SHELL Model | for Safe and Effective Human Performance

Aviation SHELL Model

Before explaining the SHELL Model Human Factors, I will explain the Human Factors. FAA defines Human Factors as a “multidisciplinary effort to generate and compile information about human capabilities and limitations and apply that information to equipment, systems, facilities, procedures, jobs, environments, training, staffing, and personnel management.
Simply I can say that Human Factors are the practice of applying scientific knowledge to reduce human error. 
In this article, I wrote about SHELL Model Human Factors, the name being derived from the initial letters of its components, Software, Hardware, Environment, Liveware). It was first developed by Edwards in 1972, with a modified diagram to illustrate the model developed by Hawkins in 1975.

Why do Humans Make Errors?

Human error is defined as a human action with unintended consequences. Humans make errors for a large number of reasons such as:  
  •  Because they have not been adequately trained to perform tasks.  
  •  Because they do not have the basic ability to perform the task even if they were trained - Because the task is beyond normal human abilities.
  •  Because they misinterpret information important to the performance of the task. 
  •  Because some event occurring during the performance of the task changes the nature of the task in a way that they have never encountered. 
  •  Because of the influences of many human factors such as stress, distraction, fatigue, illness, visual illusions, spatial disorientation, old age, immaturity, cultural beliefs, and the list goes on and on.  
The above-mentioned causes of human error, are encompassing a wide range of challenges that influence people very differently as humans do not all have the same capabilities, strengths, weaknesses, or limitations.  
The study and application of human factors are complex because there is not just one simple answer to fix or change how people are affected by certain conditions or situations.

Understanding SHELL Model Human Factors

It is helpful to use a model to aid your understanding of Human Factors, as this allows a gradual approach to comprehension. In the SHELL Model Human Factors, the following components are used:

[ L] Liveware (Humans)

Humans, the [L]- Liveware is at the center of the SHELL Model Human Factors, all other aspects (Software, Hardware, and Environment) must be designed or adapted to assist human performance and respect human limitations. If these two aspects are ignored, the humans - in this case, the aviation professionals - will not perform to the best of their abilities, may make errors, and may compromise safety. Liveware-Liveware is the interface between people (leadership, crew cooperation, teamwork, and personality interactions). Safety deficiencies may occur due to: 
  • Bad relationships between workers. 
  • Shortage of manpower.  
  • Lack of supervision.  
  • Lack of support from managers

[ H] Hardware (Machine)

The liveware-Hardware interface is the one most commonly considered when speaking of human-machine systems. For example, the design of seats to fit the sitting characteristics of the human body, of displays to match the sensory and information processing characteristics of the user, and controls with proper movement, coding, and location. e.g., tools, test equipment, the physical structure of aircraft, design of flight decks, positioning and operating sense of controls and instruments, etc.).

[ S] Software (Procedures, Symbology, etc.)

Liveware- Software, encompasses humans and the non-physical aspects of the system, such as procedures, manual and checklist layout, symbology, and computer programs.

[ E] Environment (the situation in which the L-H-S system must function).

The human-environment interface was one of the earliest recognized in flying. Initially, the measures aimed at adapting humans to the environment. The liveware-Environment interface must consider perceptual errors induced by environmental conditions such as temperature, pressure, humidity, noise, time of day, light, darkness, etc.


SHELL Model Human Factors awareness can lead to improved quality, an environment that ensures continuing worker and aircraft safety, and a more involved and responsible workforce. More specifically, the reduction of even minor errors can provide measurable benefits including cost reductions, reduction in work-related injuries, and reduction in more significant events that can be traced back to human error.  
The SHELL concept 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.  
SHELL Model Human Factors is a very interesting subject because it is one of the most significant opportunities to make aviation both safer and more efficient. Further reading: 
1-      UK CAA, CAP 719, Fundamental Human Factors Concepts.

Maged Saeed AL-Hadabi

I’m Instructor / Maged Saeed Al-Hadabi. ​ Air Cargo / IATA Dangerous Goods Regulations / Safety Management System Senior Instructor, Auditor [ Yemen Airways] . Approved IATA DGR/ SMS Instructor by Yemen Civil Aviation Authority.

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