Ways not to make the sight gauges when using rigid PVC tubing

[jaredyates]

(this is a post about personal experience, not an update from Bob)

A few months ago I made a set of fuel sight gauges out of the rigid PVC tubing as described here:

https://bearhawk.tips/3789

If there's a way to do something wrong, I'm definitely capable of finding it. In this case I found a few ways, so I thought I'd share. The first symptom was that I was getting a little whiff of fuel smell in the cabin every now and then, and I hadn't been able to find any leaks. Yesterday it was time to refuel, and I filled the left tank to around 3/4. On the next flight when briefly flying uncoordinated, I got a really strong fuel smell. Turns out the sight gauge was leaking at the top due to a crack. When the plane was uncoordinated, it allowed the fuel to come over to the wing root and fill the gauge.
IMG_20230116_161939389.jpg

The root of the problem is that pipe threads are tapered, which makes male fittings function as a wedge. The tap is also a wedge. After talking with Bob, here are some things that I did wrong when making them. Making a set of gauges requires cutting four sets of threads. When I did the first one or two, I operated the tap by hand. Then I was kind of lazy so for the last two I chucked up the tap in a handheld drill. In the latter case especially, I quite likely was not removing material cleanly, and thus was actually stressing the plastic outward, while also not cutting very deep threads. If the plastic spreads, then the threads are shallow, making the wedge function of the fitting worse.

When it came time to install the fittings, the threads have to be sealed with something, as all pipe threads do, so I used fuel lube. The down-side of this is that it further reduced friction and made it even easier to overtighten the fittings.

Bob says he has used plastic fittings in this location instead of aluminum. In my next version I think I'll still use the aluminum fittings, but I'll do a better job of getting good threads cut, and also less tightening. I might also try some type of clamping during the tapping operation, to help ensure that I'm cutting threads and not just bell-mouthing the plastic end.

In my case the crack was at the top of the gauge which made it obvious only when the fuel level at the wing root was high. This would have been much less convenient if I had been away from home and filled the tanks all the way up. There were some warning signs such as similar-looking marks at the tube ends. Only one has developed into a leaking crack so far, but I suspect it's a matter of time and I'll be replacing both tubes.

[rodsmith]

Great info Jared, I will closely be inspecting mine. Instead of fuel lube is there a preferred sealant, would like to know what others have successfully used.

[jaredyates]

I plan to use fuel lube in the next application, just keeping in mind to go easy on the torque.

[Nev]

Another way to do these is to use flexible clear hose from the hardware store. I then got some aluminium caravan awning track and cut the flange off, and pulled the tubing inside the track. Eventually I added a hose clamp to the lower end to provide a full seal. So far it's been good.

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[ttibc@msn.com]

I've read recently, I don't know where, someone suggested locktite. Is this an acceptable method superior to fuel lube?

[rodsmith]

Yes someone else mentioned it, can't remember who. That's what I used, but now wondering if that is ok

[jaredyates]

The fuel lube wasn't a chemical incompatibility but rather just a lubricant that allowed for easier overtightening. I would imagine any number of products would be fine as long as the installer doesn't overtighten, or tighten based on feel and expect there to be much torque.

[bkyser]

One of these going from cracked to broken in flight and flooding the cabin with fuel would make for an interesting flight. The fairly fragile rigid PVC doesn't seem like the best material here. I think crashworthiness should also be considered here. I can see were a broken sight gauge could turn a survivable incident or accident into a deadly post-crash fire.

FWIW: Nev's setup looks a lot more robust to me.

[Daniel]

I've got to agree here. Impossible to failure proof here but I've seen pvc crack easily and often. I would rather go with the clear fuel flex line offer through mcmaster carr. They turn yellow but replacement seems easy enough. The added aluminum that Nev did seems like a nice touch.

[Battson]

One of these going from cracked to broken in flight and flooding the cabin with fuel would make for an interesting flight. The fairly fragile rigid PVC doesn't seem like the best material here. I think crashworthiness should also be considered here. I can see were a broken sight gauge could turn a survivable incident or accident into a deadly post-crash fire.

FWIW: Nev's setup looks a lot more robust to me.

