Surely you've all been wondering, here's one answer I ran across (more at the link):
All airplanes will eventually sink if it is in water, even pressurized
planes. (more on that later) But there are several areas in the
airplane that have pockets of air that help keep the plane afloat. For
example, in the area between the outside skin of the fuselage and the
interrior there is a space that is usually insulated and has air that
needs to be displaced by the water. In most airplanes built today, the
wing is the fuel tank, and since water is heavier than fuel the fuel in
the wings help offset some of the weight of the plane…not a lot but
some.
There is also air in the cargo hold of larger planes that will help
maintain buoyancy until the air is replaced by water. Anyone who thinks
an airplane is water tight and will float because it is pressurized is
nuts! The airplane is pressurized only while the engines are running
and the air being pumped into the aircraft to pressurize it is almost
escaping the aircraft just as fast as it is being pumped in. There are
control valves in the forward and rear bulkhead that regulate the
pressure inside the plane but all pressure is lost if the engines quit
running. At the altitude that the A-320 that crashed in the Hudson
river was at when it lost it's engines, it probably didn't have much
pressurization anyway since it was only a few thousand feet above sea
level.
My favorite Airport movie was the one with the sunken plane.
Ooh, a quick check of Netflix and I learned it was a Concorde.
Airport ’77.
Just because the engines aren’t running doesn’t necessarily mean that the cabin will lose pressure…those same valves that allow air in can also be closed from the flight deck. Perhaps this is the “ditching button” that was discussed on the Today Show (to my knowledge, the A320 does not have one button for this function).
A plane floating is just a matter of buoyancy. Something to keep in mind is that commercial aircraft are incredibly light for their volume…their skin and internal components are pretty light to begin with, and then one must consider that the interior of a plane is mostly empty space. The engines are by far the heaviest part, and according to the NY Times, one of them has already been lost.
The Economist had an article in December 2002 that quoted a Mr. Jackson from “Jane’s All The World’s Aircraft” who stated this:
“No large airliner has ever made an emergency landing on water. So the life jackets, with their little whistles and lights that come on when in contact with water, have little purpose other than to make passengers feel better.”
I don’t have a subscription to the Economist, but this article was referenced here:
http://dir.salon.com/story/tech/col/smith/2004/03/19/askthepilot71/index.html
http://en.wikipedia.org/wiki/Ditching
Am I the only who finds it humorous that in Wade’s second link, there are about a dozen examples of commercial passenger planes that have ditched in the water?
Wouldn’t increased air pressure in the cabin make an airplane heavier? (More air molecules crammed into a fixed volume equals increased density and weight, right?) So for ideal buoyancy, the cabin would have lower air pressure, but be airtight.
Wade: that economist article was just dead wrong; I hope that’s what you were intending to point out.
TomHynes: I believe some people did swim yesterday, there were reports of some of the last to leave swimming to the boats. Of course you’re not going to swim very far in freezing water (5 minutes is the figure in my head) but not all the world’s water is freezing.
Sully/ThaPlumber ’12
Sorry, the relevance is that the plane at sea level has an interior air pressure also of sea level.
Tom, correct about the surface tension. I was think buoyancy and confusing that with surface area. What I should have said: wings may float, fuselage won’t, add the two together and you get a much better scenario.
Wow, all of those unpressurized dingies. How did they float?
For that matter, all of those balloons filled with pressurized gas, ditto?
(Am engineer)
(Am also smart@$$)
K, I have a degree in physics.
1) The poster who said that surface tension for floating is not important is absolutely correct. Surface tension is absolutely tiny and only works for floating things that don’t break the surface of the water, anyway.
2) Yes jet fuel is lighter than water, hence it adds to the buoyancy of the plane.
3) An airplane is more or less airtight (can be approximated as such over short timescales.) This means that it takes a while for the water to get in.
4)
For a large object, floating means that your weight is less than the volume of water you displace. Which is another way of saying that your average density is less than that of water. In most water tight objects (like ships, planes,etc) most of the weight is in the surface. Volume increases as x^3 and surface area increases as x^2. This means that the larger the object is (for large mostly hollow objects), the easier it is to float. This means that a 747 or (in this case an Airbus 320) is easier to float than a little leer jet.
As a side note, this works for objects that float in air too, if their average density is less than that of the air, they will float. This is what allowed Buckminster fuller to propose his wild ideas about giant floating-in-the-air geodesic cities. They would, in theory, work like hot air balloons, but they would be so large that the temperature difference between the inside and outside only need be 1 or 2 degrees, which is achievable by warming from the sun.
Following the Wade Nichols Wikipedia link, I thought this was the BSIRT:
“For example, Ralph Nader’s Aviation Consumer Action Project has been quoted as claiming (though not offering proof) that a wide body jet would ‘shatter like a raw egg dropped on pavement, killing most if not all passengers on impact, even in calm seas with well-trained pilots and good landing trajectories.’[1]“
The biggest problem with water landings in the modern era, is the low slung under wing engines, which eventhough they are designed to detach, produce a huge catastrophic break-up hazzard in a ditching. This is actually the first twin to survive a water landing intact, and some early speculation is that the engines must have completely flamed out to allow for this, otherwise even slow spooling fanblades would suck up water upon impact and produce drag. All the other water landings have been rear fuselage mounted engines.
Airbus engineers should be given major props for some excellent design, and are equal heroes in this remarkable intersection of luck, skill and ingenuity. The A320 does have a ditching mode in it’s flight management system that swithces of the fly by wire features, adjusts the valves, and seals vents. Another kudo’s to modern technology is the Flight Management System’s, flight envelope controls, which adjusted flight controls to keep the aircraft from stalling, as the pilot brought it down.
Great example of MAN, MOMENT & MACHINE. By the way the pilot’s an old airforce fighter pilot..nerves of steel they say.
I love the idea that you need a degree in engineering or physics to comment on a topic that used to be mastered by 14-year-olds. Did Archimedes live in vain?
Empty fuel tanks are going to be better for flotation than full ones: the rest of the volume is air, and unless they crack on impact, the tanks are certainly watertight.
Aren’t empty wing tanks more likely to crack than full tanks?
It’s obvious that’s all about the principal of archimedes and buoyant force of course. A body swims when it displaces just as much of bulk as it posseses itself.
For reference, the jet fuel in a fully loaded Air Bus A320 would only provide 4800-6000 kg of buoyancy. Take-off weight for an A320 is on the order of 74,000 kg.
Comments on this entry are closed.