Fuel Return Line

A header tank or back to the wing is probably best, but I have seen quite a few just back into the gascolator.

Hi all

The return is really required, especially with the higher pressure Corona pumps on the carb engines.  You can get away with a small header as some do but a gascolator is really not suitable as it has no place to cool the fuel on its return, you wind up circulating hot fuel. 

The real issue is engines with high flow, such as the injected versions, they are returning the bulk of the fuel to tank and in that case you need a header more to the 3 to 5 gallon size if possible.  Smaller that that the hot fuel is not really cooled enough in my opinion.  The type 914 also circulates a lot of fuel and is similar to the injected engines.  

The best is return to the tank you are drawing from.  The second best would be a correctly sized header tank, that is something you should work with your OEM on for best mounting and size.  The least is the gascolator since the size is just so small there is very little relief from the hot fuel to recirculate. 

Cheers

".....I have seen quite a few just back into the gascolator."

Roger L,  I am surprised at you. By any measure this is not an acceptable system.

It needs to be understood that a fuel return line, in a carburettor Rotax 9,  is NOT required for the engine to run. The fuel line is there to mitigate an engine design feature (the location of carburettors & their fuel supply lines on top of the engine),  that may result in fuel vapourisation, particularly in hot ambient conditions.

Countries that have winter (higher volatility) fuel may be more prone to this phenomena. 

Fuel vaporization is primarily a feature of a recently run engine, in hot conditions, where the engine has nor returned to ambient temperature ie on the ground. The effect is of hard starting, rough running and possible loss of engine power in whole or part, usually during take- off . I have heard of it occurring in flight (likly due to winter fuel)

The simple solution is to install a fuel return line.

The fuel return is specifically to mitigate the chance of fuel heating, to the point of vaporisation and to remove any hot fuel & vapour (bubbles) that happens to have developed.

The return line;

 # Facilitates the circulation of cool fuel, replacing the hot, even when the engine is not running (pump required).

 # Draws off hot fuel and vapour.

Best practise requires that the hot fuel & vapour be returned to a large volume of relativly cooler fuel . In doing this, the smaller volume of hot fuel is cooled, the vapour also cooled may return to a liquid state and vapour that does not return to liquid, is allowed to leave the fuel system via a breather.

A fuel return line that delivers hot fuel & vapour fuel back to the gascolator, or main delivery line,  can not be effective in meeting the objectives, articulated above. Such an arrangement is merely a token.

😈

Hi Sean,

 I didn't do these. It was Flight Design and it was carb engines. But it does seem to work on about 1800+ Flight Designs.

"... it does seem to work on about 1800+ Flight Designs."

While I acknowledge when a system, that shouldn't work or work well, somehow defies logic, that doesn't mean I accept it as best practice.

Logically, your Flight Design  example, should have very low efficacy, compared with a fuel return to a large cooler volume of fuel, that is at ambient pressure (has a breather).

I can see that some form of cooling (heat exchange) in the return line, may allow  a return to gascolator to work - no one has suggested such an arrangement.

It would be interesting to hear from pilots of Flight Design aircraft, so fitted, and regularilly flying in hot weather, say 40C or above. My bet they experince fuel vaporization symptoms, when trying for a hot start.😈