Why is water in the fuel tanks bad?

[triplese7en]

So a student comes up and asks why water in the fuel tanks is bad. What's your best explanation?

Playing devil's advocate:

There's moisture in the air and that doesn't cause a problem with the engine. With a dew point of 20 degrees C, the water content is 17.15 grams/cubic-metre. The water in the tanks will flow with the fuel to the engine where it will vapourize and be no different than the water already in the air. If the water were to vapourize in the fuel lines as it approaches the engine, due to the heat of the engine, the fuel pump would push the water vapour through the lines and into the engine.

[New_PIC]

Low time and not even PPL yet but I'll take a shot:
Assuming it doesn't freeze and block the line, it still displaces fuel. No fuel, engine stops.

[ahramin]

Entrained water in fuel is a problem in some fuel systems because it freezes and blocks fuel flow. Over time you also have corrosion and fungal problems.

Most spectacular example of this was relatively recent, BA 38:



There's also the obvious problem that engines won't run on water but hopefully any student already understands that before they decide to learn to fly a powered airplane.

You don't need to worry about water vaporizing in the fuel lines, fuel vaporizes far more readily.

[CpnCrunch]

Water is denser than avgas, so it collects at the bottom of your tanks. Usually (but not always - see link below) the fuel drains are at the lowest points in the tanks so you can drain out small amounts of water, but if there is enough water in the tank it will obviously go into the engine. Also if you neglect to drain the water, the engine might start okay but the water could move over the engine inlet at an inopportune moment.

Also, have a look at this and it should scare the crap out of you if you're complacent about water in fuel:

http://www.sumpthis.com/cessna150andces ... 24x768.htm

[DanWEC]

Water is denser than avgas, so it collects at the bottom of your tanks. Usually (but not always - see link below) the fuel drains are at the lowest points in the tanks so you can drain out small amounts of water, but if there is enough water in the tank it will obviously go into the engine. Also if you neglect to drain the water, the engine might start okay but the water could move over the engine inlet at an inopportune moment.

Also, have a look at this and it should scare the crap out of you if you're complacent about water in fuel:

http://www.sumpthis.com/cessna150andces ... 24x768.htm

Holy crap! Why??

So I'd suggest that the moral of this story is have someone push down on the tail of a C150 when straining the fuel. I wonder if a C172 is similar.

Now that being said, it doesn't seem to have been a huge issue thus far in the long tenure of the c150.

[photofly]

The water in the tanks will flow with the fuel to the engine where it will vapourize and be no different than the water already in the air.

There are two errors in that sentence. The first is that the water won't flow "with" the fuel - if there's enough liquid water in the tank, it will flow instead of fuel, with obvious consequences.

The second error is that avgas is much more volatile than water. The carburetor (and the rest of the induction system) is designed to vaporise avgas, and it's unlikely that much liquid water would evaporate through the same system. By illustration of the same, the vapour pressure of gasoline at 38C (I read) is close to atmospheric, which is why carburetors work well and why fuel system designers have to worry about vapour lock in fuel systems. By contrast, the vapour pressure of water at 40C is only about 2 inches of mercury; it only reaches 1 standard atmosphere by the time the temperature has risen to 100C (of course).

Diesel oil and Jet-A are even less volatile than water. That's why you need high temperatures and high pressure injectors to aerosolize and evaporate them for combustion in diesel and turbine engines.

[Beefitarian]

I would say, "Because it does not burn. In fact often it is used to put fires out. Fire is what powers the engine."

Then I'd make a funny noise like,"Neeeeeaaaaaarrr" and motion my hand in a dive followed by saying, "KA-BOOM" with both hands raised suddenly.

[CpnCrunch]

Water is denser than avgas, so it collects at the bottom of your tanks. Usually (but not always - see link below) the fuel drains are at the lowest points in the tanks so you can drain out small amounts of water, but if there is enough water in the tank it will obviously go into the engine. Also if you neglect to drain the water, the engine might start okay but the water could move over the engine inlet at an inopportune moment.

Also, have a look at this and it should scare the crap out of you if you're complacent about water in fuel:

http://www.sumpthis.com/cessna150andces ... 24x768.htm

Holy crap! Why??

So I'd suggest that the moral of this story is have someone push down on the tail of a C150 when straining the fuel. I wonder if a C172 is similar.

Now that being said, it doesn't seem to have been a huge issue thus far in the long tenure of the c150.

I think it's only an issue if you have a lot of water. Generally there should only be a little bit of water in the drains (if any). If you're seeing a lot of water, you probably need new fuel caps (or fly more often).

I'm not entirely sure what is a 'lot' of water...generally I see zero water in a hangared plane, or a maximum of about 1cm at the bottom of the fuel tester for a plane sitting outside. If I saw the water come more than half-way up the fuel tester, I would be worried. However I live in the prairies, so perhaps its normal to see more water than this if the plane is parked outside on the wet coast.

[fanspeed]

So a student comes up and asks why water in the fuel tanks is bad. What's your best explanation?

