What kind of rigid tubing can be flared with a double flare?

• A. Copper tubing 1/8 inch OD
• B. Stainless steel tubing
• C. Magnesium alloy tubing
• D. 5052-O and 6061-T aluminum alloy tubing in sizes from 1/8 inch to 3/8 inch OD

Answer: (D)

Double flare works best on rigid tubing that can be formed into two tight, interlocking cones without cracking. In aviation practice, the combination that fits this requirement well is aluminum alloy tubing, specifically 5052-O and 6061-T alloys, in the common sizes from 1/8 inch to 3/8 inch outside diameter. These alloys provide the right balance of ductility and strength: they can be formed into a double flare without splitting and will seal reliably when the flare nut is tightened.

Other metals listed don’t meet the same reliability for this specific flare method in the given size range. Copper, stainless steel, and magnesium alloys either aren’t specified for double flaring in this context, or their material properties (hardness, brittleness, or work-hardening tendency) make a double flare less dependable. In short, the double flare procedure is designed for aluminum alloy rigid tubing in that size range.