Why is water in the fuel tank bad?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

The other problem with water contamination is that it can freeze in the fuel lines at high altitude, effectively blocking the fuel line with ice, and preventing the fuel getting through. Fuel treatments such as Liquid Engineering Fuel Set are recommended for preventing water from settling in fuel storage tanks, but not for aviation. This is because the water will still be present in minute particle size,- suspended in the fuel , and burnt in the fuel. But the tiny particles of water can still ice up at sub zero temperature.

Answer 5

Although I would agree that “because of ice” and poor combustion are probably the MOST important reasons, I thought I’d add that water can wreak havoc on many fuel quantity systems.

Most fuel quantity systems are a capacitance style system in which the fuel acts as the dielectric of a capacitor. When water sits at the bottom of the tank, it can cause the dielectric value of the probe(s) to drop to essentially zero. This can cause a myriad of problems ranging from inaccuracies to total system failure.

Answer 6

fuel tank corrosion will also be a problem at the lowest spot in the tank, where the water tends to puddle under the gasoline. This is a very common problem in motorcycles during extended periods of disuse and can actually perforate the fuel tank.