The 914 Thread

[foxy]

the return line to the gascolator is also done on the Flight Design CTSW

[av8rps]

Esser,

I pulled a guy out of a bad crash in 2007 for what we believe was caused by hot fuel /vaporization on a Rotax 914. The return line on that aircraft was run to a small (1 gallonish) header tank located about 2 ft behind the passenger seat. The aircraft only had about ten hours on it, and it had some issues with running hot both with the coolant temps, and with under cowl temps. After flying the aircraft for a while to check coolant temps to see if they were improved after relocating radiator (which did improve it), he returned to the very short runway he was operating from. He then shut it off for a while to check for leaks and such, and then decided to go again only about ten or fifteen minutes later. It started right up, but was "surging" at high idle while he was running it up for a short takeoff with the brakes held. He shouldn't have gone...but he did.

As it rolled down the runway and began to climb, it seemed like the problem had cleared up. But then after only climbing to about 100 feet it started to bog and run out of power, acting as if it were gradually running out of fuel. He lowered the nose and headed back to the runway, but just as he was about to make the end of the runway the engine caught and carried him past his only opportunity to hit that little runway. So he had to go around. Unfortunately the engine kept bogging and surging until he couldn't maintain altitude and it stalled and spun into a rock embankment. He's lucky to still be with us.

So the moral of this story is...don't let your 914 run hot fuel back to the engine. Sooner or later it is likely to vapor lock. In the case of this particular aircraft, after the pilot recovered he rebuilt the airplane and reinstalled that same 914 engine (after being thoroughly checked). But this time he ran the return line to the wing tanks where it has the opportunity to blend with the other cool fuel (and get cooled off by airflow over the wings), and hasn't ever had a problem again. And that aircraft has since then flown a couple thousand hours.

So...if I had a 914 engine in my plane, I'd make sure that return line went back to somewhere that I know would cool the fuel rather than just send if off to get warmer and warmer in a never ending cycle going back to the hot engine compartment, ultimately ending in the float bowl of the carburetors.

And fwiw; The aircraft that crashed had only about 8 gallons of fuel in it, and it was very warm that day. So I'm sure that didn't help the situation at all.

Paul

[Esser]

Hey Paul,

Thanks for the info. Did he run it back to only one tank or did he have a tee fitting?