Definition
A reference line used in aircraft station diagrams that runs lengthwise along the aircraft, parallel to the centerline, used to locate points to the left or right of the longitudinal centerline. Buttock lines are measured in inches from the centerline, with separate left and right designations.
Plain English
An imaginary line running front-to-back along the aircraft, used to measure how far left or right of the centerline a point is located. Mechanics use it to pinpoint exact positions on the aircraft when reading manuals or repair drawings.
Context Anchor
Seen in aircraft maintenance manuals, structural repair drawings, inspection instructions, and parts location references.
Derivation
The term comes from naval architecture, where 'buttock lines' were used on ship hull drawings to show the curve of the hull when viewed from the side. Aircraft designers borrowed the convention because aircraft fuselages, like ship hulls, need a system of reference lines to describe their three-dimensional shape on flat drawings.
Why Pilots Care
Maintenance technicians rely on buttock lines together with fuselage stations and water lines to locate damage, components, or repair points precisely. Misreading a BL reference can lead a mechanic to inspect or repair the wrong location on the airframe.
Analogy
Think of it like a street address on a grid. The buttock line gives the side-to-side part of the address, so the technician can find the exact spot on the aircraft.
Grounding Statement
Imagine looking down at an aircraft from above: the centerline runs nose to tail, and buttock lines are parallel rulers spaced out to the left and right of it.
Intuition Check
Buttock does not refer to a body part here. In aircraft maintenance, a buttock line is a side-to-side location reference on the aircraft structure.
Example Sentence 1
The repair manual identified the crack at fuselage station 245, buttock line 12 right, just outboard of the centerline.
Example Sentence 2
Technicians aligned the replacement panel using the buttock line markings on the drawing.