Torque Setting Fuel Vent Line

[jeffwhip]

I?m not saying I?m right but after talking to several builders I elected to tighten this nut to 40 Inch/Ibs. The fuel vent line is not pressurized and if it leaked, the leak would occur inside the tank. Many people I trust told be to hand-tighten then go an extra 1/4 turn. 40 Inch/Ibs felt about right.

[jeffwhip]

Originally Posted by RV Jerry

I?m curious why more builders don?t double flare tubing , makes the flare much stronger and not as likely to get damaged being torqued

Jerry,
I?ve never heard of double flaring. Can you elaborate?

Jeff

[flion]

Originally Posted by Captain Avgas

Hi Jeff, you need to be careful here. You can get a lot of dangerous advice on VansAirforce. Firstly I think the reference to the data on Vans Aircraft (the Aeroquip data) is probably wrong for your application. That data is for grade 6061 aluminium to grade 6061 aluminium fittings. However you seem to be interested in a grade 6061 fitting to a flared aluminium tube. In that case your original assumption would be entirely correct and you should use the AC43.13 data (table 9.2) originally referenced. For -4 that would be a torque of 50-65 inch pounds. However, again, as you rightly surmised, those torques are for aluminium aviation grade 5052 tubing. On the other hand the stuff that Vans supplies is cheap 3003 grade and it?s considerably softer ( generally used for commercial aircon systems). So in that case I would use a slightly lower torque...maybe change the 50 min to be 50 max.
I think you have been very much on the right track in your original post. The number of builders who have serious problems with ?creamed? 3003 fuel and brake line tubing flares leading to failure is alarming.

Lots of good points in your post to ponder but remember that Vans stands behind the info they post. Here's another thing to think about - I've seen (and did, until I caught on) 'creamed' flares before the fittings were ever installed on the aircraft. It's very easy with a flaring tool to 'tighten the heck out of that sumbish', especially on soft aluminum, which leaves the flared portion too thin and prone to cracking at the transition.

I prefer the flats method because getting an inch-pound torque wrench on those fittings in a tight space can be a problem.

As for double-flared fittings, I have to second the points about the tooling being hard to find, afford, and use. Now, if some company with the tooling started offering pre-made lines with double-flared fittings, I suspect they'd do good business (I'm not so far into my RV-10 that I wouldn't consider swapping out) but otherwise it's hard to compete with single-flared fittings.

[RV Jerry]

Originally Posted by jeffwhip

Jerry,
I?ve never heard of double flaring. Can you elaborate?

Jeff

The double flare tool I normally use is simply a small die that fits inside tube then you tighten the flaring end onto the die then remove the die and flare as normal what the die does is roll the tubing inward slightly so you have more material on final flare this set up requires no more working room than other flare tools. I?m sorry don?t know how to post pictures.

[Captain Avgas]

Originally Posted by flion

Lots of good points in your post to ponder but remember that Vans stands behind the info they post.

And Vans data is correct. But it is only correct for grade 6061 fittings to grade 6061 adaptors where both mating surfaces are fully machined to 37 degrees. But that same Vans data is not correct if you are torquing an 818 nut onto a flared grade 5052 aluminium tube because the 5052 is considerably softer than the 6061. And the commercial grade 3003 tubing that Vans supplies is much softer again. The fact that builders often do not understand this frequently leads to over-torquing of flared brake tubing with resultant brake failure. It’s a really common problem.
Conversely if you use machined steel fittings then there is a different torque table again....steel is harder so the torques increase.
Does that make sense??

[Captain Avgas]

Originally Posted by flion

As for double-flared fittings, I have to second the points about the tooling being hard to find, afford, and use. Now, if some company with the tooling started offering pre-made lines with double-flared fittings, I suspect they'd do good business (I'm not so far into my RV-10 that I wouldn't consider swapping out) but otherwise it's hard to compete with single-flared fittings.

I don?t think any of the manufacturers of certificated singles use double flared tubing...certainly Cessna doesn?t.

[jeffwhip]

Originally Posted by Captain Avgas

And Vans data is correct. But it is only correct for grade 6061 fittings to grade 6061 adaptors where both mating surfaces are fully machined to 37 degrees. But that same Vans data is not correct if you are torquing an 818 nut onto a flared grade 5052 aluminium tube because the 5052 is considerably softer than the 6061. And the commercial grade 3003 tubing that Vans supplies is much softer again. The fact that builders often do not understand this frequently leads to over-torquing of flared brake tubing with resultant brake failure. It?s a really common problem.
Conversely if you use machined steel fittings then there is a different torque table again....steel is harder so the torques increase.
Does that make sense??

I appreciate all of the feedback. Did I over torque this fitting (40 Inch/Ibs).

[Captain Avgas]

Originally Posted by jeffwhip

I appreciate all of the feedback. Did I over torque this fitting (40 Inch/Ibs).

No, definitely not. As you know the torque range for the aviation grade 5052 -4 tubing is 50-65 inch pounds. But there is no table for the 3003 tubing because it simply is not aviation grade. In those circumstances I’d be inclined to use 45-50 for the 3003. To be honest this fitting will probably be permanent and therefore not as critical as the flared tube fittings on the brake calipers which are often removed and re-torqued numerous times. If those are overtorqued they just get a little bit more creamed every time until they fail.
Personally I did not use any 3003 on my plane. 5052 only. The difficulty with the 5052 is that it only comes in straight lengths so transport can be a problem. Vans love 3003 ‘cause it’s cheap and the rolled coils are easy to transport. And it’s the Experimental category so they can use whatever they like.

[carlrai]

Originally Posted by jeffwhip

Jerry,
I?ve never heard of double flaring. Can you elaborate?

Jeff

Google double flared tubing. Lots of photos and how to video.

[maniago]

From my experience, double flares are only for steel tubing and are at 45degs, and until recent years (past 30 or so) were only found on european vehicles. I cant say for sure, but I'd suspect that Al 5052 and 3003 wont take the folding required for the female double flare without cracking, and I doubt that the male 37deg connectors are expecting that bulk of material at the face. In either case, AL is more malleable at the interface anyway and the greater surface area of a single flare is more than enough to handle the pressures required, most notably brake pressures 500 to maybe spike pressure of 1000psi. Lesser fuel and oil pressures in no way need a double flare, and realistically could be done with house hold brass compression fittings...but of course thats not FAA approved so dont do it. FWIW, we use single flares on high pressure scuba compressors and boosters, which can push 4500psi, with no issue.

Just my .02 however.