They are certainly a fragile thing, but not as fragile much as one might think. It would be a severe crash if those tubes got broken. But they aren't going to survive any meaningful impact from the side, being a long skinny tube made of soft materials, without much structural support. I don't think I've yet seen any design which overcomes this, without making a few ugly compromises. I think severe turbulence and unsecured passengers (or pilots) poses the most risk to them. More risk than crashing, IMO.

There have been many severe accidents over the years involving aircraft with glass sight gauges installed. Still the general design has never been AD'd off the airframes and persists today in many aircraft types.

For what it's worth, mine are still going strong 10 years after construction. I've never once seen them take a knock, it's just an "out of the way" area of the cabin.

As the OP notes they do need to assembled carefully so as to avoid cracking the tube, and with a sealant. A threaded connection applies a huge amount of force on the materials involved, when tightened. Hence threads are easily stripped on metal bolts of the same size, what chance does PVC stand...

[Nev]

I was concerned that someone might grab them inadvertently.

[marcusofcotton]

PVC in this applicatin scares the daylights out of me! Don't look for long life from it and when it fails it can easily be catastrophic. Different demanding applications over the years has shown me that the Tygon brand of tubing will give you specific information about their products and they are what they say they are. Check out website and specifically LP-1500 tubing (look at their related products as well).

https://www.fluidtransfer.saint-gobain.com/tubing/thermoplastic-elastomer-flexible-tubing/tygon-lp-1500

The fact that LP-1500 is CARB and EPA B1-15 rated means that off gassing won't have long term effects on polycarbonate windows (yes, non rated actual fuel lines can have an effect!). Also UV resistant and flexible in the cold. Specifically rated for gasolines including those with ethanol.

Note: I haven't used LP-1500 myself yet, but after researching for this thread I've added a note with my plans to use it.

[Bcone1381]

Ya know, yesterday I started messing with my sight gages again. I'm wiring and need to have them assembled to keep them electrical wires from co-existing in the same space.

My sight gages were fabricated a few years ago, and yesterday were bench tested to be leak free at 40psi. But I ran into problems assembling my ridged lines going between the tanks and the gages. Errors of my own making. How did I clock the lines, which end goes into the tank, the backplate....did it go in this way or that way. It took a hour to figure it out and included connecting the wing to figure it out.

(Put your wings and fuselage on stands with HF dollies under each one....it takes 5 minutes to connect a wing.)

I think today I will be ordering the fittings and the tubing to make the tygon product sight gages. They seem so simple. And if I get two tees, I will easily install a breather line for venting the two tanks.

[marcusofcotton]

I think today I will be ordering the fittings and the tubing to make the tygon product sight gages. They seem so simple. And if I get two tees, I will easily install a breather line for venting the two tanks.

I look forward to pics and your feedback afterwards. Will you be putting refraction stripes behind it so clear gas level can be easily spotted at a glance? Here is a pic of stripes in action from Steve Pierce on Supercub.org. I've seen some other stripe patterns that are even nicer to distinguish level so experimentation may be desired.
image.png

[Bcone1381]

Yes, Mark. And I think I'll use Red hashes.

[Battson]

PVC in this applicatin scares the daylights out of me! Don't look for long life from it and when it fails it can easily be catastrophic. Different demanding applications over the years has shown me that the Tygon brand of tubing will give you specific information about their products and they are what they say they are. Check out website and specifically LP-1500 tubing (look at their related products as well).

https://www.fluidtransfer.saint-gobain.com/tubing/thermoplastic-elastomer-flexible-tubing/tygon-lp-1500

<...>
Also UV resistant and flexible in the cold.

Well.... that seems a little alarmist to me. That PVC tube is used in many aircraft for this purpose, and has been for decades.

In terms of the suggested alternative - you are right they are a very honest company. The manufacturers confirm that product will degrade over time (despite what Aircraft Spruce says!) and it will yellow as UV damage occurs, per their technical documentation it's UV resistant, not UV proof. I have not seen these issues yet with the PVC, but it will happen eventually. It remains clear and strong so far (it's around a dozen years old). What do you consider "long life"?

