SonexAus – Safety valve for top-vented fuel tanks This tip has been re-written because the valve has been greatly improved and is applicable now to all fuel vents.
Background
The fuel vent on my Sonex is just a short forward-facing tube. An advantage of this type of vent on a tail-dragger is that the fuel tank can be completely filled. A big danger is that in the event of the aircraft becoming inverted in an accident, much of the fuel can run out the vent. I also noticed that with the fuel tank more than 3/4 full, during any low-g manoeuvre, or even a moderately steep descent, fuel would come out the vent. I think this will occur whether the vent comes out the top or the bottom, but if you have the tank venting at the bottom you can’t see it happening.
I originally made the safety valve using a steel ball bearing. It solved the first problem but not the second. The steel ball has since been replaced with a PTFE (Teflon) ball which is denser than fuel so will still block the vent if the aircraft is inverted, but is light enough so that pressure of the fuel is enough to keep it against the seat. I have tried a variety of manoeuvres with more than 50 litres in the tank and observed no leakage.
The valve is made from an AN816-4D fitting which is what most people will be using for the vent connection to the fuel tank. The end is drilled out to 6 mm and a 3/16" ball inserted and retained by a pin through a cross-hole. I used a short bit of 1.2 mm stainless steel lock wire as the retaining pin. The ball is a 3/16" Teflon sphere. Teflon has a very high resistance to avgas.
A good way to drill the small cross-hole would be to screw the tapered thread into a female fitting and drill the assembly. Unscrewing the AN816 fitting should clean-up the thread.