What is the name of the joint where two parts of an airplane meet at a 90 degree angle?

The correct identification of the joint where two parts of an airplane meet at a 90-degree angle is referred to as a fillet. A fillet is a rounded or beveled joint that connects two surfaces, often used in structural designs to strengthen the connection and manage stress distribution. In aircraft assembly, fillets are commonly employed to provide a smooth transition between parts, which is especially important in reducing drag and optimizing aerodynamic performance.

This type of joint not only enhances the structural integrity of the airplane but also aids in the manufacturing process by allowing for easier welding or bonding of the components. The design of a fillet can vary, but it is fundamentally characterized by the 90-degree angle between the adjoining parts, regardless of its specific shape, which differentiates it from a simple joint, hinge, or bracket that serve different functions in aircraft assembly.