I have learnt PVC used for this application can deform under it's own weight if it gets extremely hot in the cabin (think direct sunlight in a desert all day with windows closed). It remains strong afterwards. I have posted about that issue, years ago.

The fact that LP-1500 is CARB and EPA B1-15 rated means that off gassing won't have long term effects on polycarbonate windows (yes, non rated actual fuel lines can have an effect!)

Using Lexan AKA polycarbonate for front windows or the skylight - experience shows that crazing is bound to happen, eventually. Fuel drips on the open windows from the fuel drains, and you get splashes of fuel on the skylight from the fuel bowser. It can't be avoided forever.

If people decide to use the "clear flexible" plastics for fuel sight gauges, fuel lines, brake lines, or window applications - be aware you will have to change them more often than some of the alternatives.

[marcusofcotton]

Well.... that seems a little alarmist to me. That PVC tube is used in many aircraft for this purpose, and has been for decades.

In terms of the suggested alternative - you are right they are a very honest company. The manufacturers confirm that product will degrade over time (despite what Aircraft Spruce says!) and it will yellow as UV damage occurs, per their technical documentation it's UV resistant, not UV proof. I have not seen these issues yet with the PVC, but it will happen eventually. It remains clear and strong so far (it's around a dozen years old). What do you consider "long life"?

I have learnt PVC used for this application can deform under it's own weight if it gets extremely hot in the cabin (think direct sunlight in a desert all day with windows closed). It remains strong afterwards. I have posted about that issue, years ago.




Using Lexan AKA polycarbonate for front windows or the skylight - experience shows that crazing is bound to happen, eventually. Fuel drips on the open windows from the fuel drains, and you get splashes of fuel on the skylight from the fuel bowser. It can't be avoided forever.

If people decide to use the "clear flexible" plastics for fuel sight gauges, fuel lines, brake lines, or window applications - be aware you will have to change them more often than some of the alternatives.

It's experimantal and we can each do as we please. Just as you have been very helpful and supportive in our community with your experience, that is what I strive for as well. All of us have different experiences in life. I stand by my concerns.

I'd be extremely surprised if you could point out a certificated aircraft application using PVC tubing with tapered threads cut into the end of it in any application. For that matter I'd be suprised if you found any manufacturer specifying such a method anywhere. Should that PVC have a small split at the lower end it wouldn't take much of a leak to make for a bad experience.

In searching Aircraft Spruce site I didn't find LP-1500 but they did list LP-1200 and F4040 fuel lines. From the manufacturer's information I thought LP-1500 would be more suitable and appears would have greater clarity for the application.

One specific case of the polycarbonate deterioration I saw was from quality manufacturer (Gates IIRC) modern black (not clear) automotive fuel line, but not the more expensive lower permeation SAEJ30R9 fuel injection CARB and EPA rated hose. But alas, if it's going to deteriorate any way I guess it doesn't matter if we carelessly speed it up its demise.

I sincerely thank you for your many great contributions, but respectfully disagree with you on this matter.

[Battson]

I respect your views, I figured you must have plenty of experience with plastics. I can only talk from my practical experience of this design and PVC.

[TimTall]

Regardless of what type of material used, it's a very good idea - and required in Canada - to put a restrictor in the system to reduce the flow should there be failure of the sight tube. I machined down some brass pins to make an interference fit with the AN fittings at the top and bottom of the gauges and drilled 0.040" holes in them. Once they are pressed into the fittings they can never escape and cause issues themselves, and they restrict the fuel to a much more manageable flow should the clear tube ever break. They also reduce the sloshing in the gauge as a bonus.

[marcusofcotton]

Regardless of what type of material used, it's a very good idea - and required in Canada - to put a restrictor in the system to reduce the flow should there be failure of the sight tube. I machined down some brass pins to make an interference fit with the AN fittings at the top and bottom of the gauges and drilled 0.040" holes in them. Once they are pressed into the fittings they can never escape and cause issues themselves, and they restrict the fuel to a much more manageable flow should the clear tube ever break. They also reduce the sloshing in the gauge as a bonus.

Absolutely agree!