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

You know Sean, gobsmacked is actually in the Oxford dictionary 😄.   Utterly astonished, astounded….   It seems to me that a fuel return line to a gascolator mounted forward of the firewall misses the whole point.  As you said, there is no where for vapor bubbles to go, and the fuel is essentially being cycled through a heating loop in the engine compartment. Undoubtably, the aircraft designers know things I don’t, but at first glance this defies common sense.  It’s almost like they just found a way to meet the criteria for a return line the easiest way. 

Oliver, 
It does not matter who may have plumbed their aircraft in what manner. It doesn’t matter if it’s Winter Blend fule, or Summer blend fule, or AVGAS. Routing the fuel return line to a fuel tank in order to comply with the Rotax installation manual is independent of troubleshooting your problem. Doing so is also not a monumental undertaking. A monumental undertaking is trying to explain to the insurance company why the aircraft was flown knowing the fuel system did not comply with the engine manufacturer’s installation manual, but was not accomplished because it was too difficult to do. If the engine quits resulting in a damaged or destroyed aircraft or even worse, explaining this to an attorney should someone received severe injuries or die as a result won’t brief well. 
Download the IM and tell the owner (is that you or are you flying it for someone else?) the aircraft is not in compliance with the IM and needs to be made right. A person can have all the necessary wing skin rivets drilled out, the wing skin out of the way, and the fuel tank exposed in one day if they are comfortable with drilling out rivet heads.  If the wing tank came from Zenith it will already have a tapped, threaded bung in the upper outboard corner of the tank waiting to receive the return line. Run a metal line from the tank to a maintenance access panel at the wing root so you won’t have to open up the wing every five years to replace the rubber fuel line. Routing a flexible fuel line on a Zenith high wing airplane from engine to a wing tank is simple. This is why there is a Phase 1, to identify and correct flaws in the build. Whoever signs the logbook verifying Phase 1 is complete is placing their name in the book stating the aircraft is safe to operate. 

Hi Jim

I agree with you, but I’ve always been bothered that Rotax includes the statement essentially giving the aircraft manufacturer, or Kit manufacturer, the final authority for the fuel system design, with the only hard and fast requirements being that the the fuel system provides the required fuel in all situations, and that it passes a hot test. This gives the manufacturer an out from the listed requirements if the system works, or at least works in their test aircraft. I know this is about assigning liability to the manufacturer rather than Rotax, but it dilutes the importance of these requirements. Too bad that regarding the components listed by Rotax they don’t use the words “must Include the following regardless of final design fuel system design”.  I see this kind of liberty taken by other manufacturers also.  All that said, if it were my aircraft I would do as you suggest.   

Why is it wrong when so many around the world do I without any issues.

You beat me to it Jeff- absolutely agree.

I understand why Rotax does this - they only supply the engine (& advice) plus some optional accessories eg Ring Mount, exhaust, etc The aircraft designer/builder (inc. home build) must come up with appropriate systems to supply fuel, cooling air and so on.

As for complying with something like a "hot test" - do Rotax also require this test (& others) to be conducted in the full range of ambient temperatures that their engine may be asked to operate in ? - I think not.

Using the subject of this Thread, as an example, there is the potential for a only just meeting the standard installation (pass), in a cool/moderate climate, that may be a total fail, in a hot climate.

To me the fuel return to gascolator is a "shonky" practise, no matter how many aircraft are flying successfully with it. There would be few, if any records, of pilots struggling to start their hot engine, or worse losing power on TO/Climb Out😈