The SHELL Model is a conceptual tool used to analyze the interaction of multiple system components. The SHELL Model contains the following four (4) components:
SOFTWARE (S): procedures, training, support, etc.
HARDWARE (H): machines and equipment
ENVIRONMENT (E): the working environment in which the rest of the L-H-S system must function and
LIVEWARE (L): humans in the workplace
In the center of the SHELL model are the humans at the front line of operations. Although humans are remarkably adaptable, they are subject to considerable variations in performance. Humans are not standardized to the same degree as hardware, so the edges of this block are not simple and straight. Humans do not interface perfectly with the various components of the world in which they work. To avoid tensions that may compromise human performance, the effects of irregularities at the interfaces between the various SHELL blocks and the central Liveware block must be understood. The other components of the system must be carefully matched to humans if stresses in the system are to be avoided. The SHELL Model is useful in visualizing the following interfaces between the various components of the aviation system:
Liveware-Hardware (L-H)
The L-H interface refers to the relationship between the human and the physical attributes of equipment, machines, and facilities. The interface between the human and technology is commonly considered with reference to human performance in the context of operations, and there is a natural human tendency to adapt to L-H mismatches. Nonetheless, this tendency has the potential to mask serious deficiencies, which may become evident only after an occurrence.
Liveware-Software (L-S)
The L-S interface is the relationship between the human and the supporting systems found in the workplace, e.g., regulations, manuals, checklists, publications, standard operating procedures (SOPs), and computer software. It includes such issues as the recency of experience, accuracy, format and presentation, vocabulary, clarity, and symbology.
Liveware-Liveware (L-L)
The L-L interface is the relationship among persons in the work environment. Since operational personnel function in groups, it is essential to recognize that communication and interpersonal skills, as well as group dynamics, play a role in determining human performance.
Liveware-Environment (L-E)
This interface involves the relationship between the human and both the internal and external environments. The internal workplace environment includes such physical
considerations as temperature, ambient light, noise, vibration, and air quality. The external environment includes operational aspects such as weather factors, infrastructure, and terrain. This interface also involves the relationship between the human internal environment and its external environment. Psychological and physiological forces, including illness, fatigue, financial uncertainties, and relationship and career concerns, can be either induced by the L-E interaction or originate from external secondary sources. The work environment includes disturbances to normal biological rhythms and sleep patterns. Additional environmental aspects may be related to organizational
attributes that may affect decision-making processes and create pressures to develop “workarounds” or minor deviations from standard operating procedures .
According to the SHELL Model, a mismatch between the Liveware and the other four components contributes to human error. Thus, these interactions must be assessed and considered in all sectors of the aviation system.
