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I mean, on the one hand it seems kind of obvious, if you had a fuel tank that was mostly water then you would lack combustible materials for the engine, but I get the impression that having any water in the fuel tank at all is considered a really bad idea.

Why is that? Is it all right to have a very small amount of water in the fuel tanks? Is there a defined percentage of liquid in the fuel tank that must be fuel in order to operate safely? (say 99.87% or something?) And does this differ from prop to turboprop to turbofan to jet?

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There will always be trace amounts of water suspended in the fuel; trace amounts don't cause problems. –  David Richerby Apr 16 at 18:08

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Fuel systems are in general fed by gravity, at least until the fuel leaves the tank, where sometimes (always if the tank is lower than the engine) a pump pushes it toward the engine. This means that fuel is generally extracted from the bottom of the tank (or slightly above the very bottom to avoid having small particles from the bottom of the tank clogging up the fuel filter, incidentally this also introduces the concept of unusable fuel, as the stuff remaining at the bottom of the tank, although there are a lot more factors to this).

Fuel is less dense than water, meaning it'll float on top of the water. Or if you want to think of it the other way around, water is heavier and will sink to the bottom, right where you're trying to get your fuel to feed to the engine. Water doesn't burn very well and the engine will stop. Avgas is generally 0.72 kg/l and Jetfuel around 0.82 kg/l (water being 1 kg/l, see have SI units work well that way?).

A tiny amount of water probably mix with the fuel and simply reduce the power output slightly, whereas anything more will be drained from the tank first, and it'll be very hard to restart the engine. You'd have to pump all the water through the cylinder and out the exhaust, and I don't think this is generally possible in flight, especially right after takeoff which is generally the case when you have water in the fuel (could be right after switching tanks too though).

I don't know much about turbine engines, but I suspect they're less susceptible as the water will not prevent relighting the engine (the pumps will simply push the water out, there's no cylinder that could fill up with water), and you'll generally be using up more fuel on the ground making the problem readily apparent. This is pure speculation though.

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For turbine engines, it's not as much the engines as it is the fuel system. With the lower temperatures they tend to fly in, the water can freeze into ice and clog up the fuel lines. British Airways 38 lost power and crashed on approach because of this. It can also cause issues with the fuel level sensors. –  fooot Apr 16 at 14:49
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While water will separate from gasoline (avgas) to the bottom, it mixes and remains dispersed in jet fuel unless there is really a lot of it. –  Jan Hudec Apr 16 at 17:13
    
How about algae or fungus growing in the fuel tank? Would that be more of a problem with a lot of water? –  Phil Perry Apr 16 at 21:07
    
@PhilPerry "stuff" living in the fuel is actually a real-world problem for jet fuel (and other kerosene/diesel fuels) - and does tend to get worse as the water content in the fuel rises (the "stuff" lives in the water and feeds at the water/fuel boundaries). It's enough of an issue that there are products specifically designed to deal with it.. –  voretaq7 Apr 16 at 21:17
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For anyone who's interested; a recent flame-out in Calgary was attributed to water in the fuel –  falstro Apr 25 at 11:46

Water in the fuel system is something of an over-inflated issue in aviation. Most aircraft have unusable sections of the fuel tank that will hold upwards of one gallon. That means that unless you have a gallon of water in the tanks it's unlikely that you'll be picking any up. We sump the tanks before every flight because it's easy to prevent an issue, and we're also pulling out any debris which is a much bigger issue than water. But if you're just running along and a bit of water sloshes into the fuel intake, it's unlikely that you'll even notice. Now obviously an engine can't burn straight water, and that's why we have systems in place to mitigate how much gets into the system.

Small amounts of water injected into the engine will actually boost performance. Water's heat-absorption properties allow it to (once injected) suck up much of the left-over heat from the engine's previous combustion cycle. This means that the engine is cooled and less work needs to be done to compress the fuel/air mixture. Upon combustion, the water vaporizes and expands much better than dry air, boosting compression greatly. The practice of using water (and often alcohol) in engines is known as Water Injection and has been around for a long time.

However, putting too much water in the engine can over-boost compression, which can result in blown cylinders, cracked pistons, and many other forms of damage. It can also lead to hydro-lock, which is when the expanded water vapor occupies more volume than the cylinder provides. This can at best stall the engine, or at worse blow a cylinder.

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I'm not sure I'd say water contamination in fuel is an "over-inflated issue" -- Every year we have a bunch of accidents because some dolt didn't sump their tanks (or "got a little bit of water", tossed it away, and didn't keep draining to look for more). Having your engine try to burn water 200 feet off the ground in your initial climb can really ruin your flight. –  voretaq7 Apr 18 at 5:37

Since water is more dense then kerosine, the water will sink to the bottom, where the fuel tank outlet also happens to be, meaning that if there is a sizeable amount, the engine will stop, since there's simply nothing to burn.

Approximate densities of a few fuels:

  • Water: 1 kg/l
  • Kerosine: 0.82 kg/l
  • 100LL: 0.72 kg/l
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Water mixes with kerosene quite well. –  Jan Hudec Apr 16 at 17:13
    
@JanHudec So, does that mean this answer is incorrect? –  Articuno May 25 at 19:56
    
@Articuno: It would be correct for gasoline, but as long as it says kerosine, I believe it is not. At least until there is a lot of water, than it will also gather at the bottom. –  Jan Hudec May 25 at 20:02

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