Airline pilots: Pressure differential

Factual Questions
1

First of the month, so I’m very busy at work. Apologies for not doing the research and math myself. I know the air pressure at sea level is 14.7 psi. I think passenger jets are pressurised to 8,000 feet, and I think that’s about 11 psi. And I think the air pressure at cruising altitude (35,000 feet) is about 3.5 psi. So at cruising altitude, there’s a pressure differential of about 7.5 psi.

How many square inches is the cabin door on a typical airliner? Assuming my numbers are correct (please correct me if I’m wrong), I can multiply the area of the door by 7.5 to find out how ‘heavy’ (i.e., how much you’d have to pull to open it) the door is at altitude.

2

You’d also need to account for the Bernoulli effect.

3

7-8 psi is a decent ballpark for pressure differential at altitude.

Door height is generally 6’4" or so. Width varies from ~4 feet on a 737 to not quite 6 feet on a 767/777/747. Airbus products are similar or slightly wider.

The door opening mechanism has a lot of mechanical advantage to it. So there’s a difference between “How much air pressure force is pushing the door outwards against the hull?” and “How much torque do I need to apply to the door handle to drive the mechanism to cam the door inwards and slightly away from the hull?”

As well, door handles aren’t all like they used to be. The DC-9, MD-80, 707, 727, and 737 have the traditional big handle that swings through 180 degrees to unlatch and unseat the door. Pretty much anything bigger or newer is done differently.

On the larger airplanes the larger doors don’t first pull inwards then swing outwards. Instead they first pull inwards then roll upwards into the ceiling on tracks along the side of the door frame. With a counterweight system to offset the weight of what’s otherwise a few hundred pounds of metal & escape slide.

In addition, there’s typically an electric motor assist to run them up & down after they’ve been mechanically unlatched by a handle of some sort. As well, there’s a gizmo like a garage door torsion spring which is loaded up as the door is closed. It’s available to slam the door up and open in an emergency evacuation even if the electricity is out and even if the door frame is a little bent from the accident that just happened.
Interestingly I’m wasting time in Chicago right now before continuing my workday. The airplane I brought here this morning was a 767 with the electric assist inoperative on the main entry door. Not a big deal, even 4’10" 100 lb gate agents and flight attendants can muscle the door up & down well enough without it. But what are the odds you’ll ask about something that it’s been years since the last time I’ve had an issue with it?

4

Airliners are pressurized to 10,000 feet, which means 10.1 psi inside. The atmospheric pressure at 35,000 feet is, as you say, about 3.5 psi, meaning the differential pressure is 6.6 psi.

Say a cabin door is about the same size as a door at your house, perhaps a bit shorter, 30 X 72 inches, = 2160 in2, leading to a force of about 14000 pounds. There’s a reason the doors are plug-type (i,e, fit their openings like corks from inside). The only outward-opening doors I know of are the overwing emergency exits on newer 737s, and they’re pretty small.

5

OK, let’s use 6’4" and 4’. That’s 3,648 sq. in. Multiply that by 7.5, and you get 27,360 pounds of air pressure on the door. That’s like ten times what I would have guessed. :eek: (It just popped into my head, and I hadn’t thought about it that much.)

Given that the door must first be pulled inward against the air pressure differential, and given that there is apparently some sort of mechanism to help overcome the pressure differential, let’s say some [del]suicidal nutjob[/del] mentally unstable individual decides to open the door. How many pounds would he have to pull on the door to open it at cruise altitude?

6

Airliners are pressurized to between 7000 and 8000 ft. The 787 Dreamliner is pressurized to 6000 ft.

https://apex.aero/2015/12/10/turning-down-the-cabin-pressure

7

As smithsb notes, that’s not quite right.

AIUI, when the cabin pressure drops to the equivalent of 10,000’ is around the time when the oxygen masks drop.

8

Really? Why?

9

Because:

10

It’s not that the *purpose *of the mechanism is to open the door against air pressure. Even under a very slight pressure load that would be bad. On the ground there have been injuries and a fatality or two over the years when, due to some combo of goofs and malfunctions, the airplane isn’t fully depressurized when they park at the gate or commence an evacuation. The FA goes to open the door, muscles it open against partial pressure, and is promptly swept out the door by the remaining escaping pressure. Oops.

The purpose is that the overall door latching dogs go over-center into the latched state. And have to be pulled back across that over-center condition to become unlatched. The latching dogs may only be moving 1/4" in or outboard in the over center zone. But the operator has ahold of a handle 18-24" long and is swinging it through an arc of 150-170 degrees. That’s a lot of mechanical advantage.

