Aircraft are generally built up from the basic components of wings, fuselages, tail units and control surfaces, landing gear, and power plant.
Figure 4.11: Aircraft monocoque skeleton.
The structure of an airplane is the set of those elements whose mission is to transmit and resist the applied loads; to provide an aerodynamic shape and to protect passengers, payload, systems, etc. from the environmental conditions encountered during the flight. These requirements, in most aircraft, result in thin shell structures where the outer surface or skin of the shell is usually supported by longitudinal stiffening elements and transverse frames to enable it to resist bending, compressive, and torsional loads without buckling. Such structures are known as semi-monocoque, while thin shells which rely entirely on their skins for their capacity to resist loads are referred to as monocoque.