In the "27 years of th RVator" Vans lists 40-65 inch pounds for 1/4" and 75-125 inch pounds for 3/8". That coms right out of The Standard Aircraft Handbook.
The 1/2 flat method using 3003 tubing is right at 50 in-lb. See calc discussion below.
Question for those smarter than me: Is the chart in AC43.13-1b for AN818 nut torque on aluminum flared hydraulic tubing acceptable? Is this not the definitive torque spec for our use? The paragraphs preceeding this chart talk about 3003 and 5052 tubing as well.
First, let me say, I am an engineer and did some rough calc and tested on the bench. But - YOU MUST SATISFY YOURSELF UNTIL VANS TELLS US DIFFERENT - DON'T TAKE MY COMMENTS AS GOSPEL.
Now, with that disclaimer, the -6 sleeve has about a .040" contact width. 3003 has approximately a 5000 psi yield strength, and a 9/16-18 thread with a .20 friction factor will produce a contact stress of 5000 psi with ~315 pounds of force. Translated into theoretical torque, this is about 30 in-lbs. Taken to the real world, and compared to a dry tighten, then turn 1/2 flat, the nut began to turn at about 45 in-lb. So - draw your own conclusions, but I think that yielding is beginning there, and the torque value will increase with more "flats" because the contact area is made by chamfers in the sleeve. Further displacement yields a larger contact area and takes higher torque, BUT - the contact stresses are not higher. The the material is yielding. This is less than a precise analysis, thus the difference with the real results.
So, why are all the charts off?? Is it because they don't consider 3003 as the material? They do say "ALLOY". If yield stress is 10ksi, then the torque would be twice as much, but 1/2 flat is still good.
For my airplane, and my life, I will be using the 1/2 flat or 40 -45 in-lb method for 3003 tubing and 3/8" diameter.
Critical, definitive discussion is invited. A second, third, critical look is always good for a peer review.
Let it begin.