Fuel Tank Vent lines freezing?? >>KitPlanes

[RKellogg]

I read Ken Krueger's (excellent) article about fuel system design in the recent issue of KitPlanes magazine last night. (Thank you, Ken!) The part about fuel vent lines not having a low spot that can trap moisture caught my attention.

Pretty sure my RV-7A has (standard design) vent lines with a low spot that could trap condensation and possibly freeze, creating a fuel starvation situation. It's getting to be the cold half of the year here in the upper Midwest, and I'm wondering how likely this condition is. (Not talking here about icing of the vent at the exterior of the aircraft, as one would encounter in flight in icing conditions.)

The fuel vent will (slowly) pass something less than 20 gallons of air every time the tank is emptied. Not convinced that that would clear accumulating moisture. Pretty sure a quick blast of air from the air hose would blow any accumulating moisture into the tank, if the cap were removed. It wouldn't take very much moisture to freeze an ice plug across the 1/4 inch vent line.

The low spot in the vent line is in the nominally heated cockpit, but I'm not comforted. The engine has to suck quite a bit of fuel before it makes much heat.

Any simple, reliable way to test vents as part of preflight on cold days?

One could make a temporary clear plastic hose vent line to see how much moisture accumulates in the vent line. ...maybe without even removing the aluminum vent line, just patch in a parallel temporary clear vent line to visually observe ....??

Does this sound like an issue to anyone else? Can anyone say with confidence that this is a non-issue?

- Roger

[Mike S]

Originally Posted by RKellogg

Any simple, reliable way to test vents as part of preflight on cold days?

- Roger

What I do is this.

First, remove cap, and use dip stick to check fuel level. Then, while I am under the wing sumping the tank, I slide the end of a piece of hose apx 6' long over the tip of the vent tube. Then, I blow into the hose, while leaning down to listen at the open fuel filler port. After verifying the line is open, remove the hose, and replace the fuel cap.

[lr172]

Originally Posted by RKellogg

I read Ken Krueger's (excellent) article about fuel system design in the recent issue of KitPlanes magazine last night. (Thank you, Ken!) The part about fuel vent lines not having a low spot that can trap moisture caught my attention.

Pretty sure my RV-7A has (standard design) vent lines with a low spot that could trap condensation and possibly freeze, creating a fuel starvation situation. It's getting to be the cold half of the year here in the upper Midwest, and I'm wondering how likely this condition is. (Not talking here about icing of the vent at the exterior of the aircraft, as one would encounter in flight in icing conditions.)

The fuel vent will (slowly) pass something less than 20 gallons of air every time the tank is emptied. Not convinced that that would clear accumulating moisture. Pretty sure a quick blast of air from the air hose would blow any accumulating moisture into the tank, if the cap were removed. It wouldn't take very much moisture to freeze an ice plug across the 1/4 inch vent line.

The low spot in the vent line is in the nominally heated cockpit, but I'm not comforted. The engine has to suck quite a bit of fuel before it makes much heat.

Any simple, reliable way to test vents as part of preflight on cold days?

One could make a temporary clear plastic hose vent line to see how much moisture accumulates in the vent line. ...maybe without even removing the aluminum vent line, just patch in a parallel temporary clear vent line to visually observe ....??

Does this sound like an issue to anyone else? Can anyone say with confidence that this is a non-issue?

- Roger

Something interesting to consider, but that would take quite a bit of condensation to block a 1/4" line, especially if the low point was flat. a sharp bend low spot would be more problematic. I was far more concerned with ice on the vent inlets. That seemed like a far greater threat to me. I ended up putting in a tee with a 1 PSI check valve to vent from the cabin if the inlet froze.

Larry

[Kahuna]

You will find a number of examples of alleged vent freezing and potential solutions on previous threads from over the years. There is a lot of good material there from some very smart people.

[RV3bpilot]

I designed this simple small tank just for plugged fuel vents whether it is plugged from bugs or ice or other foreign matter.
There is a one way check valve on the short hose that can take air in but not out so that the cockpit will not fill up with fumes.

[RKellogg]

Originally Posted by RV3bpilot

I designed this simple small tank just for plugged fuel vents whether it is plugged from bugs or ice or other foreign matter.
There is a one way check valve on the short hose that can take air in but not out so that the cockpit will not fill up with fumes.

Hmmmm... interesting. So, does the aluminum drum fill up with fuel when the fuel in the tank expands in the sun? Is there a way to drain it? Could it fill up with condensation, and freeze?

If the ports were on the bottom of the tank it wouldn't accumulate liquids... ?? Maybe just the "outside vent" port should be on the bottom of the tank...

Does it need to be a tank, or would a simple cross (double tee) work? Or a small block with four NPT ports tapped and cross-drilled?

Looks like this system will still have a low spot in the vent tube between the vent manifold and the tank that could collect condensation and freeze.

- Roger

[rhill]

Fuel vents and how to prevent fuel leaking in the event of an off field landing are a concern to nose wheel RVs,bad enough to be hanging from the shoulder straps,the last thing I need is leaking fuel.Any thoughts on that one?

[RKellogg]

Originally Posted by rhill

Fuel vents and how to prevent fuel leaking in the event of an off field landing are a concern to nose wheel RVs,bad enough to be hanging from the shoulder straps,the last thing I need is leaking fuel.Any thoughts on that one?

If the vent port is higher than the fuel level when the plane is belly-up, it shouldn't leak fuel, unless the fuel is expanding within the tank which could happen if the hypothetical inversion occurs on a sunny day, or there is radiant heat from a nearby campfire. The one "A-model inversion" that I have any knowledge about did not include fuel leaking from the vent lines.
- Roger

[AlexPeterson]

Originally Posted by Mike S

What I do is this.

First, remove cap, and use dip stick to check fuel level. Then, while I am under the wing sumping the tank, I slide the end of a piece of hose apx 6' long over the tip of the vent tube. Then, I blow into the hose, while leaning down to listen at the open fuel filler port. After verifying the line is open, remove the hose, and replace the fuel cap.

Seems like a good way to condense moisture in the vent lines! Maybe use a baster bulb instead of lungs?

[Carl Froehlich]

Originally Posted by RV3bpilot

I designed this simple small tank just for plugged fuel vents whether it is plugged from bugs or ice or other foreign matter.
There is a one way check valve on the short hose that can take air in but not out so that the cockpit will not fill up with fumes.

Robin - I'm looking for a check valve like this to add a 'T' to the vent lines in each wing root on my RV-10. Can you share the part number and where to get the check valve you used?

Carl