Flying Scotsman

Well Known Member
Yes, I searched the forums, but couldn't find an answer...surely this is a known question with a known answer (or two or three, depending upon the source of "official" information):

For a steel AN B-nut/flare on a hose connected to an aluminum male AN flared fitting, what is the proper torque value (-4 size)?

I don't want to start another never-ending debate (hopefully)...I just need the torque spec.

TIA!

Steve
 
"1Torque values are shown in inch pounds for aluminum fittings."

I was asking about *steel* fittings (B-nut/sleeve) to aluminum fittings (male flare).

Am I the only one of 8,000 planes that has a steel-to-aluminum AN connection anywhere on the plane? :)
 
"1Torque values are shown in inch pounds for aluminum fittings."

I was asking about *steel* fittings (B-nut/sleeve) to aluminum fittings (male flare).

Am I the only one of 8,000 planes that has a steel-to-aluminum AN connection anywhere on the plane? :)

The FAA says not to do various combinations...

Fitting combinations composed of different alloys
should be avoided to prevent dissimilar metal corrosion.
As with all fitting combinations, ease of assembly,
alignment, and proper lubrication should be assured
when tightening fittings during installation.


Lots of data here...

http://www.faa.gov/library/manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch07.pdf

...but your question isn't really answered.
 
Torque to the weakest connector; in this case aluminum.

This is correct. Which makes the Aeroquip chart referred to by Walt also correct. So that's the answer to the OP question.

As a further point: Where a lot of builders come unstuck is that they use the Aeroquip aluminium fittings chart referred to by Walt to determine the torque for aluminium flared tube to aluminium fittings. In that case the correct torque is actually LESS than the value of the aluminium fitting (the alloy of the tube is softer than the alloy of the fitting).
The consequence is that the flare on the aluminium tube often gets "creamed" and may subsequently pull out or fracture. This is a very typical failure for aluminium tube brake lines that have been torqued to the value of the aluminium B nut. With dissimilar metals, always torque to the recommended torque value of the component with the lower yield value (softer alloy).
 
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This is correct. Which makes the Aeroquip chart referred to by Walt also correct. So that's the answer to the OP question.

As a further point: Where a lot of builders come unstuck is that they use the Aeroquip aluminium fittings chart to determine the torque for aluminium flared tube to aluminium fittings. In that case the correct torque is actually LESS than the value of the aluminium fitting (the alloy of the tube is softer than the alloy of the fitting).
The consequence is that the flare on the aluminium tube often gets "creamed" and may subsequently pull out or fracture. This is a very typical failure for aluminium tube brake lines that have been overtorqued.

I agree, but then where is the reduced value torque spec. for 3003 tubing used by Vans?

The FAA usually assumes the much stronger 5052 tubing....
 
I agree, but then where is the reduced value torque spec. for 3003 tubing used by Vans?

The FAA usually assumes the much stronger 5052 tubing....

That is an excellent point Gil. As usual, you're really on the ball. The available charts are for 5052 tubing as you say, because that is the aviation grade. And that is the grade that I use exclusively. I just don't use the Vans 3003 stuff because it is low grade commercial air conditioning tube. And there have been a lot of failures of that tube on RVs, particularly on brake line flares where they can be unfastened and reinstalled a number of times.

I would say that Vans supplies the inferior 3003 alloy tube for two reasons. Firstly because it comes in rolls (5052 tube comes in straight lengths) and is therefore easier to fit into smaller boxes. And secondly because it is super cheap (McVans strikes again).

My recommendation to builders if using Van's 3003 tube would be to torque to the 5052 values as per AC 43.13 Table 9.2 but use the Minimum values as the Maximum. In the case of -4 for brake lines that would be 50 in lbs Max. Compare that to the Max torque for -4 aluminium fittings of 140 in lbs and you can see why many builders are looking at the wrong tables and dangerously creaming their 3003 tubing flares.