Free air cowl isn't as good as fan cooling, but that doesnt mean its useless - hundreds ( thousands ? ) of hours have been done on Thrusters with free air cowls without incident - you just have to manage the motor a bit more. ie Overheating on slow taxi and shock cooling on descent. the worst thing about those old 503's is not the cooling, but the non-electronic ignition. The points are hard to set and dont stay within spec for long - especially in dusty conditions. Oh and tail-up-taxi solves the overheating problem on the ground. This was acceptable practice in the bad ol' days - probably would be frowned upon these days.