What happens if cabin differential pressure is not 0?
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What happens if cabin differential pressure is not 0?
Just wondering.
I was reading that you should not open doors in aircraft unless the cabin pressure is zero.
What would happen if it wasnt?
I know if doors were opened in flight you won't be able to breathe, that's why the cabin is pressurized.
But when landed, the pressure outside is normal.
What is the problem now?
I was reading that you should not open doors in aircraft unless the cabin pressure is zero.
What would happen if it wasnt?
I know if doors were opened in flight you won't be able to breathe, that's why the cabin is pressurized.
But when landed, the pressure outside is normal.
What is the problem now?
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It doesn't take much differential before you can't open the doors.
Most doors are plug type so the pressure keeps them shut.
Normally there is a link to the squat switch which opens the dump valve and there are also moisture traps in the lower hull which open at low differentials to get rid of the condersate thats formed on the inside hull.
If that all doesn't work there is a emergency dump valve which can be opened.
But to be honest once you turn the engines off the door seals would deflate and the air would equalise. But most aircraft are tea bags with holes in them and as soon as you turn the flows off they equalise in a matter of seconds.
Most doors are plug type so the pressure keeps them shut.
Normally there is a link to the squat switch which opens the dump valve and there are also moisture traps in the lower hull which open at low differentials to get rid of the condersate thats formed on the inside hull.
If that all doesn't work there is a emergency dump valve which can be opened.
But to be honest once you turn the engines off the door seals would deflate and the air would equalise. But most aircraft are tea bags with holes in them and as soon as you turn the flows off they equalise in a matter of seconds.
American had a F/A die during an emergency evac from an A300 in MIA in 2000. They returned after TO because they had a pressurization problem and had a cargo fire warning on final. They were overpressurizied when the door opened and the F/A was blown 60 feet out on to the ramp.
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Years ago on BA's Internal German Services out of Berlin, when the flight landed and the German stewardess opened the rear door, it blew open, probably in the wind, and nearly pulled her out. Convinced the darn pilots had not depressurized properly, she stomped up to the flight deck to remonstrate. The two pilots were still sitting quietly, in peace and harmony, doing the shutdown checks, and she burst onto the flight deck in a temper and said in a loud, accusatory voice to the very confused pilots 'Captain! I have just been sucked off!'.
(told that before, but I love that story, and the one about the tray)
(told that before, but I love that story, and the one about the tray)
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F = M x A
But when landed, the pressure outside is normal.
What is the problem now?
What is the problem now?
The cork will be dislodged from the bottle with tremendous force.
As the pressure in the champagne bottle increases that pressure exerts force on the cork. Eventually the higher differential pressure inside the bottle overcomes the friction of the cork and the lower ambient pressure outside the bottle and the cork is ejected. Why does the bullet depart the casing when the firing pin strikes? In the second example we have a relatively small mass rapidly expanding generating enough force to launch a lethal projectile.
What is the volume of the interior of your airplane’s cabin, how many cubic meters of air is there in that cabin? Convert than number to square inches and we are likely dealing with very big numbers. Now multiply that mass by one, two or more pounds per square inch differential. The result is you have a lot of force. The force generated by that huge volume of air moving is likely more than could be generated by the New Zealand All Blacks on a scrum line. If I am standing in front of the door, and it opens under those circumstances, I am going to get ejected-forcibly.
I am not familiar with the American Airlines flight attendant accident in Miami back in 2000 referenced by an earlier poster, but I can see somebody being launched from a dangerous height and suffering fatal injuries.
Every jet aircraft I have flown has been designed to be completely depressurized either right before or very soon after landing we want to be able to get those doors open.
Force = Mass multiplied by Acceleration
Last edited by Northbeach; 7th Jan 2012 at 19:18.
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I'm being a bit dull here and can't get my head around it.
But. At altitude, the reason why everything gets sucked out is the cabin pressure is higher than outside, so if for whatever reason, there is a breach in the hull, it would mean the high pressure of the cabin would flow to the low pressure outside right?
