In a semi-monocoque fuselage, what are the longitudinal structural members called?

In a semi-monocoque fuselage, the longitudinal structural members are typically known as stringers and frames. Stringers serve as the primary support structures that run along the length of the fuselage, adding strength and rigidity while also helping to maintain the shape of the aircraft. They are attached to the fuselage structure and work in conjunction with the frames, which are the cross-sectional members that contribute to the overall structure's integrity by providing support for the skin of the fuselage.

The term “londerons” mentioned in the answer choice does not accurately apply to the components of a semi-monocoque fuselage structure, which is why it would not be the correct term. The other terminology, such as rivets and gussets, refers to different structural elements used in assembly but do not represent the longitudinal members themselves. Splices and frames would also not correctly identify the longitudinal members, as splices pertain to joining sections of materials rather than structural support that runs longitudinally. Therefore, when considering the primary components that provide the necessary support and shape for a semi-monocoque fuselage, stringers and frames are the most accurate terms.