What characterizes a monocoque airplane structure?

A monocoque airplane structure is characterized by the fact that the structural loads are primarily carried by the outer skin of the aircraft. This design approach eliminates the need for a heavy internal framework, allowing for a lighter overall structure. The strength of the monocoque design relies on the integrity and rigidity of the outer shell, which is typically made from materials such as aluminum, composites, or other lightweight materials that can withstand aerodynamic forces.

In this context, the monocoque design is particularly advantageous for achieving a lightweight structure while still ensuring the aircraft can handle the stresses and strains of flight. The outer skin must be engineered to provide both strength and shape, enabling the airplane to maintain structural integrity during operation.

The other options suggest features more characteristic of different structural designs. For instance, having a lightweight substructure that provides strength implies a semi-monocoque or a different construction technique where an internal framework contributes significantly to carrying loads. An internal framework carrying structural loads describes a design approach that utilizes beams and supports, which is not aligned with the monocoque principle. The unique feature of the monocoque design lies in its reliance on the outer skin for load-bearing, differentiating it from those other construction methods.