The SHELL model is a conceptual model of human factors that clarifies the scope of aviation human factors and assists in understanding the human factor relationships between aviation system resources/environment (the flying subsystem) and the human component in the aviation system (the human subsystem) (Hawkins & Orlady, 1993 4; Keightley, 2004 7).
The SHELL model was first developed by Edwards (1972) and later modified into a 'building block' structure by Hawkins (1984) (Hawkins & Orlady, 1993 4). The model is named after the initial letters of its components (software, hardware, environment, liveware) and places emphasis on the human being and human interfaces with other components of the aviation system (Johnston, McDonald & Fuller, 2001 6).
The SHELL model adopts a systems perspective that suggests the human is rarely, if ever, the sole cause of an accident (Wiegmann & Shappell, 2003 9). The systems perspective considers a variety of contextual and task-related factors that interact with the human operator within the aviation system to affect operator performance (Wiegmann & Shappell, 2003 9). As a result, the SHELL model considers both active and latent failures in the aviation system.
The SHELL Model
Fundam10.gif
As modified by Hawkins (image embedded from Atlas Aviation on 24 Aug 2009)
Each component of the SHELL model (software, hardware, environment, liveware) represents a building block of human factors studies within aviation (International Civil Aviation Organisation, 1993 5).
The human element or worker of interest is at the centre or hub of the SHELL model that represents the modern air transportation system. The human element is the most critical and flexible component in the system, interacting directly with other system components, namely software, hardware, environment and liveware (Hawkins & Orlady, 1993 4).
However, the edges of the central human component block are varied, to represent human limitations and variations in performance. Therefore, the other system component blocks must be carefully adapted and matched to this central component to accommodate human limitations and avoid stress and breakdowns (incidents/accidents) in the aviation system (Hawkins & Orlady, 1993 4). To accomplish this matching, the characteristics or general capabilities and limitations of this central human component must be understood.
Human Characteristics
Physical Size and Shape
In the design of aviation workplaces and equipment, body measurements and movement are a vital factor (Hawkins & Orlady, 1993 4). Differences occur according to ethnicity, age and gender for example. Design decisions must take into account the human dimensions and population percentage that the design is intended to satisfy (Hawkins & Orlady, 1993 4).
Human size and shape are relevant in the design and location of aircraft cabin equipment, emergency equipment, seats and furnishings as well as access and space requirements for cargo compartments.
Fuel Requirements
Humans require food, water and oxygen to function effectively and deficiencies can affect performance and well-being (Hawkins & Orlady, 1993 4)
Input Characteristics
The human senses for collecting vital task and environment-related information are subject to limitations and degradation. Human senses cannot detect the whole range of sensory information available (Keightley, 2004 7). For example, the human eye cannot see an object at night due to low light levels. This produces implications for pilot performance during night flying. In addition to sight, other senses include sound, smell, taste and touch (movement and temperature).
Information Processing
Humans have limitations in information processing capabilities (such as working memory capacity, time and retrieval considerations) that can also be influenced by other factors such as motivation and stress or high workload (Hawkins & Orlady, 1993 4). Aircraft display, instrument and alerting/warning system design needs to take into account the capabilities and limitations of human information processing to prevent human error.
Output Characteristics
After sensing and processing information, the output involves decisions, muscular action and communication. Design considerations include aircraft control-display movement relationship, acceptable direction of movement of controls, control resistance and coding, acceptable human forces required to operate aircraft doors, hatches and cargo equipment and speech characteristics in the design of voice communication procedures (Hawkins & Orlady, 1993 4).
Environmental Tolerances
People function effectively only within a narrow range of environmental conditions (tolerable for optimum human performance) and therefore their performance and well-being is affected by physical environmental factors such as temperature, vibration, noise, g-forces and time of day as well as time zone transitions, boring/stressful working environments, heights and enclosed spaces (Hawkins & Orlady, 1993 4).
Components of the SHELL Model
Software
Non-physical, intangible aspects of the aviation system that govern how the aviation system operates and how information within the system is organised (Hawkins & Orlady, 1993 4).
Software may be likened to the software that controls the operations of computer hardware (Johnston, McDonald & Fuller, 2001 6).
Software includes rules, instructions, regulations, policies, norms, laws, orders, safety procedures, standard operating procedures, customs, practices, conventions, habits, symbology, supervisor commands and computer programmes.
Software can be included in a collection of documents such as the contents of charts, maps, publications, emergency operating manuals and procedural checklists (Wiener & Nagel, 1988 10).
Hardware
Physical elements of the aviation system such as aircraft (including controls, surfaces, displays, functional systems and seating), operator equipment, tools, materials, buildings, vehicles, computers, conveyor belts etc (Johnston et al, 2001 6; Wiener & Nagel, 1988 10; Campbell & Bagshaw, 2002 2).
Environment
The context in which aircraft and aviation system resources (software, hardware, liveware) operate, made up of physical, organisational, economic, regulatory, political and social variables that may impact on the worker/operator (Wiener & Nagel, 1988 10; Johnston et al, 2001 6).
Internal air transport environment relates to immediate work area and includes physical factors such as cabin/cockpit temperature, air pressure, humidity, noise, vibration and ambient light levels.
