What is a butt line?

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.

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.