Last November an American Airlines flight attendant died after an emergency landing in Miami. The guy popped the door too soon and supposedly got sucked from the plane, landed on the tarmac and died. I can’t figure out how the pressure in the cabin could have been greater than the outside air once the plane had landed. Even if the plane hadn’t depressurized yet, the cabin altitude should have been higher than the surrounding sea level air (lower pressure), pushing the guy in instead of out. I figure the guy simply fell out, but the articles say he was sucked. I’m stumped. Someone please figure this out!
Welcome to the board.
There was a thread on this subject a while ago. Try this link:
http://boards.straightdope.com/sdmb/showthread.php?threadid=47450
Hope it answers your questions.
Note: The woman passenger who fell out of a commuter plane over Sacramento yesterday must have jumped on purpose.
They made a special landing when the door light went off the first time and had the door checked. Then she did it again after they took off again.
The specialist for the manufacturer said it takes a lot of force to open the door in flight, as the wind pushes it shut, not open. It checked out OK again when they landed.
The reporters (news gatherers) aren’t pretty smart. They like big words (“depressurized”). Sound high-tech for them. Their reports have very low common denominator, from The NY Times to The Globe. Pressurize or shmesurized, the moment he opened that heavy door, the pressure equalized. He did not fall becase of pressure gradient, and there was none. If the door opened outside, I guess I understand why he fell. But I do not want a lawsuit, so I stop here.
Peace.
I’m sure someone will correct me if I’m wrong, but I’m think that an airplane’s cabin pressure can never be lower than the surrounding air. Pressurization systems work by maintaining a pressure differential between the cabin and the outside. I don’t think that they can create a situation of a “negative pressure.”
OK, Dr. Lao, you asked to correct you, so, here I go.
The cabin pressure may, but never is, higher than the outside pressure. In flight, it is maintained equal to the atmospheric pressure at 10.000’ (3,000m). It is done to diminish stress on the fuselage. 10,000’ pressure is easily tolerated by sitting people. Pay attention during you next flight: your tympanic membranes will react to the changes during ascent/descent. Ask a stewardess for a candy and ask her if I was right.
Peace
peace:
I think you misunderstood what I was saying. I am aware that a pressurized aircraft is not pressurized to sea level when it is cruising at 35,000 ft. My question was about whether the pressure can ever be lower in the cabin that outside the cabin. For example, is it possible for an airplane to be sitting on the group in Miami with a cabin altitude of 10,000 ft? I don’t think it is. An airplane pressurizes by using bleed air from its jet engines (or turbo charger, if it is a piston powered aircraft). There isn’t a pump to remove air from the cabin, so I can’t envison a situation where the cabin altitude of an aircraft is higher than it’s pressure altitude.
On the contrary, the whole purpose of pressurizing an aircraft is to make the cabin pressure higher than the outside pressure. Take a look at your statement. If the cabin pressure is maintained at 10,000 feet (actually, I think it is maintained nearer 8,000 feet, but I believe the cabin altitude can be adjusted) and the aircraft is flying at 35,000 feet, then the cabin pressure is greater than the outside pressure.
Also, as far as “diminishing stress on the fuselage”, pressurization actually causes stress on the fuselage (because it’s pressurized). Aircraft go through pressurization “cycles”, which can over time cause fatigue. This is what caused the first jet airliners, the DeHaviland “Comets”, to crash.
Actually is easily tolerated by breathing people. Above ~12,500 ft, you need supplimental oxygen to be able to think/react sufficiently to fly a plane. At 30,000 ft, you would soon be incapacitated, and at 45,000 ft, you would quickly be dead (e.g. Payne Stewart & co. in their Lear Jet).
Q: My question was about whether the pressure can ever be lower in the cabin that outside the cabin
Sorry, Dr. Lao, if I misunderstood your question. It is lower, above 10,000(8,000)ft. Above this altitude, the blowers pump outside (conditioned) air into the cabin Normally the air pressure in the cabin is maintained equal to the outside pressure, up to 10,000ft (8,000ft). Above this attitude, the inside pressure is kept constant, equal to normal atmospheric pressure at 10,000(8,000)ft. It allows for some structural savings (see the ref. below, but the mechanichs are self-evident). When the plane descends, the pressure gradually increases, from 10,000(8,000)ft, to sea-level and kept equal with the altitude, till it reaches the ground/sea level. My first (unclear) sentence should have been: “At sea level, the cabin pressure may, but never is, higher than the outside pressure.” I meant, that although the air pressure can be slightly increased inside a parked sealed plane, it is never done, because it is not nessary. So, as far as the discussed accident: the air pressure inside and outside the plane below 10,000(8,000)ft is equal. Opening of the door would not cause a slightest sound, as there is no pressure gradient.
