Why are fuselage skin holes countersunk?

Fuselage skin holes are countersunk primarily to accommodate flush head rivets. This design feature serves multiple purposes within aerospace assembly. Flush head rivets are engineered to sit level with the surface of the fuselage, reducing aerodynamic drag and contributing to the overall smoothness of the aircraft's exterior.

Using flush head rivets is particularly important in applications where air resistance is a critical factor, as it aids in improving the aircraft’s aerodynamic performance. Additionally, the countersinking process creates a bed for the rivet to sit flush, ensuring that there are no protrusions that could lead to potential structural vulnerabilities or points of failure.

While other elements like structural integrity, weight reduction, and paint application may have their importance in specific contexts, the primary and most direct reason for countersinking is indeed to allow for the installation of flush head rivets, ensuring an efficient and streamlined assembly that meets stringent aerospace standards.