Looking at 912 failure modes......

[Head in the clouds]

It obviously isn't essential for suitable 912 ULS operation...I have been flying succesfully now for 705.9 hrs without it, and so have many others that I know of !...................................................Maj...

Good point, and then its also still worth bearing in mind how pumps function. If we used rotary vane pumps (like on a swim pool filter) they simply unload and cavitate without harm if you block the outlet. Diaphragm and ballvalved solenoidal electric pumps hammer the diaphragm and the balls and seats if flow is stopped, so the relief valving is probably as much benefit to the pump as to the float valve problem...just thoughts

 

 

[Head in the clouds]

HITC,........I feel that the confusion is because certain things have been lost in the German-English translation, and it just doesn't read as it should..They probabily ment to convey that it is 'essential' you don't suffer any vapor locks !!!...................................Maj....

I know where you're coming from and vapour locks is another issue which will have to be for tomorrow because their business of running the (incoming) lines over the engine "to prevent vapour locks" as you mentioned a while ago, is completely against proper practice...inverted 'U', (not U, but inverted U..) and all that, and heat rises etc, so the fuel lines should avoid the top of the engine, by standard practice...any U (that's not an inverted one, u'know) is fine for vapour, but any line over the top of the engine is automatically an inverted U. I quite understand it will work fine on most installations, its the 1% when the fuel vapours and no-one can understand why, that is the issue.

 

 

[storchy neil]

My 912ULS maj my manual show that 912uls fuel will be return to tank also 914 neil

 

 

[storchy neil]

I'm thinking the 'return line' really doesn't need to return to tank but just recirculate back to behind the pumps to act as a (rather poor) form of pressure relief valve. Any thoughts about that?

 

i personaly would make it return to tank as stated in manual leagal neil

 

 

[metalman]

Mine is a new engine and has the return line going back to the collector tank, seems to work okay but I've only done ground runs at the moment

 

 

[flyhi]

The return line back to the left tank in my Tecnam is shown in the fuel system diagram and described in the aircraft service manual so some manufacturers may consider it part of the aircraft.

 

 

[pmccarthy]

On a trip my return line delivers a significant quantity to the right tank when left is selected. At the least, it must be relieving load on the fuel pump.

 

 

[Head in the clouds]

The return line back to the left tank in my Tecnam is shown in the fuel system diagram and described in the aircraft service manual so some manufacturers may consider it part of the aircraft.

I agree, the return line would be a component of the aircraft rather than the engine, but it still has to have something to connect to, and that's the very clearly described (clearly described when you read the installation manual more carefully than I did) "essential" fuel manifold. And the manifold can hardly be a part of the aircraft, it's a part of the engine, so I'm going to contact Floody today and find out why it's not supplied (or even mentioned - when you spend over $20K on a complete new engine installation, not just a replacement engine where you might be expected to re-use all the existing ancillaries), since it clearly states in the manual that it's essential to the fuel system. And as Neil pointed out, the manual applies to every model of the 912, not just the certified S.

 

 

[Guest Maj Millard]

HITClouds, A few things to ponder.....The 912 does not have a history of stopping in the big picture. The 912 fuel system does not have a history of suffering from vapor lock, even without the fuel-return function. The vapor/fuel return was probabily fitted as a requirement by some aviation authority, for engine certification. The 1% chance of stoppage you mention ( I think way less), is when you use all that emergency-landing practise you've done. By modifying an already very good and well proven fuel system, you may easily take that chance out to 2%-5% even 10% ..who knows, you have now become a test-pilot !!....tread very carefully, this same scenero has been carried out many times before, with mostly mixed results.......................................Maj...

 

 

[storchy neil]

maj what in the hell are you on about test pilot

 

rotax manual in your mind sould be not used why would rotax write it

 

in no way is hih modifying a system neil

 

 

[Guest Maj Millard]

Neil, I'm not referring to his incorporating of the vapor-return, but more to his suggestion of re-routing of fuel line from on top to below the engine...Then he is indeed modifying the engine !............................Maj...

 

 

[storchy neil]

return line going back to the collector tank, my thoughts is that it is right what you have done as that will releive presure on the pump and and float needle neil

 

 

[Head in the clouds]

Hang on, we're getting a bit mixed here. The return line is not a vapour return line, it's an over-pressure control system, it's in the manual Maj, the return line barb on the top of the manifold has a flow restrictor in it (a 0.014 inch diameter hole) and that would massively reduce the load on the fuel pump when the float valve is closed, and also prevent overpressuring of the float valve.

 

Regarding the routing of the fuel lines the manual makes no mention of running the lines on top of, or below the engine. Logically the output from the mechanical (engine) pump would go shortest route which is across the top of the engine to the (non-existent) manifold or to a splitter to feed each carby (and the fuel pressure gauge). But the fuel line coming into mechanical pump, according to the manual, can run any way that doesn't result in an inverted U, so under the engine is fine, or over it if there's no high point to encourage vapourisation. However the manual is very specific about lagging of the fuel lines to keep the their temp less than 36C (which is pretty low for the engine bay...) and if lagging won't achieve that then they require a cold air fed conduit to run through, which I've certainly never seen, even in this hot country of ours.

 

I'm not being alarmist here, and I quite appreciate that 912s aren't making any kind of habit of stopping but there have been those odd occasions when the engine splutters, won't run right etc and then suddenly all is fine again prompting that uneasy feeling of 'what was that all about...?' For all we know, and being a new aircraft, Luskintyre could have been something like that, low throttle setting (full carby bowls) prompting an overpressure/flooding, vapour maybe, sudden throttle application.... and then there's turbo spooling time if it was a 914.