Repairs or splices involving stringers on the lower surface of stressed skin metal wings are usually:

Repairs or splices involving stringers on the lower surface of stressed skin metal wings are considered more critical than those on the upper surface primarily due to the structural load distribution and potential impact during operation. The lower surface of the wing experiences tension during flight as the aircraft generates lift, making the integrity of this area highly important for maintaining aerodynamic performance and structural safety.

When stringers, which provide longitudinal support to the wing structure, are compromised, the consequences can be significant, especially at the lower surface where stress is concentrated. As a result, repairs must be executed with great care and under strict adherence to engineering and safety standards. This heightened level of scrutiny ensures that any repair maintains the wing's ability to withstand the forces it encounters during flight.

In contrast, repairs on the upper surface may be less critical since this area is generally subjected to compressive loads when the aircraft is in flight. Therefore, while repairs and splices on the lower surface are permitted, they are treated with more caution and required to meet more stringent criteria to ensure continued airworthiness. This understanding underscores the importance of careful engineering judgment when considering repairs in different sections of the wing structure.