External air transport environment includes the physical environment outside the immediate work area such as weather (visibility/turbulence), terrain, congested airspace and physical facilities and infrastructure including airports as well as broad organisational, economic, regulatory, political and social factors (International Civil Aviation Organisation, 1993 5).
Liveware
Human element or people in the aviation system. For example, flight crew personnel who operate aircraft, cabin crew, ground crew, management and administration personnel.
The liveware component considers human performance, capabilities and limitations (International Civil Aviation Organisation, 1993 5).
The four components of the SHELL model or aviation system do not act in isolation but instead interact with the central human component to provide areas for human factors analysis and consideration (Wiegmann & Shappell, 2003 9). The SHELL model indicates relationships between people and other system components and therefore provides a framework for optimising the relationship between people and their activities within the aviation system that is of primary concern to human factors. In fact, the International Civil Aviation Organisation has described human factors as a concept of people in their living and working situations; their interactions with machines (hardware), procedures (software) and the environment about them; and also their relationships with other people (Keightley, 2004 7).
According to the SHELL model, a mismatch at the interface of the blocks/components where energy and information is interchanged can be a source of human error or system vulnerability that can lead to system failure in the form of an incident/accident (Johnston et al, 2001 6). Aviation disasters tend to be characterised by mismatches at interfaces between system components, rather than catastrophic failures of individual components (Wiener & Nagel, 1988 10).
SHELL Model Interfaces
Liveware-Software (L-S)
Interaction between human operator and non-physical supporting systems in the workplace (Johnston, McDonald & Fuller, 2001 6).
Involves designing software to match the general characteristics of human users and ensuring that the software (e.g. rules/procedures) is capable of being implemented with ease (Hawkins & Orlady, 1993 4)
During training, flight crew members incorporate much of the software (e.g. procedural information) associated with flying and emergency situations into their memory in the form of knowledge and skills. However, more information is obtained by referring to manuals, checklists, maps and charts. In a physical sense these documents are regarded as hardware however in the information design of these documents adequate attention has to be paid to numerous aspects of the L-S interface (Wiener & Nagel, 1988 10).
For instance, by referring to cognitive ergonomics principles, the designer must consider currency and accuracy of information; user-friendliness of format and vocabulary; clarity of information; subdivision and indexing to facilitate user retrieval of information; presentation of numerical data; use of abbreviations, symbolic codes and other language devices; presentation of instructions using diagrams and/or sentences etc. The solutions adopted after consideration of these informational design factors play a crucial role in effective human performance at the L-S
The SHELL model is a conceptual model of human factors that clarifies the scope of aviation human factors and assists in understanding the human factor relationships between aviation system resources/environment (the flying subsystem) and the human component in the aviation system (the human subsystem) (Hawkins & Orlady, 1993 4; Keightley, 2004 7).The SHELL model was first developed by Edwards (1972) and later modified into a 'building block' structure by Hawkins (1984) (Hawkins & Orlady, 1993 4). The model is named after the initial letters of its components (software, hardware, environment, liveware) and places emphasis on the human being and human interfaces with other components of the aviation system (Johnston, McDonald & Fuller, 2001 6).The SHELL model adopts a systems perspective that suggests the human is rarely, if ever, the sole cause of an accident (Wiegmann & Shappell, 2003 9). The systems perspective considers a variety of contextual and task-related factors that interact with the human operator within the aviation system to affect operator performance (Wiegmann & Shappell, 2003 9). As a result, the SHELL model considers both active and latent failures in the aviation system.The SHELL ModelFundam10.gifAs modified by Hawkins (image embedded from Atlas Aviation on 24 Aug 2009)Each component of the SHELL model (software, hardware, environment, liveware) represents a building block of human factors studies within aviation (International Civil Aviation Organisation, 1993 5).The human element or worker of interest is at the centre or hub of the SHELL model that represents the modern air transportation system. The human element is the most critical and flexible component in the system, interacting directly with other system components, namely software, hardware, environment and liveware (Hawkins & Orlady, 1993 4).However, the edges of the central human component block are varied, to represent human limitations and variations in performance. Therefore, the other system component blocks must be carefully adapted and matched to this central component to accommodate human limitations and avoid stress and breakdowns (incidents/accidents) in the aviation system (Hawkins & Orlady, 1993 4). To accomplish this matching, the characteristics or general capabilities and limitations of this central human component must be understood.Human CharacteristicsPhysical Size and ShapeIn the design of aviation workplaces and equipment, body measurements and movement are a vital factor (Hawkins & Orlady, 1993 4). Differences occur according to ethnicity, age and gender for example. Design decisions must take into account the human dimensions and population percentage that the design is intended to satisfy (Hawkins & Orlady, 1993 4).Human size and shape are relevant in the design and location of aircraft cabin equipment, emergency equipment, seats and furnishings as well as access and space requirements for cargo compartments.Fuel RequirementsHumans require food, water and oxygen to function effectively and deficiencies can affect performance and well-being (Hawkins & Orlady, 1993 4)Input CharacteristicsThe human senses for collecting vital task and environment-related information are subject to limitations and degradation. Human senses cannot detect