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Permalink Reply by Matt Pope on October 8, 2023 at 11:52am

I spent many hours researching and thinking about this - I'm not saying my way is the best design or even a good design, just that it's the best I could come up with to retrofit my 701 with a return line. It's not flying yet. 

I found a helpful video series on YouTube showing Skytek's design of the fuel system for the 912iS installation in the 750 STOL. I didn't copy the design, but I incorporated many parts of it. 

Here's a description of my system from the wing tanks down to the engine and back:

Both wing tanks have snorkel vents on the filler caps. 

The fuel lines coming from the wings are downhill all the way (or at worst flat) to prevent air bubbles from blocking fuel flow. Both fuel lines have emergency shutoff valves mounted high in the cockpit that could also be closed to balance the fuel or prevent fuel from flowing from one wing to the other and out the vent when parked on a slope.

The wing tanks drain into a ~1 gallon header tank that is mounted behind the seat. (I found a cylindrical aluminum tank that someone was selling from a Highlander kit.) This header tank also acts as a big gascolator - there is a fuel drain mounted at the lowest point that is accessible through a hole in the bottom skin for pre-flight sumping, and there's a serviceable 100 micron fuel filter mounted on the fuel outflow line. The header tank also ensures uninterrupted fuel flow in the event that the wing tank ports become unported during an extended forward slip with low fuel in the wings.

From the header tank, fuel flows through the coarse fuel filter and then into the electric fuel pumps, which also have their own 30 micron filter (it's a 912 ULS but I have added a fuel injection kit). The fuel pumps are mounted on the bottom of the fuselage behind the seats.

From the fuel pumps, fuel runs under the pilot's seat and into the cockpit, where there is an emergency shutoff valve down below the door (the other emergency shutoff valves wouldn't stop fuel from flowing out of the header tank). The fuel line continues through the firewall to the final fuel filter and then to the engine.

From the engine, the fuel return line runs to the pressure regulator (again, it's fuel injected), and then back through the firewall, along the passenger side wall of the cockpit, under the passenger seat, and then through a check valve before entering the header tank. The check valve prevents fuel from draining through the return line during servicing or after a crash. The return line enters the header tank a few inches above the outflow port to prevent any bubbles in the returning fuel from being drawn into the engine.

The cool fuel from the wings enters the top of the header tank, while the warm returning fuel enters lower in the tank so they mix and any vaporized fuel should condense back into liquid. Any gases that do accumulate in the header tank are vented from the top of the tank to a high point on one wing tank and then out through the snorkel vent. The vent line between the header and the wing tank goes uphill all the way, with maybe one or two short flat sections, so any fuel in it shouldn't block the gases from escaping.

I also mounted an optical liquid/gas sensor an inch or two below the top of the header tank so I get a warning if air starts to accumulate in there - it's basically a "you better be on the ground in fifteen minutes or things might get interesting" light.

Cheers!

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Permalink Reply by Shaun D on October 8, 2023 at 12:05pm

Ah, my plane is getting heavy, I was hoping to avoid the weight and complexity of a header tank, but it is definitely the safest option, I might just have to do that.

Thanks for the response Matt! 

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Permalink Reply by Matt Pope on October 8, 2023 at 12:27pm

No problem!

Yeah, the weight is a concern, but for retrofitting an aircraft with finished wings as I was, adding a header tank was much easier than adding return lines to both wing tanks, and the type of fuel selector valve you need for that costs over $1000 for some reason (at least it did when I was looking). It's a proven design though, so I'd consider it if you still have your wings open.

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Permalink Reply by Shaun D on October 9, 2023 at 8:08am

I have the retrun lines in my tanks, and I also have the duplex fuel valve.

Do you think that is the better option vs having the lines mix?

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Permalink Reply by Matt Pope on October 9, 2023 at 1:14pm

It sounds like you're halfway there already, so that might be the best way to go. I guess it depends on your risk tolerance, but for a fuel system I'd choose the design that is used in certificated aircraft over one that is known to cause venting issues and engine stoppage (!) even if the double return is more complicated and slightly heavier. Also, with a bit of creativity you might be able to mount the valve somewhere that doesn't require fuel to flow back and forth to the gascolator - for example, I've seen people add pretty long extensions between the valve itself and the selector knob. Or you could run the fuel lines along the forward door frames and mount the fuel selector valve just below the center of the instrument panel, and then mount the gascolator forward of the firewall, eliminating the low point behind the seats. Tthat's what my 701 had before I went and complicated everything with the fuel injection kit, and the fuel lines inside the cockpit were covered with painted sheet aluminum covers that weighed next to nothing and actually looked pretty good.

As with every project, installing everything for the header tank was a much bigger job than I anticipated, with lots of time spent in the hell hole figuring out where to place things to avoid the flight controls while keeping wight as close as possible to the center of gravity. It would have been much easier if I used flexible fuel lines though - I used aluminum versatube and had to make and remake every part more than once to get everything to fit together properly while looking at least half decent. Anyone attempting that should buy versatube by the 50 foot roll.

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Permalink Reply by Ken Ryan on October 9, 2023 at 8:24am

Here are a some things to think about when considering weight:

1. When using a header tank there could be fewer fittings and fewer inches of fuel line (or hose).

2. When using a header tank, you can feed both tanks to the header and use a lighter and simpler on/off valve.

3. When using a header tank, because both tanks feed the header it can be easier to manage necessary fuel quantity and you might be able to carry less fuel (or get a longer range from full tanks) than if you are returning fuel to individual tanks.

There is also a benefit of eliminating the possibility of taking off on the wrong tank.

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