What is the purpose of the butt line in aircraft drawings?

• A. To indicate the reference position on the centerline.
• B. To show the wiring harness path.
• C. To annotate fastener sizes.
• D. To serve as a reference line offset to the right or left of the center line.

Answer: (D)

In aircraft drawings, the butt line is a reference line that runs parallel to the centerline but is offset to the left or right. It provides a lateral position reference used to locate features that aren’t on the centerline, such as frames, ribs, or stringers, on plan views. You’ll often see designations like a specific butt line number to indicate where these features lie relative to the centerline. This is exactly what makes the butt line describe a reference line offset from the center line, rather than the centerline itself. It isn’t used to show wiring harness paths or to annotate fastener sizes, which use other drawing conventions.