Hi All,
After reading this thread I did some looking at the King Cat and the Composite Bob Cat fuel system. The inside diameter of the UAT nipple where the fuel is pumped in is .103 inches as is the inside diameter of the UAT nipple going to the Sullivan header tank. The brass tubing, as supplied by BVM, for the Sullivan header tank has an inside diameter of .125 inches for both the inlet and outlet. Now comes the problem - the outlet of the Sullivan tank routes to a Tee fitting, which goes to both main tanks. The inside diameter of this BVM white plastic Tee fitting is .083 inches - big problem!! The main kevlar tanks, which have the threaded in aluminum fittings, have an inside diameter of .092 inches - no problem since there are two of these fittings. So what we have here is a restrictor right in the middle of the fuel system. If you calculate the cross sectional areas of the parts you will see that the Tee fitting has 35% less area than the UAT input fitting. So during fueling once the UAT and Sullivan header tanks are full unless the flow rate from the caddy is REALLY LOW you will build up pressure in the UAT and Sullivan tanks. During defueling if any fuel is in the two main tanks you will cause the UAT and Sullivan tank to suck in until there is no more fuel in the mains. In order to alleviate this pressure problem it is mantatory that all lines and fittings downstream of th UAT input nipple have an inside diameter at least equal to the inside of the UAT input nipple. This ignores the pressure you will get just pushing the fuel through the system, which should not be high enough to deform the UAT or Sullivan header tank. The BVM Tee fitting needs to be replaced with one which has an inside diatmeter at least as big as the UAT input nipple. The same white Tee fitting is used in the vent system from the main tanks, so if you use an overflow tank, as we all do, you must change that fitting also.

I was just ready to do the final installation of the tanks in my composite Bob Cat, so that is where I measured these parts. I will use bigger Tee fittings in that airplane. My King Cat is a couple of years old and has the BVM white Tees. I have not noticed flexing during fueling/defueling, but I have not paid much attention either. I do notice that during a full power run-up that the UAT sucks in a little!! I will change the Tees on the King Cat before it flies again. It is raining and blowing so hard here in the Northwest I need something to fill in the time anyway.

Al Watson