It can be helpful to use a model to aid in the understanding of human factors, or as a framework around which human factors issues can be structured. A model that is often used is the SHELL model, a name derived from the initial letters of its components:
- Software (e.g. maintenance procedures, maintenance manuals, checklist layout, etc.);
- Hardware (e.g. tools, test equipment, the physical structure of aircraft, design of flight decks, positioning and operating sense of controls and instruments, etc.);
- Environment (e.g. physical environment such as conditions in the hangar, conditions on the line, etc. and work environment such as work patterns, management structures, public perception of the industry, etc.);
- Liveware (i.e. the person or people at the centre of the model, including maintenance engineers, supervisors, planners, managers, etc.).