In reply to David Ribbe, my BVM UAT instructions ( and I have UATs in both of my BobCats, StarJet and Sabre) state that before the first flight the UAT must be purged and outlines the procedure, which must be repeated if the UAT is ever drained. I believe the UAT works best if mounted vertically or at least at 45 degrees or so as per BobCat. In comparison the simple header tanks in my F4 and F15 and Mig 29 are automatically purged on every refuel. Please be assured that I am not "having a go" at the UAT (as you see, I do use them) but all three of my flameouts in the last year have been with UATs and none have been experienced with the simple header tank. Apparently the folds in the sack CAN trap air bubbles. That is my experience to which I should add that, to date, the 20 flights I have logged with my second, JetCat P70 powered, Bobcat with a UAT have been flawless. That combination (BoBcat/ P70) is a pure delight.
Regards,
David Gladwin