There are more outward opening doors on airliners than most folks realize. As Rocketeer said above, the overwing exits on late model 737s open outward. As do the overwings on the 767 variants that have them, and the overwing exits of all models of Airbus I’m aware of. The vast majority of below decks cargo doors open outward. Some variants of 757 have a narrow full height emergency-only door aft of the wing. That also opens outward.

About the only doors which are plug-type and first move inwards and pivot, then move outwards through the opening, are the 4 corner doors on the typical 737/A320/MD80 type aircraft and the 4, 6, or 8 non-overwing doors on a 757/757/777/787/A330/A340.

Our bottom line understanding out in the field is that no human is going to have the strength to get a door open against cruise pressurization. I don’t know what mechanism prevents the outward opening emergency exists from being opened by a nutcase, but there is one.

Altitudes:

Typical airliners end up with cabin equivalent altitudes of 7000-8000 feet at typical cruise altitudes. Very short flights at lower aircraft altitudes will of course also have a lower cabin altitude. We fly one short segment between Miami and Orlando. We only climb to 19,000 and the cabin adjusts directly between sea level at one end and 100 feet at the other end.

Yes, the 787 is real proud of their 6,000 foot cabin during high cruise; I’ve not had a chance to sample it

The cockpit warning horn goes off if the cabin climbs above 10,000. The passenger oxygen masks typically auto-deploy at 14,5000. Or whenever we hit the switch.

11

That’s the question. No human is going to have the strength to open a door against cruise pressurisation. But what is the ‘virtual weight’ of the door that is too ‘heavy’ for a human to open it?

12

I’m not sure that’s an answerable question. There is no phase in the opening process where the human is simply pulling the door uniformly inwards against the uniform outward pressure differential with no mechanical interference or mechanical advantage.

The question amounts to something like saying "When I use a particular Nautilus workout machine set to “20”, how many pounds am I lifting? IMO the answer is “Who knows? It varies by machine and it varies over the length of the movement. If the machine is well-designed, then we can say that ‘20’ is twice as hard as ‘10’ and half as hard as ‘40’. After that we’re sorta done.”

Racer72 works as an airplane assembler for Boeing. He may be able to look at some engineering specs I have no access to.

13

I think it’s answerable, as long as people don’t get bogged down in technicalities. The ‘mechanical advantage’ complicated it. To make it more abstract, let me rephrase the question:

Assume a room that is pressurized to 7.5 psi above the exterior air pressure, and it has a large hatch that must be pulled inward to open. How many pounds must be applied to the hatches handles to open the hatch? ISTM the general answer to the GQ (using the dimensions of a 737 door) is about 27,000 pounds.

For the Nautlius, I’d answer it with a crane scale. :wink:

14

That all would have been a relief for my mom years ago-after I went to an airline door as a preschooler and tried to open it…

15

All Boeing aircraft are set to drop masks at 14,000 to 14,350 feet. Airbus uses the same switches for their aircraft so they are likely the same. I have tested the 737 oxygen systems a few thousand times over the years, the setting for the auto drop portion of the test have never changed.

16

If you simply mean “Assume the door has a towel-bar type handle across it and the person is simply tugging inboard on that towel bar against the differential pressure”, well then you’ve got your answer.

~7.5 psi time height x width is all there is to it in that simplified case.

I probably overstated the size of the doors a bit in my earlier post. Figure 74" tall by 36" wide for a 737 = 2,664 in[sup]2[/sup] x 7.5 lbf-in[sup]-2[/sup] = ~20K lbf total. For widebodies try 74" x 52" = 3,848 in[sup]2[/sup] x 7.5 lbf-in[sup]-2[/sup] = ~28+K lbf total.

That number is definitely a valid calculation for how much force the door is exerting against the door frame just sitting there under pressure. So, one way or another, that’s the total amount of force a person must generate to unseat the door from the frame. As you say, how exactly they generate that total force, which levers or gears or whatever are available, is just details.
Aside 1: The old DC-9 / MD-80 series had a small pressure relief flap at the bottom of the main entrance door. The first movement of the door handle towards open would move this flap and open a 1" tall x ~3’ wide passage to the outside world. The idea was to allow any residual pressure differential to escape before the door got open enough to risk it being blown open the rest of the way and pushing the FA out at the same time. That improves safety and door opening ease in the narrow case of an evacuation in an almost-but-not-quite depressurized airplane.

I assume, but do not know, that the mechanical linkages were rigged so the same “feature” couldn’t be used in cruise; I expect they’d design it so that’d take more oomph on the handle than people have.
Aside 2: With the traditional Boeing and McD-D narrow body door designs, once unseated, the doors pivot out and forwards. In normal ground use, they rotate through 180 degres and latch open more or less flush with the fuselage forward of the door opening.