the whole range of sensory information available (Keightley, 2004 7). For example, the human eye cannot see an object at night due to low light levels. This produces implications for pilot performance during night flying. In addition to sight, other senses include sound, smell, taste and touch (movement and temperature).Information ProcessingHumans have limitations in information processing capabilities (such as working memory capacity, time and retrieval considerations) that can also be influenced by other factors such as motivation and stress or high workload (Hawkins & Orlady, 1993 4). Aircraft display, instrument and alerting/warning system design needs to take into account the capabilities and limitations of human information processing to prevent human error.Output CharacteristicsAfter sensing and processing information, the output involves decisions, muscular action and communication. Design considerations include aircraft control-display movement relationship, acceptable direction of movement of controls, control resistance and coding, acceptable human forces required to operate aircraft doors, hatches and cargo equipment and speech characteristics in the design of voice communication procedures (Hawkins & Orlady, 1993 4).Environmental TolerancesPeople function effectively only within a narrow range of environmental conditions (tolerable for optimum human performance) and therefore their performance and well-being is affected by physical environmental factors such as temperature, vibration, noise, g-forces and time of day as well as time zone transitions, boring/stressful working environments, heights and enclosed spaces (Hawkins & Orlady, 1993 4).Components of the SHELL ModelSoftwareNon-physical, intangible aspects of the aviation system that govern how the aviation system operates and how information within the system is organised (Hawkins & Orlady, 1993 4).Software may be likened to the software that controls the operations of computer hardware (Johnston, McDonald & Fuller, 2001 6).Software includes rules, instructions, regulations, policies, norms, laws, orders, safety procedures, standard operating procedures, customs, practices, conventions, habits, symbology, supervisor commands and computer programmes.Software can be included in a collection of documents such as the contents of charts, maps, publications, emergency operating manuals and procedural checklists (Wiener & Nagel, 1988 10).HardwarePhysical elements of the aviation system such as aircraft (including controls, surfaces, displays, functional systems and seating), operator equipment, tools, materials, buildings, vehicles, computers, conveyor belts etc (Johnston et al, 2001 6; Wiener & Nagel, 1988 10; Campbell & Bagshaw, 2002 2).EnvironmentThe context in which aircraft and aviation system resources (software, hardware, liveware) operate, made up of physical, organisational, economic, regulatory, political and social variables that may impact on the worker/operator (Wiener & Nagel, 1988 10; Johnston et al, 2001 6).Internal air transport environment relates to immediate work area and includes physical factors such as cabin/cockpit temperature, air pressure, humidity, noise, vibration and ambient light levels.External air transport environment includes the physical environment outside the immediate work area such as weather (visibility/turbulence), terrain, congested airspace and physical facilities and infrastructure including airports as well as broad organisational, economic, regulatory, political and social factors (International Civil Aviation Organisation, 1993 5).LivewareHuman element or people in the aviation system. For example, flight crew personnel who operate aircraft, cabin crew, ground crew, management and administration personnel.The liveware component considers human performance, capabilities and limitations (International Civil Aviation Organisation, 1993 5).
The four components of the SHELL model or aviation system do not act in isolation but instead interact with the central human component to provide areas for human factors analysis and consideration (Wiegmann & Shappell, 2003 9). The SHELL model indicates relationships between people and other system components and therefore provides a framework for optimising the relationship between people and their activities within the aviation system that is of primary concern to human factors. In fact, the International Civil Aviation Organisation has described human factors as a concept of people in their living and working situations; their interactions with machines (hardware), procedures (software) and the environment about them; and also their relationships with other people (Keightley, 2004 7).
According to the SHELL model, a mismatch at the interface of the blocks/components where energy and information is interchanged can be a source of human error or system vulnerability that can lead to system failure in the form of an incident/accident (Johnston et al, 2001 6). Aviation disasters tend to be characterised by mismatches at interfaces between system components, rather than catastrophic failures of individual components (Wiener & Nagel, 1988 10).
SHELL Model Interfaces
Liveware-Software (L-S)
Interaction between human operator and non-physical supporting systems in the workplace (Johnston, McDonald & Fuller, 2001 6).
Involves designing software to match the general characteristics of human users and ensuring that the software (e.g. rules/procedures) is capable of being implemented with ease (Hawkins & Orlady, 1993 4)
During training, flight crew members incorporate much of the software (e.g. procedural information) associated with flying and emergency situations into their memory in the form of knowledge and skills. However, more information is obtained by referring to manuals, checklists, maps and charts. In a physical sense these documents are regarded as hardware however in the information design of these documents adequate attention has to be paid to numerous aspects of the L-S interface (Wiener & Nagel, 1988 10).
For instance, by referring to cognitive ergonomics principles, the designer must consider currency and accuracy of information; user-friendliness of format and vocabulary; clarity of information; subdivision and indexing to facilitate user retrieval of information; presentation of numerical data; use of abbreviations, symbolic codes and other language devices; presentation of instructions using diagrams and/or sentences etc. The solutions adopted after consideration of these informational design factors play a crucial role in effective human performance at the L-S
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