What is the Location Numbering on Aircraft Structures

Modern aircraft are made up of thousands of structural components, each of which must be precisely located for routine inspection, maintenance, and repair. Without a standardized method to reference where these components are situated, even simple tasks could become time-consuming and error-prone.

This is why aircraft location numbering systems were created, allowing every inch of an airframe to be mapped using consistent reference points. With these systems, technicians are able to communicate and work with accuracy, whether identifying a flap hinge point or a stringer deep within the cabin. In this blog, we will explain some important aircraft location numbering conventions, uncovering why they are so vital to safe and efficient maintenance practices.

Primary Aircraft Reference Systems

1. Fuselage Stations (Fus. Sta. or FSs)

FSs are numerical reference points measured in inches along the aircraft’s longitudinal axis. This starts from an origin point known as the reference datum, an imaginary vertical plane usually positioned at or near the tip of the aircraft’s nose labeled Station 0. All fuselage station measurements are taken from the reference datum, extending aft along the centerline of the aircraft.

• Example: FS 137 indicates that the structure or component is located 137 inches behind the reference datum.

Some manufacturers may refer to these as Body Stations (BS) instead of FS, but the principle remains the same.

2. Buttock Line (Butt Line or BL)

The BL is a vertical reference plane that runs longitudinally along the aircraft’s exact centerline, used to measure lateral positions in inches to the left or right. By convention, positive BL values indicate positions to the right of centerline, while negative values indicate positions to the left.

• Example: A component located at BL +40 is 40 inches to the right of the centerline, while one at BL -25 is 25 inches to the left.

Buttock lines are especially important for determining the precise lateral placement of structural elements such as wing attachment points, horizontal stabilizers, or landing gear assemblies.

3. Waterline (WL)

The WL is a system of vertical reference points measured in inches above or below a designated horizontal plane on the aircraft. This plane often referred to as WL 0 may be aligned with the aircraft’s cabin floor, fuselage base, or another easily identified structure depending on its unique design.

• Example: A stringer located at WL 120 is positioned 120 inches above the established horizontal reference plane. Conversely, WL -20 would indicate a point 20 inches below that same reference.

Waterlines establish the height of structural components like bulkheads, stringers, and floor beams within the airframe. As such, these measurements enable technicians to maintain the structural symmetry of elements and uphold proper load distribution, especially in multi-deck aircraft where components are distributed across different elevations.

Reference Stations by Component Type

In addition to general fuselage measurements, aircraft manufacturers employ specific station numbering for key control surfaces and systems.

Aileron Stations (AS)

 AS measurements begin at the inboard edge of the aileron and extend outward, perpendicular to the aircraft’s rear wing beam.

• Example: If a technician is directed to inspect a bracket at AS 15, they will measure 15 inches outward from the inboard edge of the aileron, along a line perpendicular to the rear wing beam, to locate the correct mounting point.

These values are for validating that the aileron is properly positioned relative to the wing structure.

Flap Stations (KS)

KSs are used to determine the location of components along an aircraft's flap, ensuring accurate alignment. Like aileron stations, they are measured outboard from the inboard edge of the flap, extending perpendicularly from the wing’s rear beam.

• Example: A fastener located at KS 10 would be 10 inches outboard from the flap’s inboard edge, following a line perpendicular to the rear wing beam.

Nacelle Stations (NCs or Nac. Sta.)

NC measurements are employed to locate structural elements within or around the engine nacelle, such as mounts and cowlings. These measurements are taken forward or aft of the front spar of the wing, and are aligned perpendicular to a designated waterline within the nacelle area.

• Example: A technician tasked with checking a support bracket at NC 20 would measure 20 inches aft of the nacelle’s front spar, perpendicular to the nacelle’s reference waterline, to find the exact inspection point.

Specialized Structural Reference Systems

On larger or more complex aircraft, several additional reference designations may be used, including:

• Horizontal Stabilizer Stations (HSSs): These measure positions along the span of the horizontal stabilizer, starting from a central point—typically the intersection with the vertical stabilizer—and extending outward.
  • Example: An inspection point at HSS 30 would be 30 inches outboard from the stabilizer's centerline.

• Vertical Stabilizer Stations (VSSs): These define vertical measurements along the height of the vertical stabilizer, usually taken from the stabilizer’s base upward.
  • Example: A component located at VSS 40 would be 40 inches above the base of the vertical stabilizer structure.

• Powerplant Stations (PSSs): These indicate reference points along the length of an engine assembly, starting at a designated engine reference plane and extending forward or aft along the engine nacelle.
  • Example: A sensor mount at PSS 55 is positioned 55 inches aft of the engine’s reference plane.

Maintain Accuracy in Aircraft Repairs with Quality Parts

In summary, aircraft location numbering systems are essential for enabling precise identification, communication, and servicing of structural components. However, for maintenance tasks to be carried out with the utmost accuracy and safety, it is just as imperative to source high-quality parts that meet rigorous performance standards. ASAP Semiconductor is proud to support these needs through its online platform, Aviation Sourcing Solutions.

With access to an extensive inventory of aircraft structural parts and more, all of which are sourced from leading global manufacturers and suppliers, this website makes it easy to secure all of the components you require in one convenient place. In addition to our commitment to quality and compliance, we offer competitive pricing, rapid lead times, and tailored solutions to ensure seamless fulfillment. If you would like to learn more about how we can support your specific procurement needs, connect with one of our knowledgeable team members at your convenience.