As a rough test to be sure the vent line will actually vent the header, one can run the transfer pump(s) until fuel begins to overflow thru the vent tube (be careful and stick a length of plastic tubing on the vent and run it back to one of the wing tanks to catch the overflow).

My LNC2 is an early 235 (first flight in '93 and still flown regularly) and I located the external tip of the vent according to the plans in effect at that time - just about straight down from the vent exit in the header. My external tube protrudes about 3/4 inch and has the 45 degree cut - so there must be a reasonably good pressure field under there - or 3/4 inch is long enough to get outside the boundary layer - whichever, mine works as required.

Also the plans called for plumbing all three fuel tank vents (wing and header tanks) with 1/4 inch tubing. One-quarter inch ID works OK as long as a mud dauber wasp (or some other big bug) doesn't take up residence where you can't see the plug (don't ask how I know) so keeping the vents clear should be added to your pre-flight check. A large diameter pipe cleaner stuck in the vent about an inch, with a red ribbon attached to the end that hangs out, works OK - as long as you are diligent in your preflight.

Dan Schaefer
N235SP --
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