Fuel Tank Vent/Water Contamination

[Robert Eilers]

I fly an IFR C170B. I am also new to the C170B - which I am learning to love. Following a flight in heavy rain the other day, while wiping the airplane down, I got a good look at the overhead fuel vent between the tanks on the top of the cockpit. The way the vent is shaped it causes me wonder if the vent might not contribute to water contamination in the fuel during prolonged flight in heavy rain. Anyone heard of or exxperienced water contamination as a result of the overhead vent?

Thanks

[GAHorn]

Hi, Robert!
You are more likely to experience more water in the tanks from common humidity in the air which replaces the fuel you consume.
As you burn fuel down, the air which vents into the tanks is chilled by the fuel which is evaporating in there. As you climb the temp drops and water vapor will condense. As you descend, it will even coalesce onto the fuel tank inner-walls and run down into the fuel. That's the major source of water in your fuel sampler cup during your preflight.
The amount of water captured by the vent "gooseneck" from flying in rain is miniscule compared to what occurs naturally due to condensation and from what enters at the fuel caps while sitting in the rain and when washing the airplane. (And in any case,...that's what your gascolator is for,...to capture any water on it's way to the carburetor so you can drain it out at the next preflight.)
(I have flown my airplane in long, cross-country downpours, (notably after the Wilmington convention on the way down thru the Carolinas and Georgia and Alabama, where we flew in virtually constant moderate and heavy rain for over 10 hours) and found no more water in the fuel than I do normally in dry Texas.)

[Dr. Dave]

Hello.

Haven't logged on in quite a long time, but this thread caught my attention. I'm flying a 55 Cessna 170B out of Victoria on Canada's southwest coast where "visible" humidity is a regular occurrence. I keep my plane hangared and regularly check for water contamination before "every" flight. I also try to always leave the plane hangared with full tanks. This comes from my previous float experience on a 185 where water in the fuel was a regular occurrence.

I've owned this plane for almost 3 years now and have "never" seen so much as a drop of water when checking the tanks or draining the gascolator? I have often wondered if I am perhaps doing something wrong and just missing the water that just about has to be there? Is anyone else similarly finding little or no water in the tanks on pre-flights?

Thanks

Dave

[flat country pilot]

Our plane is kept inside and I never find visible water when sumping the tanks. The plane sat for a month with the tanks less than half full while we seeded our crop. Last week I finally took it back out. Sumped the tanks before I moved it , rolled outside and did a preflight before sumping again. No visible water.

I just don't see any moisture in the gas, ever. I do think about it more in the winter, and wonder if the sump drain could freeze open if water was present when I sump the tank.

Anyway, no water.

Bill

[GAHorn]

I've seen only trace amounts of water after the airplane has sat for awhile with less than full tanks.
It's unlikely that freezing water would open a sump drain since it's already closed and therefore no water beneath it. (But I have seen similar drains frozen shut on other airplanes. The drains wouldn't open during pre-flight. We added fresh fuel and moved the airplane into a heated hangar. An hour later they'd open but we got no water to drain out. (I imagine the frozen water let go then floated around in the tank. I guess I wasn't very wise to go fly since the ice particles might have blocked/clogged the lines somewhere..... but nothing out of the ordinary happened... and you guys still have to put up with me.)

[Dr. Dave]

Thanks for your responses and it gives me a little bit of comfort to know that at least some of you are rarely finding water in the fuel. I suspect those that do have planes kept outdoors and have less than secure fuel caps.

Short-story regarding flying in freezing temperatures even in the summer with even minimal fuel contamination. I was in a Cessna 185 on floats at 10,000 feet over the coastal mountains of western BC in June, when the engine stopped abruptly! And I mean abruptly! It did not even sputter it just quite!!! Longtime Bush pilot friend of mine with lots of Arctic experience happened to be sitting (dozing)in the right hand seat. While I was busy concentrating on setting up appropriate glide and scanning for a possible place to crash as softly as possible, and desperately trying to dredge up from memory an appropriate emergency response check checklist he.... snapped awake and with one hand, , firewalled the controls, (throttle, mixture and RPM) and with the other hand switched the fuel feed to the other tank. The engine almost instantly roared back to life!

