SHELL Model (ICAO)
The SHELL model was developed by Elwyn Edwards in 1972 to structure human-machine interfaces and expanded into today's block diagram by Frank Hawkins in 1987. ICAO Doc 9683 Human Factors Training Manual (1998) adopted it as standard teaching material.
The five elements
| Letter | Component | Examples in GA |
|---|---|---|
| L (centre) | Liveware — the human (pilot) | Perception, reaction, stress, fatigue |
| S | Software — procedures, checklists, symbology | POH, AIP, charts, RT phraseology |
| H | Hardware — cockpit, instruments, controls | Switches, seat, cockpit visibility |
| E | Environment — surroundings | Weather, noise, vibration, day/night, regulatory environment |
| L (outer) | Liveware — other humans | ATC, instructor, other pilots, passengers, family |
Interface logic
Safety arises from well-fitting interfaces between the central L and the other elements. A mismatch creates risk:
- L–H: poorly arranged switches, confusing gear lever with flaps
- L–S: misleading checklist, ambiguous chart
- L–E: sun glare on instruments, heat, night
- L–L: unclear ATC communication, instructor pressure, passenger pressure ("get-home-itis")
Relevance for the PPL
SHELL is the rationale for HPL training covering all five areas — not just the pilot. A PPL student should check the interfaces before every flight: aircraft (H) vs me (L), POH (S) vs me, weather (E) vs me, ATC/instructor (L) vs me.
Related models
- Reason's Swiss Cheese — model of systemic error (Reason 1990), complements SHELL with latent conditions.
- SHELL in context: ICAO Doc 9859 SMM integrates SHELL into the Safety Management System.