I'm not flaming or nothing ,but how can a plane go downhill and not accelerate,what your saying here is the fuel will out run the plane .Sorry don't think so .Now if the plane is stopped for some reason,and pointed down then I could see your point ,but where both the fuel and plane are going at the same rate of decent if the fuel is at the back of the tank I''m sure it will stay there. Inertia will take over gravity any day of the week.