Contradiction from Van's re: AN fuel fitting torque

[alpinelakespilot2000]

There seems to be a contradiction from Van's regarding torque on AN fittings. They post the Aeroquip guide for torque values on their FAQ site (see below). Most of our hard aluminum lines use size 6 fittings. According to the guide, these require 150-195 in-lb. That's what I've started to use.
http://www.vansaircraft.com/pdf/Torq...m_Fittings.pdf

However, when I recently did my fuel tank service bulletin, I notice that they specify 110-130 in-lb for the fuel pick up tube, which is also a size 6. That's a very significant difference! In fact, at the extremes of the ranges, it's almost twice the torque!
http://www.vansaircraft.com/pdf/sb06-2-23.pdf

In trying to reconcile this apparent contradiction, one of the main things I wonder is whether the torque for our soft aluminum tubing should be different from the perhaps harder materials used in Aeroquip hoses. Anything to that theory? Or is this just a contradiction we have to live with and make up our own mind about?

Anyway, if anyone could clear this up for me, that would be great. Making up my own rules on fuel lines is too exciting for me!

(FWIW, on the occasions where I can't get a torque wrench on the fittings, I often use one of the alternate tightening strategies, such as hand tight + 1/4 turn or bottom the seats then a certain number of additional flats. Still I'd like to know what the torque values really should be when I do use a torque wrench.)

[DanBaier]

I can't shed any light on Van's instructions.

What I can suggest it that these fittings are commonly overtightened. It's not unusual to see the flares cracked or otherwise damaged - even to the point where the tightening has taken much of the flare out and pulled the end of the tube away from the fitting. It seems counter intuitive, but too much tightening of the nut will promote a leak.

Using the "flats" approach per the Aeroquip spec sheet can do the job and it works well. It's especially helpful when using a torque wrench is impractical.

Dan

[schristo@mac.com]

as suggested... flats...

from my experience the best way to appropriately torque these fittings is marking the point where the fitting bottoms out and using the suggested flat number to final tighten... it leaves a clear mark on the fitting and ensures that the fitting seals and you don't over tighten.

[Captain Avgas]

Originally Posted by alpinelakespilot2000

In trying to reconcile this apparent contradiction, one of the main things I wonder is whether the torque for our soft aluminum tubing should be different from the perhaps harder materials used in Aeroquip hoses. )

Steve, you've hit a home run.

The torque values (inch pounds) Vans stipulate for aluminium AN-818 nuts being tightened onto grade 6061 aluminium fittings are sourced from the Aeroquip Fittings Catalogue. -4 is 100/140 min/max and -6 is 150/195 min/max.

The torque values (inch pounds) Vans stipulate for aluminium AN-818 nuts being tightened onto grade 5052 aluminium tubing (flared) are sourced from AC43.13. -4 is 50/65 min/max and -6 is 110/130 min/max.

Both are correct.

It is logical that the torques required for grade 5052 aluminium tube (flared) are considerably lower that the torques recommended for grade 6061 fittings because 5052 is a considerably softer alloy than 6061.

Many RV builders persist in wrenching up all AN-818 nuts on their flared aluminium tubing (fuel and brake lines) to the much higher torque values specified for 6061 fittings.

The end result is that the soft tubing flares are usually "creamed". In the particular case of the -4 aluminium brake lines this overtorquing is often the cause of sudden failure in service at the flare. This of course results in loss of braking on one wheel and has the potential to cause a serious brake fire.

The problem of overtorquing aluminium tubing in RVs is exacerbated by the fact that the grade 3003 tubing Vans supplies for fuel and brake lines is not in fact aviation grade. Compared to aviation grade 5052 aluminium tubing, 3003 tubing is considerably softer, and is .003" less in wall thickness. That makes it even more susceptible to damage from over torquing.

Of course needless to say Vans supplies the grade 3003 tubing because it is a general commercial grade and therefore much cheaper (surprise, surprise).

Hope I've been of assistance.