What is a butt line?

• A. A reference line perpendicular to the center line of the aircraft.
• B. A line marking the location of the jet intake.
• C. A reference line along the fuselage length.
• D. A reference line to the right or left of the center line of the aircraft.

Answer: (D)

Butt line is a lateral reference line used on aircraft drawings to specify locations to the right or left of the centerline. It provides a way to describe where frames, ribs, or equipment sit across the width of the fuselage, helping technicians locate features relative to the center of the aircraft. It isn’t a jet intake marker, nor a line running along the fuselage length, and it’s not defined by being perpendicular to the centerline—it's defined by its offset to the left or right of the centerline.