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From CP50, Page7 (October, 1986)

     A missing fuel cap on a gravity fuel system can be a very real problem. We have talked of this problem before in the CP as well as at airshows, and this year at Oshkosh was no exception. In fact, one VariEze pilot had had a first hand experience and was able to verify what we have said in the past.
     In a VariEze (or Cessna 150) which have gravity fuel systems, both tanks feed together to the engine and there is no mechanical engine driven fuel pump and, also, no electric boost pump. This system is simple and works very well as long as the fuel tank vents are open, allowing ram air to pressurize each tank equally, and as long as both fuel tank caps are on.
     Now, if one cap comes off (or even stays on but leaks badly) the fuel in that tank will be siphoned overboard (low pressure on the top of an airfoil), because a gravity fuel system has both tanks plumbed together. This means that the fuel in the tank with the cap on will be sucked across and into the tank with no cap. Of course, it will also continue to supply fuel to the engine until it is empty. When it is empty, however, your engine will quit, even though you still have most of one tank still with fuel in it (the one without the cap). The low pressure over the missing cap hole will not allow this tank full of fuel to gravity feed down to your carburetor. Selecting the header tank will allow engine restart within about 10 seconds.
     This problem does not exist on an airplane with a pumped system (such as the Long-EZ or Defiant) since the two tanks are not plumbed together.
     Check your fuel caps yourself, carefully and conscientiously every single time you get fuel, whether you put it in yourself or have the line boy do it.
     Another associated problem, particularly on a VariEze, is that the loose fuel cap will go through the prop and can damage it, sometimes seriously enough to cause an emergency landing. It is an excellent idea to attach your fuel caps to the fuel tank with a 6 inch length of light chain.
     Wes Gardner stopped by RAF just yesterday and he was still shaking in his boots from just such an experience. He was very, very lucky and managed to stretch it to an airport and, therefore, did not suffer any damage other than a large ding and some cracks in his prop - be careful.