912 ULS: Partial power loss on takeoff - vapor lock?

Yes the return line restrictor belongs in the fuel manifold per the Rotax IM. You’re in Phase 1 flight test correct? Recommend you speak to your DAR and get his thoughts on adding a fuel tank under the instrument panel before you spend any time or money doing so. 

If running a return fuel line into the gascolator was such a big issue then why do thousands of planes have them that way with no issues? I know one company that has over 2K planes setup that way and never have issues. There may be other things that are contributing to this that need to be considered.  It's rarely a one size fits all issue.

All

A return line to solve vapor issues assumes that hot fuel is being returned.  If the gascolator volume is too small the hot fuel will just circulate and nor really do its job.  The requirement to return to tank was for that problem.  Returning to a gascolator is always the less desirable way to do it.  Indeed in injected engines it simply will not work, the hot fuel will bubble to the point of causing cavitation of the fuel pumps.  Many type 914 engines that had fuel return to gascolator suffer with that issue.  (duall electric pumps and higher volume fuel return) 

The argument from OEM companies has always been it is too difficult (they don't want to spend the money) to run dual lines from tanks.  This can be solved by using duplex fuel selector valves in the case of twin fuel tank for example.  The fuel drawn in is also returned to the tank you are drawing from.  This is a mandatory requirement in a dual tank aircraft with injection and a good idea in even the carb engine.  

To prove you have an issue with winter fuel simply try a fill of Avgas.  If that solves your problem then buy better fuel as a good start. 

Cheers

Roger Lee wrote:

If running a return fuel line into the gascolator was such a big issue then why do thousands of planes have them that way with no issues? I know one company that has over 2K planes setup that way and never have issues. There may be other things that are contributing to this that need to be considered.  It's rarely a one size fits all issue.

I believe we found the problem:

Our hose kit has the 'fuel restrictor jet' sitting right on top of the gascolator and not at the fuel manifold on top of the engine. Since the hoses are crimped to the manifold I can't simply move the restrictor jet to the other side.

There are two problems with that:

1.) As per the Rotax installation manual it is MANDATORY that the return line is routed into a tank. Nowhere else, even though it is apparently quite common that it is only routed back into the gascolator. If it only goes back to the gascolator, and the fuel is at a boiling temperature, the return line would actually worsen the vapor lock issue, as the vapor would still have to go through the carbs, while the fuel pumps would hardly be creating any pressure since they are not designed to pump vapor.

2.) Having the restrictor jet sit at the gascolator, with the pumps right after it, causes cavitation (bubbles resulting from the rapid pressure drop from around 4.5 psi to almost 0), with no time for the bubbles to liquify again.

As a quick fix, we added several feet of fuel line between the restrictor jet and the gascolator. Despite of extensive testing, at similar temperatures, neither the drastic drop of fuel pressure nor the almost dying engine occurred again.

We will change our installation to a return line to one of our wing tanks.

Thank you very much again for all of your input!

Oliver