TXL, are you playing with words? Sitting (not running) people are breathing. They are breathing air at the same pressure as the air is, up to 10,000(8,000)ft, outside. At cruising altitudes, the air pressure inside, as I said, is equal to 10,000(8,000)ft. Read more about it here:
http://kudos.traveldesk.com/s/Piedmont_Airlines_Inc/1.html
http://www.sci-ctr.edu.sg/ScienceNet/cat_physical/cat_gen04268.html
Air pressure inside the airplane is kept at standard atmospheric pressure (at sea level)(although airlines may typically have it lower as to save on the structural costs of the airplane). It has to be standard atmospheric pressure so it’s comfortable. As airplane ascends, outside pressure drops, and hence inside pressure becomes higher, and if there is a hole, materials inside the airplane will get sucked out.
peace:
I am not playing with words. Just clarifying… some people have posted here and on your second reference that airliners are pressurized for passenger comfort. It seems that arriving at your destination without being dead from asphyxiation is a little more than a comfort issue.
If the air relief valves/vents do not function properly, there can be significant overpressurization of the cabin.
It is quite possible for the pressure in the cabin to exceed sea level pressure, especially if the pressurization system malfunctions. That was the primary cause for the emergency landing, according to the preliminary NTSB report for the incident referred to above- see: http://www.ntsb.gov/aviation/mia/01a029.htm .
sorry, folks, i didn’t look back far enough in the archives to see that this had already been discussed. i appreciate the answers, though. what galls me is the ntsb said they were investigating the press. system-- “The pressure is supposed to equalize the minute the airplane touches the ground,” said Jorge Prellezo, regional director for the National Transportation Safety Board. “That’s why we’re investigating.” those guys should know better.
TXL, thank you for the link. Although preliminary and confusing, it provides some information. Apparently, there was some “pressurization” problem in the air. I used “” marks to indicate that the verb is used to indicate any change of air pressure inside the cabin, compared with outside. I guess, we agree that below 10,000(8,000)ft is is equal to outside pressure. Furthermore, I gather that at no point the inside air pressure did not differ much from the outside air pressure or from that of 10,000(8,000)ft: people had no trouble breathing and emergency equipment was never activated. I also think that the door malfunctioned or something, because a really high pressure would be needed to build up inside to "explode"it. Whatever happened after landing did not lead to any significant build up of the aur pressure inside (still sealed) cabin: people would feel it otherwise and it would’ve been registered by the instruments/dials. Agree?
Peace
It is a comfort issue, because asphyxiation isn’t the alternative. The alternative to pressurization is oxygen masks for all of the passengers and crew members. That is what the bomber crews of the Eighth Airforce did over Europe during WWII. It works, but it is not a whole lot of fun.
peace:
Cabin pressure at lower altitudes is not necessarily equal to outside pressure. The preliminary report above refers to a ram air switch controlling depressurization on the ground. I don’t think the cabin pressure is normally equalized until the plane is safely on the ground (a ram air switch senses that the plane is no longer moving fast enough to fly, and opens the outflow valves). This could be inportant in the case of a missed approach, where the plane has to climb & go around again.
Still, the differential pressure should not be enough to “suck” a person out of the door, but could be enough to tug on the door (larger area=larger force), pulling the crew member off balance. The hapless crew member then tumbles out of the door and falls 30 feet to the concrete runway (or taxiway, or apron), cracks his head & dies.
Dr. Lao:
I guess that is a good point (pressurization vs. O2 masks). I don’t think there would be as many air passengers if you had to get trained in the use of O2 masks before being allowed to fly.
TXL, returning to the OP: whatever happened (it could be months before the NTSB, etc. completes the investigation), the pressure gradient was not sufficient to “suck” or to propell a person out of the plane. The poor FA has somehow lost his balance and fell out.
As far as Dr. Lao’s concern: I hope you understand, that passengers could not be more comfortable that when they breath air at the sea level pressure. The airlines keep “cruising” pressure slightly below, at 10,000(8,000)ft. So, I guess, when airlines say “for passengers’ comfort” they mean “not as it would have been otherwise, let’s say at 35,000ft”. 10.000(8,000)ft is not sea level, but is quite “comfortable” for a passenger sitting in a chair.
Peace
The woman in California fell from a DeHavilland Twin Otter, which is NOT pressurized, at a comfortable breathing altitude of about 5000 feet. Her fellow passengers report that she opened the door herself, which would take a little force with the airflow blowing it back, but is quite possible. The passengers couldn’t make the flight crew understand what had happened due to cabin noise.
The woman has since been reported to have been depressed and suicidal. No reports on why she didn’t get more help sooner, but we all know everyone’s personal stories are different and are subject to hindsight.
Completely useless info: the woman that jumped from the DeHaviland turned out to be Dutch.
Pressurization: First off, I’m a complete ignoramus on this matter (and this matter alone, thank you ;)). But am I the only one noticing pressure changes in many commercial aircraft during the taxi period to the runway?
I think they might already start decreasing cabin pressure during taxiing in order to make depressurization during ascend less drastic. Is it just me, or is there any evidence for this?
As has been pointed out, the pressurization systems are only designed to increase pressure in the cabin. What you may be experiencing is the equivalent of what I feel in my car when I switch from ‘fresh air’ to ‘recirculate’. My ears can definitely tell there is a change in pressure in the car, but it is not significant compared to the change you’ll experience when the plane climbs to cruise altitude.
Indeed, the pressure may start to increase before take-off, but I’d be surprised. I think it is a verification that the pressurization system is working, by closing some valve somewhere - a minor change in pressure that just indicates everything is working OK.