So if on the ground, and the aircraft is still pressurized for whatever reason. Wouldn't that mean the pressure outside the aircraft is MORE than the pressure inside the aircraft? So wouldn't it then flow in the reverse direction?
I know that if you open a door when it is still armed however, you will get a very different response!
But. At altitude, the reason why everything gets sucked out is the cabin pressure is higher than outside, so if for whatever reason, there is a breach in the hull, it would mean the high pressure of the cabin would flow to the low pressure outside right?
So if on the ground, and the aircraft is still pressurized for whatever reason. Wouldn't that mean the pressure outside the aircraft is MORE than the pressure inside the aircraft? So wouldn't it then flow in the reverse direction?
I know that if you open a door when it is still armed however, you will get a very different response!
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We do not attempt to open cabin doors whilst any positive differential pressure remains because in the first instance we probably couldn't. additionaly, any positive differential pressure is a hint that an engine may still be running and we may not want the doors opened. Theoreticaly, as the pressure decreases there comes a point at which we would be able to open the door with a tiny amount of positive diff and IF ! that occured the door would blow open very quickly and take us with it because we were holding onto it, NOT because there would be enough pressure to blow us out !
Those are the historical and very sensible reasons for not attempting to open doors with positive diff. Also don't forget the poor souls standing just outside the door !
PS. these are generalisations, armchair experts need not educate me about weight switches, throttles and outflow valves thank you.
Those are the historical and very sensible reasons for not attempting to open doors with positive diff. Also don't forget the poor souls standing just outside the door !
PS. these are generalisations, armchair experts need not educate me about weight switches, throttles and outflow valves thank you.
Last edited by moonburn; 13th Jan 2012 at 16:02. Reason: additional consideration
In an aircraft's pressurisation system is working normally the cabin pressure ( aka cabin altitude) varies dependant on altitude, it's not a fixed value in psi/bar/hectopascals throughout the flight.
Correct, at high altitude there's a lot more more pressure inside than outside and in the event of a breach of the hull air will flow out.
As you descend towards sea level the outside air pressure is increasing but the pressure inside the aircraft is deliberately increased as well . FWIW if the pressurisation doesn't increase the internal pressure most aircraft have inward opening valves that open to allow air into the hull to prevent the hull from being crushed during the descent.
If the aircraft pressurisation system works properly when you land the pressure inside and outside the hull should be pretty much the same, but just in case many aircraft have a feature that fully opens up the cabin air exhaust, or outflow valves when the aircraft touches down, to equalise inside and outside pressure..
a breach in the hull, it would mean the high pressure of the cabin would flow to the low pressure outside right
if on the ground, and the aircraft is still pressurized for whatever reason. Wouldn't that mean the pressure outside the aircraft is MORE than the pressure inside the aircraft? So wouldn't it then flow in the reverse direction?
If the aircraft pressurisation system works properly when you land the pressure inside and outside the hull should be pretty much the same, but just in case many aircraft have a feature that fully opens up the cabin air exhaust, or outflow valves when the aircraft touches down, to equalise inside and outside pressure..
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lilfly - most aircraft have positive pressure cabin differential on landing which needs to be dissipated before doors are opened. Generally, however, there is a big clue for c/crew as they start moving the door handle.
A) It will be hard to move
B) There will be one of Not-so's famous 'sucking noises' as they do. Time to take it easy and check?
A) It will be hard to move
B) There will be one of Not-so's famous 'sucking noises' as they do. Time to take it easy and check?
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Resurrecting an old thread I know, but I have the same question, that I am trying to get my head around.
So to make sure of my understanding then, due to the cabin trying to keep a positive differential, it potentially, not necessarily will, take the cabin to a slightly lower altitude than the one you are at on the ground?
So to make sure of my understanding then, due to the cabin trying to keep a positive differential, it potentially, not necessarily will, take the cabin to a slightly lower altitude than the one you are at on the ground?
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Resurrecting an old thread I know, but I have the same question, that I am trying to get my head around.
