Human factors models (in aviation)

SHEL model

The SHEL model is a theoretical framework developed by Edwards in 1972. The model places the person as the focus of interest of the discipline "Human Factors". However, humans are not independent and unrelated elements in the system but interact with other elements. Therefore, the main interest of "Human Factors" also expands to the interrelations between that person and other elements in the system: software, hardware, and the environment. The SHEL model thus takes its name as an acronym of its constituent elements (Software, Hardware, Environment, Liveware). Nowadays, the model is practically "lost" in time and buried in academic history. It, however, survives in its offspring, the SHELL model.

The SHEL Model (image embedded from [] on 15 September 2010)

SHELL model

The SHELL model was proposed by Hawkins in 1975 (see Hawkins & Orlady, 19931) as a modified version of the SHEL model. Hawkins introduced a further "liveware" element to the original model in order to represent group processes (or the interface liveware-liveware). The model acronym thus reflect the extra element (Software, Hardware, Environment, Liveware, Liveware). Hawkins also presented the model in graphical form, with the main operator (one of the "liveware" elements) in the center of the model, interacting with the remaining four elements, which are placed as outer elements. This graphical representation enhanced the central concept of the model: the human (operator) in interaction with those other elements in the system. Notice that the focus of the model is on this central human operator and his interactions (referred to as interfaces), not on the outer elements per se.

The SHELL model has played an important role in ICAO's human factors documents and training policy since the late 1980's.

SHELL-T model

The SHELL-T model was proposed by Cacciabue in (###?) as a modified version of the SHELL model. Cacciabue wanted to emphasize the team element across multiple SHELL units relatively autonomous among them except for this team element.

SCHELL model

The SCHELL model was proposed by Keightley in (###?) as a modified version of the SHELL model. Keightley represented "culture" as an autonomous element in his model.

Swiss-cheese model

The Accident causation model was proposed by Reason in 19902 as a systems approach to understanding organizational accidents. The model has also being incorporated by ICAO within its human factors documentation and training policy (in aviation, the model is often represented, and referred to, as the "Swiss-cheese model").


HFACS (Human Factors Accident Classification System)


TEM (Threat and Error Management).


LOSA (Line Operation Safety Audit)

1. HAWKINS FH & ORLADY HW [ed] (1993). Human factors in flight (2nd ed). Avebury Technical (England, UK), 1993.
2. REASON James (1990). Human error. Cambridge University Press (UK), 1992.

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Authors / Editors

Anthony FryerAnthony Fryer

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