He calmly turned to me and said "no problem we just had a small pellet of ice sucked into the fuel inlet and cut off our fuel supply. Happens all the time".! "Let's just drop down 1000 feet into that next valley to get below this freezing level. Some time later when we had been flying at much warmer temperatures for a while he calmly flipped the fuel selector back to of affected tank, there was a barely noticeable momentary drop in engine rpm and then everything ran as normal. Back on the water, I slumped both tanks again and found nothing. His explanation was that there is often a very small amount of water perhaps a quarter of a teaspoon rolling around as a bead on the bottom of the tanks that gets missed when sumping.. this amount of water is of no consequence to the engine if it gets sucked in to the fuel intake but if you're flying at cooler temperatures for a little while that little bead of water freezes into a pellet of ice. Now you have the potential for a ball valve effect which can cut off your fuel supply instantly.

I've never forgotten this and so am pretty careful about rocking the wings a little bit to try to move water down towards the sump when I do a preflight.

Best regards,

Dr. Dave

[Robert Eilers]

Thank you all for the responses. Having access to a other owners, with abundant experience in type, is invaluable. Maybe I will get the chance to meet a few of you in Kelwona.

[tshort]

I always sump each drain once before rocking the wings ... assuming that I should get any water at the drain before shaking it up and moving it around. As a matter of fact, I have never sumped any water from the 172; I don't rock the wings in that plane. The 170, however, has produced water a couple of times and I usually sump first, rock the wings, then come back after the rest of the preflight and sump again.

Those of you that rock the wings - do you just do that first? My concern was that it would disperse water and might hide some that was sitting on the drain.

Sorry for the thread creep.

Thomas

[flat country pilot]

I always sump the tanks before I move the plane.

I only rock the wings to make sure they move together. An old timer told me that "if you rock the left wing up and down, the right wing should move about the same amount." This is pretty important for structural integrity.

Bill

[Bruce Fenstermacher]

Thomas

I've had a fair amount of water in my tanks lately for reasons that are particular to my fueling practices which I've discontinued.

Anyway I would first drain the sumps. Nearly always finding water I would then rock the wings a little. Some times I'd sump right away and finding more water then I'd repeat the procedure till no water was found. Some times I'd come back after the rest of the walk around and find a pinch more.

At this point I'd go fly figuring if any water remained the gasolator would capture it and there would not be enough to fill the gasolator. This is in fact what I found never finding more than a tablespoon of water in the gasolator after the flight.

[N170CT]

This one is easy. '56 170B, never any water sumped anywhere. And being a firm believer in the old saying about "No old, bold pilots" I always sump before first flight of day and after refueling IF I take time for lunch before departing. Even after washing the airplane, water is never seen in the fuel system. Unfortunately, I find more FUEL leaking OUT of that infernal vented tank cap than water going in. Chuck

[cpolsley]

My C-170B will get small amounts of water in the tanks while flying through rain for extended periods of time. After fueling and allowing time for the water to settle I would get about a 1/2 to 1" in the fuel tester tube. I think this and ice build up, is one reason they moved the vent to behind the wing strut on later model Cessna's. This was during a trip to Alaska, it took me a couple days to figure out where the water was coming from.

[GAHorn]

According to factory test pilot W. Thompson, the reason they moved the vent to beneath the wing is to correct the complaints of fuel smells in the cabin after heavy braking with full tanks. (The gooseneck will puke fuel onto the cabin roof in a wheel landing with full tanks and heavy braking.)
The reason they placed it behind the strut is to (in order of importance) 1) avoid head injuries and 2) to avoid icing (not due to actual impact ice, but due to water trailing off the strut and into the vent.) The actual placement/position behind the strut is critical in such installations and is described in most service manuals and several service letters. (Half the vent must be hidden and half exposed to the relative wind.)

[4stripes]

Keeping the aircraft hangared, I rarely get any water from the drains. Very occasionally there might be a few drops. I also fly rain or shine, with no different results. I think the dramatic tempertaure changes if parked outside would cause more condensation.
Cheers Eric

[Bill Hart]

I keep mine outside and have not had any water in my tanks in the six months that I have owned the airplane. It has been my experience with the three Cessna’s I have owned that replacing the fuel cap seals will stop most water from getting in the tank. I live in the southeast where humidity rules and have had all of my airplanes outside, I try and keep my tanks full so condensation has not been a big problem for me.