So to make sure of my understanding then, due to the cabin trying to keep a positive differential, it potentially, not necessarily will, take the cabin to a slightly lower altitude than the one you are at on the ground?
So to make sure of my understanding then, due to the cabin trying to keep a positive differential, it potentially, not necessarily will, take the cabin to a slightly lower altitude than the one you are at on the ground?
On the ground the pax and crew would not really notice this increased pressure, until of course an attempt was made to open a door.
An incident in PNG comes to mind, exactly this situation at POM, the FO when notified by cabin crew selected manual dump.
Nose bleeds, ruptured eardrums and much wailing and gnashing ensued.
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You are correct that in normal operations most aircraft have the cabin slightly lower than the outside during landing. There are situations where we turn the bleeds off, this has the effect of giving us slightly more power from the engines in case we need it for go-arounds. An example would be for some types taking a tail wind of more than 10 knts.
The reason why they do this is because the additional pressure has a stiffening effect on the hull which leads to a decrease in engineering stress on the structure which leads to less fatigue history. Think empty plastic bottle that's closed that gets heated in the sun.
If the cabin pressure stays the same as at attitude the cabin will be less pressure than outside. There is a safety valve which allows air into the aircraft to balance things out so the hull doesn't crush like a plastic bottle if you suck the air out.
If by some bizarre combination of events you get on the ground with negative pressure internally there is a possibility of the doors being pushed in when opened but as they all swing out by some method there is a mechanical stop to stop them getting blow in the way.
The only way I could imagine a door being opened with the internal pressure being more than the outside would be when someone opens the doors with an engine running and other system failures present. The doors are approximately 2 meters square. anything more than 300N force required to open (ie lift 30kg) is going to mean that the person opening is going to know there is something different to usual. over the door that equates to 150N/m2 which is 0.0015 Bar or 0.002PSI where bar is standard atmosphere. So even the slightest pressurisation means the door is solid.
In cruise at FL 350 the cabin is pressurised to 8000ft which is 0.75 bar. Outside pressure is 0.24 bar giving 0.51bar differential which over 2m2 equates to 102 tons of force required to move the door.
The reason why they do this is because the additional pressure has a stiffening effect on the hull which leads to a decrease in engineering stress on the structure which leads to less fatigue history. Think empty plastic bottle that's closed that gets heated in the sun.
If the cabin pressure stays the same as at attitude the cabin will be less pressure than outside. There is a safety valve which allows air into the aircraft to balance things out so the hull doesn't crush like a plastic bottle if you suck the air out.
If by some bizarre combination of events you get on the ground with negative pressure internally there is a possibility of the doors being pushed in when opened but as they all swing out by some method there is a mechanical stop to stop them getting blow in the way.
The only way I could imagine a door being opened with the internal pressure being more than the outside would be when someone opens the doors with an engine running and other system failures present. The doors are approximately 2 meters square. anything more than 300N force required to open (ie lift 30kg) is going to mean that the person opening is going to know there is something different to usual. over the door that equates to 150N/m2 which is 0.0015 Bar or 0.002PSI where bar is standard atmosphere. So even the slightest pressurisation means the door is solid.
In cruise at FL 350 the cabin is pressurised to 8000ft which is 0.75 bar. Outside pressure is 0.24 bar giving 0.51bar differential which over 2m2 equates to 102 tons of force required to move the door.
In addition Boeing's idea was if you ran the cabin down a couple hundred feet during the taxi out the cabin altitude would be under control during the takeoff and smoother on passenger's ears. Old DC-9s took off with 0 cabin differential pressure and while I never felt the difference the outflow valve control wheel would sometimes spin around at rotation.
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In addition Boeing's idea was if you ran the cabin down a couple hundred feet during the taxi out the cabin altitude would be under control during the takeoff and smoother on passenger's ears. Old DC-9s took off with 0 cabin differential pressure and while I never felt the difference the outflow valve control wheel would sometimes spin around at rotation.
