Definition
A reference line used in aircraft drawings and structural repair manuals that measures the lateral (left-right) distance from the aircraft's centerline. Buttock lines run parallel to the longitudinal axis of the aircraft and are numbered outward from zero at the centerline, with values given in inches to the left or right.
Plain English
An imaginary line running nose-to-tail along the aircraft, used to measure how far left or right of the centerline a part or point is located.
Context Anchor
Seen in aircraft maintenance manuals, structural drawings, repair diagrams, and equipment-location references.
Derivation
The term comes from naval architecture, where 'buttock lines' were vertical fore-and-aft slices used to describe the curved shape of a ship's hull. Aircraft designers borrowed the convention because aircraft, like ships, need a consistent way to describe points on a complex three-dimensional shape.
Why Pilots Care
Pilots themselves rarely use buttock lines, but mechanics and inspectors rely on them to pinpoint exact locations on the airframe. Understanding the term helps when reading maintenance records or discussing repairs with a mechanic.
Analogy
Think of graph paper laid across the airplane from left to right. The centerline is zero, and each buttock line is a measured line to the left or right of that middle line.
Intuition Check
Buttock line does not refer to a seat, a body part, or a visible line painted on the aircraft. It is a drawing and measurement reference for left-or-right location.
Example Sentence 1
The repair drawing called for a doubler installed at buttock line 22 on the right side of the fuselage.
Example Sentence 2
Lofting drawings show the fuselage profile using several buttock lines spaced evenly from the centerline.