Which type of structural repair is considered less critical regarding strength in tension?

Stringer repairs on the lower surface are indeed considered less critical regarding strength in tension. This is because the primary tension forces in an aircraft structure typically act on the upper surface during flight. The upper surface experiences tension due to the aerodynamic loads when the aircraft is in the air. Consequently, repairs made to the upper surface need to maintain high strength and structural integrity to withstand these forces.

In contrast, repairs on the lower surface do not experience the same level of tensile stress during normal operating conditions. Thus, while the lower surface still requires repairs that are structurally sound, it does not face the same critical tensile requirements as the upper surface. This makes lower surface stringer repairs relatively less critical when it comes to handling tension loads. Therefore, the correct choice aligns with the understanding of how aircraft structures are subjected to different types of stress based on their location and function in flight.

Patching small holes and lap joint repairs are more related to integrity throughout the structure, but they do not specifically denote the tension considerations that a stringer repair involves.