What is BL in Aviation? (Butt Line)

In the world of aviation, there are numerous terms and acronyms that may seem unfamiliar to those outside of the industry. One such term is the Butt Line, often referred to as BL. The Butt Line plays a crucial role in aircraft design and maintenance, ensuring the safety and efficiency of the aircraft. In this article, we will explore what the Butt Line is, its significance in aviation, and how it is utilized in various aspects of the industry.

The Importance of the Butt Line (BL)

The Butt Line, or BL, is a reference line used to establish and maintain a consistent measuring point on the aircraft. It is typically located along the centerline of the aircraft, extending from the nose to the tail. The position of the Butt Line is determined during the initial design phase of the aircraft and serves as a baseline for various measurements and calculations.

One of the key functions of the Butt Line is to ensure proper weight and balance distribution of the aircraft. By using the BL as a reference point, engineers and technicians can accurately determine the center of gravity (CG) of the aircraft. The center of gravity plays a critical role in flight dynamics, affecting the stability and control of the aircraft. Maintaining the proper balance is essential for safe and efficient flight operations.

Furthermore, the Butt Line is also utilized in the manufacturing process of aircraft components. During the production of various aircraft parts, such as wings, fuselage sections, and tail assemblies, the position and alignment of these components are established based on the Butt Line. This ensures that all parts are properly integrated and aligned, contributing to the structural integrity and aerodynamic performance of the aircraft.

Utilization of the Butt Line in Aviation

The Butt Line serves as a fundamental reference point that is utilized in various areas of aviation, ranging from aircraft assembly to maintenance and repair operations. Let’s take a closer look at how the Butt Line is involved in these different aspects of the industry:

Butt Line in Aircraft Assembly

During the assembly process of an aircraft, the Butt Line is used as a primary reference for aligning and positioning major components. By aligning these components to the Butt Line, the overall structure of the aircraft is maintained in a consistent and accurate manner. This alignment is crucial for maintaining the desired aerodynamic properties and structural integrity of the aircraft.

For example, when aligning the wings during assembly, technicians will reference the Butt Line to ensure that both wings are symmetrical and positioned correctly. This is important for maintaining the structural strength and stability of the aircraft during flight.

Butt Line in Maintenance and Repair

The Butt Line is also widely used in aircraft maintenance and repair operations. When conducting inspections or performing repairs on an aircraft, technicians often refer to the Butt Line to assess the condition and alignment of various components.

For instance, when inspecting the fuselage for any signs of damage or misalignment, technicians may measure the distance between certain points and the Butt Line to determine the extent of any discrepancies. This enables them to accurately assess the condition of the aircraft and carry out necessary repairs or adjustments.

The Role of the Butt Line in Weight and Balance Calculations

Weight and balance calculations are crucial for ensuring the safe operation of an aircraft. The Butt Line plays a critical role in these calculations, as it provides a consistent reference point for determining the center of gravity and distributing the weight of the aircraft.

By accurately measuring the distance between the Butt Line and various components, such as the wings, engines, and landing gear, technicians can calculate the moment arm and weight distribution. These calculations are essential for determining the proper loading of cargo and passengers, as well as adjusting the aircraft’s trim settings to maintain stability during flight.


The Butt Line, or BL, is a crucial reference line used in the aviation industry. Its significance lies in establishing and maintaining a consistent measuring point for various measurements and calculations. From aircraft assembly to maintenance and repair operations, the Butt Line plays an essential role in ensuring the safety, efficiency, and structural integrity of aircraft. Understanding the importance of the Butt Line allows aviation professionals to perform their duties with precision and adhere to industry standards.

For More: What is SESAR in Aviation? (Single European Skyatm Research)