What does the yellow mark on an aircraft tube indicate?

• A. Light
• B. Left
• C. Center
• D. Heavy

Answer: (D)

A yellow mark on a tube is a color code that identifies a heavy-walled, high-capacity line designed to withstand higher internal pressure. This marking helps technicians quickly recognize that the tube is meant for more demanding parts of the system (such as primary hydraulics or gear actuation) and should use the correct higher-strength fittings, routing, and bend radii. It also aids in selecting the proper replacement if a tube needs to be swapped. The other options don’t fit because the mark isn’t about light gauge, orientation, or position—the color code is specifically tied to the tube’s strength and pressure rating, which is why heavy is the correct interpretation.