Shell Model in Aviation

ICAO Shell Model is an conceptual tool to analyse interaction of multiple system components.

It is described in ICAO Doc 9859, Safety Management Manual . Shell diagram is to illustrate this conceptual model uses blocks to represent the different components of human factors. As described in model contains four components:

a) Software (S) (procedures, training, support, etc.);

b) Hardware (H) (machines and equipment);

c) Environment (E) (the working environment in which the rest of the L-H-S system must function); and

d) Liveware (L) (humans in the workplace).

In the centre of the model are the humans who work at the front line of operation. 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. To avoid the stress in system other blocks of model must be optimised carefully. The SHELL system is very useful tool for visualizing interfaces between the various components of the aviation system:

shell model chart
Shell model diagram
  • Liveware-Hardware (L-H). The L-H interface refers to the relationship between the human and the hardware (equipment, machines, etc,).
  • Liveware-Software (L-S). The L-S interface is the relationship between the human and the supporting systems (regulations, checklists, manuals, computer software, standard operating procedures (SOPs))
  • Liveware-Liveware (L-L). The L-L interface is the relationship among persons in the work environment.
  • Liveware-Environment (L-E). This interface involves the relationship between the human and both the internal and external environmentsInternal workspace environment includes noise, light, air quality, temperature, vibration. External workplace environment includes weather factors, aviation infrastructure and terrain. The aviation 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 “work – arounds” or minor deviations from standard operating procedures.

Accordimg to SHELL Model, a mismatch between liveware and other blocks can cause human error.


This SHELL block diagram does not cover the interfaces which are outside human factors (hardware-hardware; hardware-environment; software-hardware) and is only intended as a basic aid to understanding human factors.


The diagram is useful in giving a visual sense of how elements of these systems interact and affect one another and in creating an awareness of how these factors influence ones decision making process. It is useful to reduce errors and prevent accidents and incidents.

Brief History:

Shell model was first developed by Elwyn Edwards in 1972, with a modified diagram to illustrate the model developed by Frank Hawkins in 1975. Hawkins (1993) modified Edwards’ model to include the interactive nature of the person to person relationship (Liveware-Liveware) and called it SHELL.

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One thought on “Shell Model in Aviation

  • 03/07/2021 at 00:57

    Human factors in aviation are very interesting subject because it is one of the most significant opportunity to make aviation both safer and more efficient. It is not easy to explain all human factors components because it is comprised of many disciplines and diversity in approach by researchers. In this article, I will explain a conceptual model of human factors [ the SHEL concept].


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