Playing devil's advocate:

There's moisture in the air and that doesn't cause a problem with the engine. With a dew point of 20 degrees C, the water content is 17.15 grams/cubic-metre. The water in the tanks will flow with the fuel to the engine where it will vapourize and be no different than the water already in the air. If the water were to vapourize in the fuel lines as it approaches the engine, due to the heat of the engine, the fuel pump would push the water vapour through the lines and into the engine.

Good point. Whereas water is 1000000 g/ cubic meter...

[trey kule]

Good grief.. Do students really ask that question? And do instructors need to look for an answer?

...engines don't run on water...anyone who does not understand that should not be flying.

Now, with jet fuel, the problem of water is different. Water and avgas , unlike scotch and mountain dew, do not mix well. Let the tanks settle for a bit and the water will mostly settle out. And being denser than avgas will settle to the bottom where the fuel outlets are located.
With jet fuels, the water and fuel mix a bit better (but I still prefer scotch and Mountain Dew), and the water is held in suspension unless it is a serious contamination. The typical problem is that if the jet fuel gets cold enough, for example at high altitude , the moisture will turn into ice crystals and plug the fuel lines. Very large aircraft have fuel preheaters to deal with this. Smaller aircraft, where the fuel heater may be inadequate add a chemical to keep the water in suspension at low temperatures.
There was a very tragic accident in the US not all that long ago involving a PC 12.

In any event, a ppl student does not need a discourse on jet fuel, even if their instructor is awaiting the impending call from a major,
As to piston engines. K.I.S.S. they don't run on water. ( and yes I know about water injection....do you really want to get into that with a student pilot who is asking such a question in the first place)

[Chuck Ellsworth]

Here is another question about water.

If friction produces heat, how long would it take a dog to hump a pail of water to a boil?

[azimuthaviation]

Here is another question about water.

If friction produces heat, how long would it take a dog to hump a pail of water to a boil?

Apply for a research grant, you might find out. But all in all it relly depends on the size of the dog.

[azimuthaviation]

Water is denser than avgas, so it collects at the bottom of your tanks. Usually (but not always - see link below) the fuel drains are at the lowest points in the tanks so you can drain out small amounts of water, but if there is enough water in the tank it will obviously go into the engine. Also if you neglect to drain the water, the engine might start okay but the water could move over the engine inlet at an inopportune moment.

Also, have a look at this and it should scare the crap out of you if you're complacent about water in fuel:

http://www.sumpthis.com/cessna150andces ... 24x768.htm

Holy crap! Why??

So I'd suggest that the moral of this story is have someone push down on the tail of a C150 when straining the fuel. I wonder if a C172 is similar.

Now that being said, it doesn't seem to have been a huge issue thus far in the long tenure of the c150.

Theres an STC for 6 point fuel sampling on 152's. Probably what the makers of that experiment are selling. The water still will go to the gascolator before the engine and thats where you want to test for water the most. It isnt a bad idea ro rock the wings then wait a few minutes before you test for water, that will probably do more than anything else to detect water in your fuel.

[Big Pistons Forever]

If I had a student ask "why is water in the fuel tanks bad ?" I would want to ask some further exploratory questions to find out if the student is in fact so ignorant of the basics of how an internal combustion engine works that they can't figure out that feeding it water instead of fuel would be a bad thing.

While we are on the subject I think sampling the fuel is "monkey see monkey do" at most flight schools without any thought about how to do it properly.

Some things to think about.

1) The attitude of the aircraft is important when sampling an aircraft that has only one drain at the back inside corner like all but the post 1996 new build Cessna's. If the the aircraft is parked on a slope such that it is in a nose down attitude then it is impossible to get water out of the tank unless you lower the tail as all the water will be at the front of the tank and will be not go out the drain. Similarly if the aircraft is tilted to one side because it is on a slope than the wing that is on the down side can't be drained.

2) Leaking fuel caps can let in a lot of water. If the aircraft has been sitting outside in a rain storm draining the tanks is a really good idea. if you are consistently finding water in the tank of your airplane get new fuel cap seals. If it is a rental snag it in the Journey Log.

3) There is absolutely no point in sampling an aircraft that has just been refueled as it will take 20 to 30 minutes for the water stirred up by the incoming fuel to settle out again. If you have doubts about the fuel then sample it before you put it in the aircraft by sampling the fuel truck or fuel supply tank.

4) If there is no water in the fuel when you sampled the tanks before you takeoff then when you land an hour later on you cross country then there won't be any water when you stop for 10 minutes and go again. So sampling fuel on at the stop on your PPL cross country which I have seen several times is IMO just silly.

5) When you take your sample make sure you actually look at it. The blue die in 100 LL can be pretty pale and it is actually not that hard to mistake a strainer full of water with a strainer full of uncontaminated fuel.

[trey kule]

Cat

As the answer to your question is one that all student pilots should know, and therefore their instructors should know it in depth, I would like to answer it.
But unfortunately, I don't know how to upload the 15 power point presentations graphs, and charts, addressing the question, and it would take me pages to write the answer when I could explain it clearly in little over an hour during a pre flight briefing, I will have to pass.

Kudos for asking though. As we all know there are absolutely no dumb questions

[Beefitarian]

Here is another question about water.

