(a) As discussed in WEEK 1, human (Liveware) is at the centre of the model. Human is generally considered the most critical as well as the most flexible component in the system. Yet people are subject to considerable variations in performance and suffer many limitations, most of which are now predictable in general terms. The other components of the system (indicated in the SHELL model) must be carefully matched with them if stress in the system and eventual breakdown is to be avoided. In order to achieve this matching, an understanding of the characteristics of this central component is essential.

(b) Important characteristics of the Liveware are as follows:
(i) Physical size and shape: In the design of workplace and equipment, a vital consideration involves body measurements and movements, which may vary according to factors such as age, ethnicity and gender. Human Factors inputs must be provided at an early stage in the design process, and data for these inputs are available from anthropometry, biomechanics etc.

(ii) Physical needs: These are people’s physical requirements such as food, water and oxygen which are indicated in human physiology and biology.
(iii) Input characteristics: Humans possess various sensory systems for collecting information from the world external, enabling them to respond to events and carry out the required task. But all senses may be subjected to degradation for one reason or another. Sources of knowledge on these characteristics are Physiology and Psychology.
(iv) Information processing: Sensory input data are processed in the human brain. In this processing function, human memory processors are involved. In this functions, there are various limitations. Factors such as stress, motivation and short- and long-term memory are involved. Instrument and alerting system design must take into account the capabilities and limitations of human information processing. Psychology and cognitive sciences are the sources of background knowledge here.
(v) Output characteristics: Once information is sensed and processed, decisions are made and messages are sent to muscles to initiate the desired response. Responses may involve a physical control movement or the initiation of some form of communication. Acceptable control forces and direction of movement have to be known, and biomechanics, physiology and psychology provide the background knowledge. (vi) Environmental tolerances Environmental factors such as temperature, vibration, pressure, humidity, noise, time of day, amount of light and G-forces can affect human performance and well-being. Heights, enclosed spaces and a boring or stressful work environment can influence human behaviour and performance. Background information is available from medicine, psychology, physiology and biology.

(c) As the aircraft engineer is the central part of the aircraft maintenance system, it is therefore, very useful to have an understanding of how various parts of his body and mental processes function and how performance limitations can influence his effectiveness at work.