General Question

colog's avatar

Why do airplanes float in water?

Asked by colog (73points) January 20th, 2009

When US Airways Flight 1549 landed in the Hudson, it remained on the surface of the water for quite some time before sinking. Why does a heavy aircraft float for even a minute?

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15 Answers

TitsMcGhee's avatar

There is air inside it, making it somewhat buoyant. Think about how a submarine works.

colog's avatar

But a submarine sinks! And it has engines that propel it to the surface! Bad example.

wundayatta's avatar

Subs only sink when they take water into their balast tanks. Otherwise, they are lighter than water, and float.

The airplane sinks as water seeps inside, taking the space of the air that keeps it bouyant.

Sakata's avatar

Not to mention that every intake and outlet valve on the entire plane seals up when a crash landing is immanent making the cabin of the plane air/water tight. That plane could’ve sat there in the water for quite some time before they had to actually evacuate.

Side note: Cheers to the flight attendants for not opening the back door. They would have killed everyone on the plane in a matter of seconds. Nice work team.

Bagardbilla's avatar

They are designed to do so.
The bottom half is a sealed compartment (full of air), they do eventually sink, but by NTSB saftey guidelines they are required to remain aflote for a certain period of time.

jasongarrett's avatar

Like anything else that floats, it weighs less than the water it displaces.

mea05key's avatar

like the titanic. when the ship was cracked into half, the other half stayed like 1 minute before sinking into the deep sea. i would say its due to the buoyant force by the water acting on the ship is temporarily higher than the weight of ship because the ship breaking into two releases the water contained in the in the ship. Or probably the sinking of the other half of the ship displaces the water to generate force to support the remainning half. OUt of context sry. LOL

robmandu's avatar

A plane will eventually fill up with water. It’s not a water-tight vessel.

Flight 1549 stayed afloat for so long because it crash landed in a relatively populous area with a lot of water craft. Several of those lashed onto the aircraft to help keep it afloat for far longer than it otherwise would’ve been capable of.

The New York Times reported :
Soon, a small armada of police boats, fireboats, tugboats and Coast Guard craft converged on the scene, and some of them snubbed up to keep the jetliner afloat.

cyndyh's avatar

That and a certain amount of surface tension.

Grisson's avatar

Speaking of surface tension, if you put a small amount of soap in the water, would the jetliner float longer, or sink faster?

Elumas's avatar

@Grisson It would not float as long because the soap would disrupt the hydrogen/oxygen bonds that create surface tension.

bpeoples's avatar

Also, the particularly well sealed part of the plane is the fuel tanks in the wings. Assuming they’re full of kerosene (Jet fuel is similar to Kerosene), each gallon of kerosene will hold up about 1.75# of plane.

If they’re full of air (as they are when the airplane is empty) each gallon of air will hold up about 8.33# of airplane.

So—the maximum fuel capacity of an A320 is 7842 Gallons (per wikipedia), fully loaded, the fuel tanks will hold up 13,000 pounds of boyancy. This is in addition to whatever boyancy is in the bottom hold, and other airspaces throughout the craft. Empty, the tanks will hold up about 65,000# of craft. The maximum takeoff weight is 169,000#, and the typical empty weight is about 95,000# pounds, so regardless, the tanks won’t keep it afloat, even if full of air.

However, that’s a fair amount of boyancy to keep things up for a while, while the other unsealed parts of the plane take on water.

cyndyh's avatar

@Grisson: I didn’t remember for sure, but I think Elumas is right.

bpeoples's avatar

With something akin to an airplane floating in the water, surface tension is going to have very little to do with whether it floats or sinks.

Buoyancy, as it were, is essentially the ratio between an objects density, and the density of the water (or whatnot). Soap isn’t going to affect the density of water much. It’ll make a water strider sink (since they’re denser than the water), but it won’t make a cork sink.

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