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What is the Location Numbering on Aircraft Structures

Even on small aircraft, a way to precisely locate structural components is essential for maintenance and repairs. Various numbering systems are used to locate specific wing frames, fuselage bulkheads, or other structural components on an aircraft, with most manufacturers using a system of station marking. For example, a nose may be designated “zero station,” and all other stations are located at measured distances in inches behind the zero station. When a blueprint reads fuselage frame 137, that station can be located 137 inches behind the nose of the aircraft.

To locate structures to the left or right of the center line of an aircraft, a similar method is employed. Most manufacturers consider the center line of the aircraft to be a zero station from which measurements can be taken to the left or right to locate an airframe member. This is most often used for the horizontal stabilizers and wings.

When trying to locate a structure member, always double-check a manufacturer’s numbering system, as they can differ from company to company. However, most use designations similar to one another.

  • Fuselage stations (Fus. Sta. or FS) are numbered in inches from a reference or zero point known as the reference datum. The reference datum is an imaginary vertical plane at or near the nose of the aircraft from which the fore and aft distances are measured. The distance to a given point is measured in inches parallel to a center line extending through the aircraft from the nose through the center of the tail cone. Some manufacturers also call the fuselage station the body station, or BS.
  • Buttock line or butt line is a vertical reference plane down the center of the aircraft from which measurements left or right can be made.
  • Waterline (WL) is the measurement of height in inches perpendicular from a horizontal plane usually located at the ground, cabin floor, or some other easily referenced location.
  • Aileron station (AS) is measured outboard from, and parallel to, the inboard edge of the aileron, perpendicular to the rear beam of the wing.
  • Flap station (KS) is measured perpendicular to the rear beam of the wing parallel to, and outboard from the inboard edge of the wing.
  • The nacelle station (NC or Nac. Sta.) is measured either forward or behind the front spar of the wing and perpendicular to a designated water line.

In addition to these, larger aircraft may also have horizontal stabilizer stations (HSS), vertical stabilizer stations (VSS), and powerplant stations (PSS). In every case, the manufacturer’s terminology should be consulted.

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July 15, 2019