Double flare tubing materials include which of the following ranges?

• A. 1/8 inch to 3/8 inch outer diameter aluminum alloy tubing
• B. 1/4 inch to 1/2 inch outer diameter aluminum tubing
• C. 3/8 inch to 1/2 inch outer diameter aluminum 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 tubing uses ductile aluminum that can be formed without cracking and still seal reliably under pressure. The commonly used sizes for aviation tubing are 1/8 inch to 3/8 inch outer diameter, which is the range that fits standard flare fittings and is easy to form without work-hardening issues. The alloys listed—5052-O and 6061-T—cover the typical needs: 5052-O is a soft, highly ductile alloy that flares smoothly and resists corrosion, while 6061-T provides greater strength for certain lines. Together, they represent the practical combination of ductility, strength, and compatibility with aviation fittings within the standard size range. The other options either give out-of-range diameters or don’t specify the appropriate alloy types, so they don’t align with how double flare tubing is typically selected and used.