Posted for "Christopher Zavatson" <Christopher.Zavatson@udlp.com>:

<<Which brings up a question - is there a redundant vent path built into the
design?  Are the caps vented via a "relief valve?"  I have thought about
adding a hole in the vent line at the high spot inside the wingtip (my
wingtips will be screwed on so they will leak).  My Cessna has a feature
like this plus vented caps.>>

I added such a feature to my 360 after I started flying IFR.  My concern was getting the vent iced up even in a mild icing encounter.  Inside the right wing tip, I added a T-fitting and a spring loaded check valve.  My vents slightly pressurized the fuel tanks ~0.15 psi.  If the vent is blocked, and the fuel system tries to pull a vacuum in the tank, the check valve opens.  It has a very low cracking pressure (I forget the number).  Internally it has a small spring keeping a flapper valve closed, so no leakage will occur inside the wing tip, either air or fuel, during normal operation. On a related topic.  When I first installed the check valve, the new vent tube ended up sticking out a little lower below the wing than the original.  On subsequent flights, when the sun light was just right I  could see the wing rib locations out on the wing.  The wing was being pressurized and the skins were deflecting enough to see it.  I hooked in a spare airspeed indicator to the vent line and found the vent was capturing nearly full ram pressure.  The vent tubes were 0.25 in aluminum tubes bent forward 45 degrees. I filed off the material from the bottom until the pressure dropped to about .15 psi at cruise speed.  The large rib spacing in the wing tanks does not support excessive pressure very well.  Full ram pressure is roughly equivalent to pressure exerted by the fuel on the lower skin at high Gs.  (Experiment for when you're really board:  Stick a piece of hose on the vent and blow as hard as you can.  -this should produce 1-1.5 psi-  If you look down the surface of the skin you can see it expand.)

Chris Zavatson
N91CZ
360 std
800hrs