Rotax 912ULS fuel system question

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flyvulcan

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We are installing a Rotax 912ULS into our Lightning Bug and we note that in the Rotax installation manual that between the fuel pump and the carbies is a fuel manifold that splits the single fuel line into two which go to the individual carbies. This manifold also has a fitting for a fuel pressure sensor, as well as a return line to the tank with a restrictor in it (items 13-25 in the attached wrongly oriented diagram).

image.jpg

My question is whether this component (including fuel return line) is a requirement to be fitted or is it optional?

The reason that I ask is that we have seen some 912 installations that do not appear to include this fitting in their fuel systems. We wonder whether these installations are incorrect or whether the use of this component is optional. If so, under what conditions should it be used, or not used?

Of course I will be consulting the Rotax agents, but I'd appreciate some feedback from Rotax 912 experienced forumites.

Thanks,

Dave
 
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Jan Olieslagers

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This has recently been a hot topic among German microlighters. The return line to the tank has been made mandatory by Rotax only a couple of months ago, and one German microlight factory has made its addition a mandatory modification to their (quite popular) design. If you have some German, you can find several discussions on www.ulforum.de and likely on other forums too.

My own opinion, worth as much as you are paying for it: do not feel obliged to install this fuel return unless you see particular danger of vapour locking, perhaps because you fly in hot climate and you use MoGas. However I heard that the risk of vapour locking is greatest at the exit side of the fuel pump, which is normally in the flow of cold intake air (not accidentally, I think). Or, of course, if you think your local authorities are likely to make this Rotax recommendation into a local requirement, sooner or later.

Myself fly behind a 912UL, not S, always use MoGas, and have no intention to add the fuel return, even if it would be less hard on my plane than on some others.
 

TFF

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Aircams just have the T. no return or fuel pressure; at least out of the box.
 

flyvulcan

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Thanks Jan and TFF. I'll be calling Lockwood today. I'm curious what they will say. I suspect it will be the Rotax line and they will insist on the use of the manifold with return line.

I'll let you know what they say when I get their answer.
 

Head in the clouds

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Hi flyvulcan. It's been a hot topic on the Recflying forum too at times. It is indeed an anti-vapour lock thing and has always been mandatory for the 912S (certificated) engine but left up to the owner/installer for the non-certificated 912ULS engine. Non-fitment of the return line and restriction valve has been deemed to be the cause of more than one engine out and crash including, as I understand it, that rather highly publicised double fatality one that spent years going through our Courts with the eventual very expensive outcome for our Association, among others ...

Having had a bit of a dig around regarding this fuel installation I have decided I would not go without fitting the return line/restricter in my current build. Having it does also have a second benefit in preventing an over-zealous electric fuel boost pump from over-pressuring the fuel system and unseating the float valves causing flooding of the carbies and in some cases engine stoppage, or at the least very rough running and power loss.

The electric pumps run very close to, and sometimes well over the max fuel pressure permitted, and since they are primarily used for takeoff and initial climb it's a mighty bad time for your pump to actually cause an engine stoppage rather than preventing one ...
 

flyvulcan

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Thanks HITC,

Given the work we've put into the Bug to get it to this point, I will bite the $500ish bullet, err on the safe side and use the component with the associated fuel return line. Other than cost and time to install, there doesn't appear to be a downside, but the upside is positive wrt preventing vapour lock etc..

Cheers,

Dave
 

Jan Olieslagers

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@HitC: there might well be something to be said for your point re the electrical fuel pump. I never thought of that, as my own has none and doesn't need one either, the fuel being carried in the high wing and delivered with remarkable reliability thanks to the force we call gravity.
 
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