So even if somebody somehow got the door fully unseated and all the pressure differential escaped, then they’d need to shove that great big door panel out into the multi-hundred mph breeze and hold it there against the slipstream to be able to get the door open far enough to fall or jump out. That’d need *really *super-human strength. And there’s no mechanical advantage there at all; you’re just pushing directly on an ordinary, albeit heavy, door to pivot it around its hinges.

17

I’m confused. What is this depressurizing on the ground? Why would a person be pushed out of the plane?
The air pressure on the ground is 14.7 psi. During the flight, the pressure inside the plane is much less–about half, to match the surrounding air pressure at high altitude.

I assume that as the plane descends, the pilot increases the air pressure inside the plane* ,to match the surrounding air. But surely the cabin pressure doesn’t increase beyond 14.7 psi ?(if so—why?)

So when you open the door after landing, why would air rush out of the plane, and with enough force to sweep the Flight Attendant out with it?

*(That’s what makes my my ears hurt during the descent, right? )

18

You’re getting confused between *absolute *pressure and the *differential *pressure between inside and out. They’re separate but related ideas.

In flight the airplane is pressurized to a greater pressure than the outside. The system is set up so that after you tell it the altitude you’re going to land at, during descent it will slowly adjust the pressure inside to make the differential close to zero shortly before landing. But the differential will be positive (greater inside than out) all the way to touchdown. Ideally at touchdown the difference is tiny like 0.05 psi. But positive nonetheless. And then upon landing, some vents should automatically open to ensure any small mismatch in either direction is imperceptibly drained away in a few seconds at most.

So, all you need to create a problem is an error in the set-up or a defect in the system and you can touch down and roll out with the cabin still pressurized up to the max above local pressure. IOW, at sea level the outside world is about 14.7 psi absolute and the inside is 14.7+8 = 22.7 absolute. Or, at Denver the outside is 10psi absolute and the inside is 10+8 = 18 absolute. Either way the airplane doesn’t care about absolute; all it’s looking at, and all the structure is “feeling”, is the difference between inside and out. Which is up to 8 in favor of inside greater than out.

Under that circumstance, if the FA somehow gets the door open enough to clear all the mechanisms, then the entire cabin-full of air, umpteen thousand cubic feet, will instantly all try to go out that partly open door powered by the 8 psi differential between it and the lower pressure outside. Which will instantly slam the door fully open and probably drag the FA along with it since she’s got ahold of the handle. As well the sudden wind will grab her and blow her out. From there she’ll hit the ground from 10, 15, or 20 feet up long before the escape slide has even begun to unfurl.
The most common way airplanes land pressurized is somebody goofs and sets the wrong landing altitude into the system. Either somebody misreads the dial, or is working from memory and mis-remembers the correct altitude at destination ABC.

For about 6 months I and *all *of my workmates flew exclusively from one coastal city to another. That knob was *always *set to 10 feet, day after day after day. Then the assigned routes changed and suddenly we were occasionally flying the airplane from the coast into the Midwest or the Rockie Mountains. But still mostly going from seaside destination to seaside destination. You can probably see where this is going.

Arriving in Chicago (600 feet above sea level) with the cabin set to 10 feet produces a bit of an ear rush at touchdown when the dump valves open. Arriving in Denver (5,000 fet above sea level) or a ski resort (7 or 8000 feet above sea level) with the cabin at 10 feet is a big enough ear & throat experience that nobody on the airplane misses it. Cue instant crying babies, scared scaredycats, a blown eardrum or two, etc.

The opposite mistake, setting the landing altitude too high, results in the airplane descending to meet the cabin at, say, 5000 when you’re actually landing at sea level. So from then on down, the cabin altitude matches the airplane altitude.

Which also means the cabin rate of descent matches the airplane rate of descent. We typically descend the cabin at a mere 300 feet per minute, which is gradual enough that nobody gets inconvenienced unless they’ve got stuffed up head/ears. In this case the cabn will be descending with the airplane at 1000, 1500, and briefly 2000 feet per minute. i.e. 5 to 7x faster than is comfortable. Cue *lots *of uncomfortable ears, crying babies, etc.

19

So is it correct that while the cabin can be pressurized at higher than outside, it can’t be pressurized lower? Because if you can do that I’ve got a great new gimmick for my next movie.

20

Correct. Lest the cabin be crushed like hose mythbusters episodes with the railroad tank cars. There are good sized simple spring-loaded valves on the skin that will be pushed open inwards by the outside pressure if it’s even a smidgen higher than that inside.