Selection of a check valve -Fuel return line

Bill Hertzel wrote:

I can marginally under stand why they suggest a check valve to bypass pressure in a closed system with a Returnl Fuel Valve in the Upper example.

Wth a single Fuel Valve and an alway open retun line, the only need for a check valve would be to prevent fuel from draining out the return hose in the event of a catastropic accident where the return hose becomes severed.

It would appear that any low value check valve that prevents reverse flow would be sufficient.

- - -

In reviewing the diagrams, in all the other installation examples, fuel WILL backflow through the Dual Fuel Valves if the event of severed hoses Unless the Fuel valves are manualyl closed.

So why is the check Valve Specified ??? Safety Third!

 

Hi Bill,

Thank you for your accurate observation. As Jeff has pointed out, it is an error in the manual. In our case, since we are using single Fuel Valve, we will use low pressure check valve. 

Best regards,

Ray

Aircraft Spruce sells it but they leave the “M” off of the part number.  So for the check valve with AN-06 fittings on both ends you would choose the CK375  which is Aircraft Spruce part number 05-00716

https://www.aircraftspruce.com/catalog/appages/andaircheckvalve.php

FYI, 375= 3/8” fittings,  the -B suffix adds a bleed hole in the valve, and the -H suffix means the valve has hose fittings instead of AN.  

You can also order directly from Andair as part number CK375M.

http://www.andair.co.uk/product/check-valve-ck375/

And Wicks Aircraft sells it..

https://www.wicksaircraft.com/shop/andair-3-8-fuel-check-valve/

I believe this check valve has as much to do with engine maintenance as anything else.  When using a single fuel valve (on the supply line only) there is no easy way to shut off the return line.  So anytime you would have to disconnect a return hose for maintenance reasons there would be a danger of draining fuel from the tank.  That’s why Rotax does not show an in-line check valve on the return line when a dual selector valve is used.  But in that case you need to build the high pressure bypass circuit.  

Thanks Jeff,

The installation of the check valve would be more dependent on the airframe than the engine.

If the return hose enters the tank above the highest possible fuel level or the tanks are lower than the engine, gravity will prevail.

In my particular case both conditions are present.

The tanks are low and the hose returns half way up the side of the aluminum Fuel Fill Pipe, about 6 inches from the fuel cap but only about an inch below it.

Bill,

That makes sense about the tank position, Rotax likely just shows all POSSIBLE elements of the fuel system in their general diagram.  This was an interesting topic for me because my plane came equipped with the wrong pressure check valve installed in the return line.  The manufacturer had used the higher pressure valve -  that was designed for the bypass circuit - but it was installed directly in the return line.  This had the return line pressure running at 20 PSI, which is almost 3x the Rotax specification.  It worked under normal conditions but when both pumps were running the pressure would sometimes get over 50 PSI because of the restricted return line fuel flow. It took me a bit to figure out and get an LOA to change it, which is when I discovered the error in the pressure specification shown in the Rotax 912iS install manual.  It seems to me that there was a learning curve for the SLSA manufacturers regarding the iS fuel system.