SHELL Model Aviation | A Basic Aid to Understand Human Factors

shell model aviation

SHELL Model Aviation (the name being derived from the initial letters of its components, Software, Hardware, Environment, Livewire) was first developed by Edwards in 1972, with a modified diagram to illustrate the model developed by Hawkins in 1975.

The term "human factor" is used in many different ways in the aviation industry. Most people have known it in the context of aircraft cockpit design and Crew Resource Management(CRM)

However, those activities are only a small part of aviation-related human factors, as broadly speaking it covers all aspects of human involvement in aviation.

Human Factors cover a range of issues including perceptual, physical, and mental capabilities, the interaction and effects on individuals of their job and working environments, the influence of equipment and system design on human performance, and finally the organizational characteristics which influence safety-related behavior at work.

In addition, Human Factors is the practice of applying scientific knowledge with the intention of reducing humanerror.

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 SHELL concept].

Human Factors Definition

(FAA), is defined 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 for safe, comfortable, and effective human performance”.


After reading my introduction, I am sure that you clearly understand the definition of human factors and why it is important to improve aviation safety. 

During writing this article from many references I found that it is important to explain to you about the human error because human factors tools aimed at reducing human error.

Why Humans Make Errors?

Human error is defined as a human action with unintended consequences. Humans make errors for a large number of reasons. For examples, they make errors:

-  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 influences such as stress, distraction, fatigue, illness, visual illusions, spatial disorientation, old age, immaturity, cultural beliefs, and the list goes on and on.

Many human factors can affect human performance and work performance in aviation. They encompass 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 humanfactors 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 Aviation

It is helpful to use a model to aid in the understanding of Human Factors, as this allows a gradual approach to comprehension. In the SHELL model, the following interpretations are suggested: [ L] liveware (human), [ H] hardware (machine) and [ S] software (procedures, symbology, etc.), [ E] environment (the situation in which the L-H-S system must function).

SHEL Model aviation
A modified diagram to illustrate the SHEL model developed by Hawkins

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.

[ L] – Liveware (Humans)

Humans, the " [ L] - Liveware," can perform a wide range of activities. However, humans have limitations. Since [ L]- Liveware is at the center of the model [SHELL], 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 human - in this case the aviation professionals - will not perform to the best of their abilities, may make errors and may compromise safety.

To achieve this matching, an understanding of the characteristics of this central component is essential. Some of the more important characteristics are the following:

a)      Physical size and shape

In the design of any workplace and most equipment, a vital role is played by body measurements and movements, which will vary according to age and ethnic, and gender groups. Decisions must be made at an early stage in the design process. 

b)      Physical needs

People’s requirements for food, water, and oxygen, etc.

c)       Input characteristics

Humans have been provided with a sensory system for collecting information from the world around them, enabling them to enable to respond to external events and to carry out the required task. But all senses are subject to degradation for one reason or another.

d)      Information processing

 These human capabilities have severe limitations. Poor instrument and warning system design have frequently resulted from a failure to take into account the capabilities and limitations of the human information processing system. Short- and long-term memory are involved, as well as motivation and stress. 

e)      Output characteristics

Once information is sensed and processed, messages are sent to the muscles to initiate the desired response, whether it be a physical control movement or the initiation of some form of communication. Acceptable control forces and direction of movement have to be known. 

f)        Environmental tolerances

Temperature, pressure, humidity, noise, time of day, light, and darkness can all be reflected in performance and also in well-being. Heights, enclosed spaces, and a boring or stressful working environment can also be expected to influence behavior and performance.


Adapting and matching remaining components [S, H, E, L] to [ L] – Liveware (Humans) the central component.

1-      Liveware-Hardware

2-      Liveware-Software

3-      Liveware-Environment

4-      Liveware-Liveware

I will start with

1-      Liveware-Hardware

This interface is the one most commonly considered when speaking of human-machine systems: 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, of controls with proper movement, coding, and location.

2-      Liveware-Software

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

3-      Liveware-Environment

The human-environment interface was one of the earliest recognized in flying. Initially, the measures taken all aimed at adapting the human to the environment.

L-E interface must consider perceptual errors induced by environmental conditions, for example, illusions during approach and landing phases. 

4-      Liveware-Liveware

This is the interface between people (leadership, crew co-operation, teamwork, and personality interactions).



I will assume that you read this article and understand a conceptual Model of Human Factors [The SHELL concept].

Which component of [SHELL Model] may involve these safety deficiencies:

-  Relationships with other people.

-  shortage of manpower,

-  lack of supervision,

lack of support from managers

Please write your answer in the comments section.



Human Factors is the practice of applying scientific knowledge with the intention of reducing human error. A model which is often used is the SHELL model aviation, a name derived from the initial letters of its components:

- [ S] software (e.g., work procedures, operational manuals, checklist layout, etc.)

- [ H] hardware (e.g., tools, test equipment, the physical structure of aircraft, design of flight decks, positioning and operating sense of controls and instruments, etc.)

- [ E] 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.)

- [ L] liveware - (i.e., the person or people at the center of the model, including pilots, maintenancetechnicians, supervisors, planners, managers, etc.).

Humans, the " [ L] - Liveware," can perform a wide range of activities. However, humans have limitations. Since [ L]- Liveware is at the center of the model [SHELL], all other aspects (Software, Hardware, and Environment) must be designed or adapted to assist human performance and respect human limitations.

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.


Thank you for reading.

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. We hope you find Aviation Professional website not only informative, but interesting and helpful as well.

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