If friction produces heat, how long would it take a dog to hump a pail of water to a boil?

Apply for a research grant, you might find out. But all in all it relly depends on the size of the dog.

It'll never happen, even with a JackRussell terrier and those little sons of bitches are the highest energy dogs available.

[CamAero]

...engines don't run on water...

http://www.youtube.com/watch?v=J9iWaCMbw60

[digits_]

3) There is absolutely no point in sampling an aircraft that has just been refueled as it will take 20 to 30 minutes for the water stirred up by the incoming fuel to settle out again. If you have doubts about the fuel then sample it before you put it in the aircraft by sampling the fuel truck or fuel supply tank.

I am aware of this sentence. However, I really doubt it's accuracy. The difference in density between water an fuel is quite big. Fuelling does not create huge currents, only a little turbulence in the fuel. Where does these 20 - 30 minutes come from? If you mix some fuel in a bottle, add water to it, you will see that the water goes to the bottom of the bottle almost instantly.

[Beefitarian]

...engines don't run on water...

http://www.youtube.com/watch?v=J9iWaCMbw60

You know what they say. "If it sounds too good to be true..." I'd like to convert my wife's Avalanche over to running on water. Can anyone go check out this potential Snake Oil tonic first hand?

http://www.hytechapps.com/

[trey kule]

Well, it seems that the engine itself does not actually run on water, but that water is the feedstock to provide the hydrogen and oxygen.
But I could be wrong about it, and an willing to admit that engines can run on water. But I am a bit cautious. So, here is an idea Cam....next time you go flying, in a avfueled piston engined plane, add bout 20 litres or so of water to each tank...let us know how it works out or you.

Now on to a bit more serious point. I must admit I was always taught to let the fuel settle after refuelling with avgas, before sampling it. I don't remember an actual time ever being offered. I just took it on faith and it seemed to have worked out ok over the years. But your claim that the separation is immediate and there is no need to wait is interesting. Anyone have an authorative reference?
Something say, from the fuel companies.

[Beefitarian]

5) When you take your sample make sure you actually look at it. The blue die in 100 LL can be pretty pale and it is actually not that hard to mistake a strainer full of water with a strainer full of uncontaminated fuel.

Here's a advantage to being old. Has anyone ever poured fuel through a chamois to separate the water from it while pumping out of barrels?

[trey kule]

Many many moons ago, yes, but finding real chamois today, would be difficult I think. And mostly the concern was foreign particles from the lining of used barrels. Besides, for the last 15 years or so that I know of, most people have filters on their pumps if they are using barrels. The heli folks even have electric pumps and special little plugs on some of their machines.
The days of climbing up on the wings with a bucket go gas are thankfully, I hope gone

[Beefitarian]

Many many moons ago, yes, but finding real chamois today, would be difficult I think.

They are not on the shelf at the general store or the Walmart super center that wiped out most of those for a 50 mile radius. (The kids still know what a radius is yeah?)

I think one could find a genuine sheep chamois if they put Ina little effort. I might even google it if I don't get distracted. Would I fail another business if I made a fuel strainer with a small piece of chamois in it to separate water. If it gets to the lower half of my strainer it's fuel. If it's trapped on top, it's water.

Come visit my booth at OSH 2014?

[Colonel Sanders]

Here's a problem with water in the fuel system.

I know of an aircraft that didn't get it's fuel drained
regularly. It accumulated a bit of water in the lower
part of the fuel system, around the drains. Because
water is heavier than fuel, it was no operational
biggie - it stayed there.

Until it got cold, and the water froze and expanded,
and cracked the fuel lines and started p1ssing fuel.

Not good.

You don't want water in your fuel system for a whole
bunch of reasons.

Here's another one. You're flying across a great lake,
and you decide like a good boy you're going to climb
up so you can stay within gliding distance of land. Say
9500 feet. You don't normally fly that high. Freezing
level is at 7000 feet. Water in your fuel freezes. Oops.

You also don't want water going through your fuel
system at the wrong time. I once had enough water
in the fuel system of a Mooney that the engine quit
taxiing. Heavy rain in Florida, old o-rings in the fuel
caps, parked outside. Drained over a quart of water
out.

I can't believe anyone asked this question. How about
I ask the question, "Why is it bad to go to prison?"

There was a very tragic accident in the US not all that long ago involving a PC 12

One word:

PRIST

You never, ever buy Jet-A unless you pay extra
for the prist. Doesn't matter if you're sweating
like a pig in the hot sun on the ground.

[trey kule]

I can't believe anyone asked this question. How about
I ask the question, "Why is it bad to go to prison?"

Now, now, don't be like that. As we who do not suffer fools gladly are constantly being reminded...there are no stupid questions. ,

and if you can't understand a student asking this question, what about the instructor that came on here to ask what the answer was.....or some of the posts discussing the various properties of an avgas/fuel mixture....

To the original poster:

So a student comes up and asks why water in the fuel tanks is bad. What's your best explanation?

The student is nearly as stupid as the instructor that would talk about dew points, evaporation for an answer Best explanation I can think of.