What does the capital letter "C" in the southwest quadrant of the NAS symbol indicate?

The capital letter "C" in the southwest quadrant of the NAS symbol signifies that the hole will need to be countersunk. Countersinking is a machining process that creates a conical surface at the opening of a hole, allowing for the head of the fastener, such as a screw or bolt, to sit flush with or below the surface of the material. This is essential in aerospace applications to reduce drag and improve the aerodynamics of the structure, as well as to prevent injury from protruding fasteners and ensure a smooth finish.

When the NAS symbol is referenced, it is crucial to recognize the specific designations as they relate to manufacturing standards and procedures in aviation. Understanding that "C" denotes countersinking helps technicians and mechanics apply the correct surface preparation and fastener installation techniques, maintaining the structural integrity and performance required in aerospace assembly.