The International Cessna^® 170 Association

I know, I know... Non 170 questions... Sorry, but I need the expertise of this great site once again.

My Champ has been leaking fuel through the vent line ever since I bought it, and its starting to really pi$$ me off. I tried bending the vent tube out of the airflow, but that didn't help. I have included a diagram of my setup:


All the fuel lines enter the header tank through a manifold/fitting at the top of the tank. The 2 front fuel lines are separate, and the rear lines come together underneath the floorboards before entering the header tank. The vent line is plumbed into the top-side of the header tank. I don't know the exact routing of the wing tank vents, so their routing is probably not correct. (Wing tanks = 13 gals each, Header tank = 2 gals)

When in flight, fuel siphons out of the vent line, that happens to exit through the windshield, near the top of the plane. I can look up in flight and see a mist, as well as the tell-tale residue on the fuselage afterwards. The skylight is also cracking and spider-webbing from having the fuel on it.

First, do I really need this vent ?? I don't think so. Both wing tanks are vented, so what good does the header tank vent do ? Plugging it would obviously be the easiest solution... Ive been told that in other installations similar to mine the header tank was completely removed, but I don't want to give up the extra fuel capacity.

Second, if I cant plug it, what can I do ? I tried bending the end slightly more, so that the open end is directly facing the relative wind when flying, hoping that this will keep the fuel from siphoning out, but I haven't tried this yet, and wanted some feedback before flying with it...

Help !!!

Bob K.
Anchorage, AK

Have you tried orienting it directly into the relative wind? On the 170s at least, the tube up top is to equalize the air pressure in the fuel tanks so that when the volume of fuel is decreased, the air pressure remains at ambient atmospheric pressure. The fact that the vent tube is directed into the relative wind like a pitot must give the tanks a little positive pressure too.

If you've deflected it at all you could be getting some venturi action, which is sucking some fuel out.

Trip-
Yes, thats what I was thinking, and had witnessed on Cubs and Cessnas around the field. All the fuel vent lines were facing into the relative wind, whereas mine was not. I think I'll try and bend it a bit to see if that helps.

Id really like to just yank the darn thing if I dont need it though, because that would allow me to seal the hole in the windshield where the vent passes through. Just one more place for a leak...

Thanks !!!

Bob K.
Anchorage, AK

I hope this doesn't sound like a stupid response to your problem - but, I once owned a Champ equipped with the original cowl tank and two wing tanks. Located inside the cockpit just under the instrument panel on the left side was a tank selector valve. As the fuel from the cowl tank was consumed I could replace it by slecting a wing tank. However, I had to be certain that at least 5 gallons had been consumed from the colwl tank before back filling from a wing tank or the vent (which on mine had a float type fuel gauge coming out of the top of it) would begin to leak fuel from the overfilled cowl tank. Do you have a tank selector? Or do the wing tanks constantly feed the cowl tank?

Robert,

Not stupid at all. Somehow my plane doesn't have any kind of tank selector valve. Ive spoken with other Champ owners with the wing tank setup, and some have the valves, and some don't. The only fuel valve I have is the old fuel shutoff valve on the left side of the firewall, between the header tank and the gascolator, with the quirky u-joint that is supposedly impossible to replace... So to answer your question, my wing tanks do constantly feed the header tank.

On a side note, I wonder how much my fuel burn will decrease once I fix this leaky vent tube... Now thats a happy thought !

Hasta ~

Bob K.
Anchorage, AK

Bob

The correct answer is to make the fuel and vent lines as original. Any modifications to the original would require an approval of some sort.

Having said that it would seem from your drawing that the vent is not needed but based on my observation from your drawing alone I wouldn't remove it.

The problem I see with removing the header vent is that any air in the header would compete with fuel-flow in looking for an escape. (This sort of problem has been blamed for C-210 fuel starvation/engine stoppage.)
I wonder if the wing tank vents are facing the relative wind? If so, it may be that air pressure is pressurizing the header and forcing fuel overboard. (The reason Cessna places underwing vents behind the strut is to prevent this as well as protect against icing.)
Your system was designed and approved by somone (presuming it's not been subsequently altered.) I'd compare it to the original, approved configuration and see where it differs.

I would think that the vent on the header tank would be necessary, to allow air to escape when refilling the system after the header tank is fully drained. Without it, you might get an air bubble in the header tank that has no way to get out.

In an afterthought.... if I were to redesign the system, I'd vent the header to the ullage* of one of the wing tanks, I think.



*http://en.wikipedia.org/wiki/Ullage

Could your vent line be creating a siphon??? From your diagram, it looks like the wing tanks would flood the cowl tank and push up thru the vent tube. Cowl tank vented?

Ta-dah !!! Its fixed ! Thank you !

I cut off about 3" of the vent line that was slightly kinked from trying to bend it to point into the relative wind, and managed to bend the remainder into the wind without kinking it... I sucked on the line to make sure all was connected, and got a mouthful of Avgas. So I knew it was still venting...

No leaks !!! The pressure from the relative wind is all that was needed to keep things from siphoning out. It really makes sense when I sit and think about it, but I didnt want to go and do something that would leave me with a hiccuping engine in the pattern... Well, I suppose in the pattern would be better than out over the frozen tundra !

Thanks again ! Much appreciated !

Bob K.
Anchorage, AK

Next time use a siphon-blow-gun (like mechanics use to wash your engine down with solvent) hooked up to the vent. That way you don't suck up gas, or inhale (very dangerous to your pulmonary health) gasoline fumes.