sight gauge connection

[Sierra Skydog]

When connecting a short rubber hose between the sight gauge fittings and the 1/4” aluminum tubing coming from the tanks, I think I need a beaded lip or slight flair on the 1/4” tubing to help secure the rubber hose. I haven’t found a cheap beading tool that small. I’m thinking maybe a slight flair might work. What have you guys done here that works? Thanks

 

[svyolo]

When connecting a short rubber hose between the sight gauge fittings and the 1/4” aluminum tubing coming from the tanks, I think I need a beaded lip or slight flair on the 1/4” tubing to help secure the rubber hose. I haven’t found a cheap beading tool that small. I’m thinking maybe a slight flair might work. What have you guys done here that works? Thanks

I used the "slight flare". Rounded the outer edge smooth to make insertion easier.

 

[jaredyates]

I did not use any rubber lines in the fuel system between the tanks and the firewall. I didn't trust them to last in such an important application. Are you sure you need them?

 

[Sierra Skydog]

not the firewall…I mean as recommended between the wing tank and sight gauge to allow for slight movement of the tanks

 

[Sierra Skydog]

thanks for the quick response

 

[gregc]

I used a simple beading tool. One of these I think: https://www.pegasusautoracing.com/productdetails.asp?RecID=31585

 

[Redneckmech]

If you want to DIY it, this is a good option. I have used similar for inter-cooler piping (automotive application) when the tool cost exceeded the amount I wanted to spend.

https://www.kitplanes.com/the-creative-homebuilder-17/

 

[alaskabearhawk]

The tube in that location isn't going anywhere. Just smooth the cut end of the AL tube and be done. The clamps will seal just fine. That's what I did and I don't have any issues.

 

[noema]

I will probably connect mine directly with a strain relief S-curve. Has plenty of flexibility. Don't see a problem forgoing a flexible hose. Open to feedback.

​

 

[jaredyates]

I didn't use anywhere near that much of a loop, but same idea.

 

[jaredyates]

We are talking about the same place.

 

[Sierra Skydog]

thanks to all for the feedback, a lot of good solutions

 

[alaskabearhawk]

Looks like a water trap…ie. built-in potential low point. No?

 

[noema]

With the S-shape installed horizontally, this should effectively prevent that.

 

[Astro John]

While we are talking about sight gauges, what would you do if one failed and started leaking in flight?

I know it’s a simple system, and anything extra is just another point of possible failure, but what about putting some sort of shutoff valve at the tank fitting?

 

[TimTall]

I put a restrictor in the fittings where the sight gauges come through the wing root into the cabin. I made my gauges out of clear PVC with 90° AN fittings. I drilled the center of a grub screw out to 0.040", tapped the AN fitting for the grub screw thread, and tightened the grub screw into one end of the fitting.

If the PVC gauge fails, the flow would be restricted to what can make it through a 0.040" hole. There would still be gas in the cabin but a lot less than full flow through an AN4 tube. I'm not really worried about a catastrophic failure of the sight gauge though. If it fails it'll probably start leaking slowly before it completely comes apart. I tested mine to more than double the pressure they'd ever see in use and there was no leakage around the threads in the pvc.

As a bonus, the restrictor dampens the movement of the fuel in the gauge which makes the gauge more useful in flight.

 

[jaredyates]

I also use restrictors at the 90-degree fittings. If for some reason the bottom aluminum line failed, I'd endeavor to get some vice grips in there to crimp the line. It's a bad day if it gets to that point.

 

[rodsmith]

I did not use any rubber lines in the fuel system between the tanks and the firewall. I didn't trust them to last in such an important application. Are you sure you need them?

I agree with Jared. I used the 1/4" blue fuel line to connect my sight glasses. When I first filled the tanks, I had a leak on the right hand lower connection where the fuel line connected to 1/4" aluminum tubing. I had to tighten the clamp to where I thought it would crush the aluminum to stop the leak. Yesterday when I walked into my hangar I could smell avgas. Found a small puddle on the floor under the trailing edge at the wing root which was dripping fuel. Had to remove the left main tank cover to find the leak. It was the same type of connection as the initial leak, but on the left upper sight glass connection. I had poor access to the clamp, got it as tight as I could and it was still dripping. I ended up installing a second clamp and again tightening to where I thought it was going to crush the aluminum tube to stop it. I had borrowed a tool to bead the aluminum tubes but the beads weren't real prominent. When I do my initial condition inspection I am going to replace the aluminum tubes and hose with #4 braided stainless racing hose assemblies from Pegasus auto racing, assuming I can get enough access on all of them. I used #3 racing hose assemblies for all the brake lines between rudder pedals.

 

[svyolo]

The type of clamp make a big difference as well. The smaller the tube, the more critical this gets. Constant tension (spring) work well if you can get a good fit, which can be tough the smaller the lines get. Crimped also work well, but you can't tighten them if they leak. For smaller lines my favorite are this type. Searching "fuel line clamps" will find them.

I also use two clamps per connection. Gravity fed fuel over my head scares me a bit.

 

[gregc]

I won't disagree that some/all of the above solutions are good ones. At the same time, I don't think that rubber hose based connections are inherently bad. Many certified airplanes uses short sections of rubber hose at the wing root and fuselage connection and these aircraft don't routinely fall out of the sky in flames. The main weakness of a hose connection is that it has a finite lifetime and will need to be tracked and replaced at some interval. I general I think we should be cautious when 'improving' on standard designs and techniques, especially if, like me, you don't have decades of knowledge and experience. Often my improvements end up being heavier, more complicated and possibly less reliable than what's suggested in the build manual or is a traditional solution. I'm learning (slowly I admit) to look more carefully at the trade-offs of different approaches.

 

[rodsmith]

Those look like they would provide a more even clamping pressure than the worm type clamps.

 

[svyolo]

The ones I have are also friction type nuts so they don't loosen up. I don't know if they are all like that.

 

[Bcone1381]

Ha! My install was iaw Bob's drawing and had 8 constant tension clamps. One leaked. As a curiocity I asked my auto mechanic what he thought was the best hose clap to use. Its exaclty what you show here and thats what I installed in all 8 locations.

 

[svyolo]

I like constant tension if I can use them, but I could not find good ones small enough for the wing root - not enough tension.

 

[svyolo]

When I did mine, I was not comfortable with a slightly moving tank, and solid connections. But with a loop in the tube, and if it is pretty "normal" in aircraft, I am getting open to the idea. It is just not what I was used to in the past. I used tubing stubs - 3-5 inches from the tank, and Aeroquip Startlight line. It is very high quality, similar to what is used high pressure fuel injection lines in the auto/marine world. It doesn't collapse, and lasts an incredibly long time. It is used in cars and normally never changed out unless you do a restoration. So hose clamps on the studs, and then Aeroquip fittings connected to the solid lines at the wing root. Primarily, it is "what I knew".