manual control for those specific airports. I can’t say the exact method since it would depend on the airplane and its operator, the route the would choose to prevent mask deployment.
Conor_J_Sweeney@reddit
That’s going to be one of a handful of locations where the pressure is going to come up as soon as the engines start.
mckenzie_keith@reddit
I asked about this once. Most planes maintain the pressure at some fraction of the landing airport pressure. The fraction depends on how high you are. As soon as you start the take-off roll in Cusco, the pressure in the cabin immediately starts going up because the ambient pressure in Cusco is lower than what that formula calculates. Somehow, either automatically or manually, the altitude for the landing airport is stored or set so the pressure system knows what it is. Or will be.
From the practical standpoint, I can confirm this because I was actually watching my barometer app on my phone when I took of from Cusco (as a passenger) and I could see the pressure go up immediately when we started our takeoff roll.
I got altitude sickness while in Ollantaytambo. So I was in the habit of keeping an eye on my barometer app.
I am curious how it works going from Lima to Cusco though. In terms of the actual pressure profile throughout the flight.
armspawn@reddit
I should have said “normal not fancy-pants modern jets…”
faster_tomcat@reddit
Cusco is trippy, when we were landing they lowered the gear at 14000 feet. I was following along from a window seat with a GPS app on phone.
praetor450@reddit
I think I understand your question, normally when a plane takes off from low altitude, let’s keep simple and say sea level, as it climbs to altitude it is trying to keep the cabin pressure to a schedule (altitude) based on pressure differential between ambient conditions and the inside of the cabin.
So for example at cruise altitude, depending on the exact altitude and conditions the cabin can be around 6000-8000 feet altitude. That means that the pressure inside of the cabin is one if you were at that altitude.
So when the airplane is climbing from sea level, it starts at that altitude of the departure airport, and as it climbs the outflow valves allow for some of the pressure to be let out to allow the internal cabin pressure to decrease corresponding to an increase in cabin altitude. This is done because otherwise the pressure differential would exceed the design of the airplane. When descending the airplane allows a little less of the air to be let out to allow the cabin pressure to increase correlating to a decrease in cabin altitude (the exact altitude is set based on the airport elevation).
So now on to your question about high altitude airports, which the ambient pressure is higher than the normal cruising cabin altitude. At take off the cabin pressure will be of the field/aiport. As the airplane climbs, the airplane will keep the outflow valves a bit more closed (not always fully closed to allow air to be cycled) this allows to cabin to build up pressure. As the pressure builds up inside the cabin altitude is now decreasing to the normal cruising cabin altitude of 6000-8000 (depending on airplane, exact cruising altitude, ambient conditions). It will keep doing so until it reaches what would be the normal cabin pressure no different than if it would have taken off from sea level. If it will land at sea level (Lima is very close to sea level) then it will follow the normal schedule for pressurization on descent.
To land at high altitude airports where the elevation is higher than the cabin altitude, then the schedule is a little different. What will happen on descent is the airplane cabin will need to be increased to match airport elevation, and it does so be slowly allowing the outflow valves to allow more air out, instead keeping more in like for a sea level landing.
MalachiteKell@reddit
The last sentence is the error. Pressurized airplanes have outflow valves, which allow air inside the cabin to exit. As long as those outfit valves operate normally, the inside pressure should never exceed outside pressure
thesuperunknown@reddit
/r/confidentlyincorrect
Zn_Saucier@reddit
Planes are pressurized above ambient when above 8-10k feet. Thats why the oxygen masks are in case of decompression.
bug_eyed_earl@reddit
Cabin pressure in passenger aircraft is always higher that the outside pressure at cruise, otherwise we would all pass out at 30,000’
EnvironmentMost@reddit
They take air from the middle stages of the engine compressor.
Zn_Saucier@reddit
Or CACTCS pack on a 787 which has a bleedless architecture.
sniper4273@reddit
The "bleed air" is from the engines, which do a whole lot of air compression as part of their cycle. This air is "bled" from the compressor stage before fuel is injected so it's *essentially just compressed outside air.
The air is cooled and filtered before being sent into the cabin for pressurization and air conditioning.
*"Essentially" is doing some heavy lifting here, as there are tiny amounts of oil and other nasty things. Or, in some engine failure cases, very large amounts of oil.
BigJellyfish1906@reddit
It would pressurize to the proper altitude shortly after start up, and then intercept a normal pressurization schedule from there.