The SHELL model is a conceptual model of human factors that clarifies the scope of aviation human factors and assists in understanding the human factor relationships between aviation system resources/environment (the flying subsystem) and the human component in the aviation system (the human subsystem). The SHELL model was first developed by Elwyn Edwards (1972) and later modified into a 'building block' structure by Frank Hawkins(1984). The model is named after the initial letters of its components (software, hardware, environment, liveware) and places emphasis on the human being and human interfaces with other components of the aviation system. The SHELL model adopts a systems perspective that suggests the human is rarely, if ever, the sole cause of an accident. The systems perspective considers a variety of contextual and task-related factors that interact with the human operator within the aviation system to affect operator performance. As a result, the SHELL model considers both active and latent failures in the aviation system. Description Each component of the SHELL model (software, hardware, environment, liveware) represents a building block of human factors studies within aviation. The human element or worker of interest is at the centre or hub of the SHELL model that represents the modern air transportation system. The human element is the most critical and flexible component in the system, interacting directly with other system components, namely software, hardware, environment and liveware. However, the edges of the central human component block are varied, to represent human limitations and variations in performance. Therefore, the other system component blocks must be carefully adapted and matched to this central component to accommodate human limitations and avoid stress and breakdowns (incidents/accidents) in the aviation system. To accomplish this matching, the characteristics or general capabilities and limitations of this central human component must be understood. Human characteristics Physical size and shape In the design of aviation workplaces and equipment, body measurements and movement are a vital factor. Differences occur according to ethnicity, age and gender for example. Design decisions must take into account the human dimensions and population percentage that the design is intended to satisfy. Human size and shape are relevant in the design and location of aircraft cabin equipment, emergency equipment, seats and furnishings as well as access and space requirements for cargo compartments. Fuel requirements Humans require food, water and oxygen to function effectively and deficiencies can affect performance and well-being. Input characteristics The human