to changes in air transport demand and capacity during times of economic boom and economic recession. Biased crew decision making and operator short-cuts as a consequence of economic pressure brought on by airline competition and cost-cutting measures linked with deregulation. Inadequate or unhealthy organisational environment reflecting a flawed operating philosophy, poor employee morale or negative organisational culture. Liveware-Liveware (L-L) Interaction between central human operator and any other person in the aviation system during performance of tasks. Involves interrelationships among individuals within and between groups including maintenance personnel, engineers, designers, ground crew, flight crew, cabin crew, operations personnel, air traffic controllers, passengers, instructors, students, managers and supervisors. Human-human/group interactions can positively or negatively influence behaviour and performance including the development and implementation of behavioural norms. Therefore, the L-L interface is largely concerned with: interpersonal relations leadership crew cooperation, coordination and communication dynamics of social interactions teamwork cultural interactions personality and attitude interactions. The importance of the L-L interface and the issues involved have contributed to the development of cockpit/crew resource management (CRM) programmes in an attempt to reduce error at the interface between aviation professionals Examples of mismatches at the L-L interface include: Communication errors due to misleading, ambiguous, inappropriate or poorly constructed communication between individuals. Communication errors have resulted in aviation accidents such as the double Boeing 747 disaster at Tenerife Airport in 1977. Reduced performance and error from an imbalanced authority relationship between aircraft captain and first officer. For instance, an autocratic captain and an overly submissive first officer may cause the first officer to fail to speak up when something is wrong, or alternatively the captain may fail to listen. The SHELL Model does not consider interfaces that are outside the scope of human factors. For instance, the hardware-hardware, hardware-environment and hardware-software interfaces are not considered as these interfaces do not involve the liveware component. Aviation System Stability Any change within the aviation SHELL system can have far-reaching repercussions. For example, a minor equipment change (hardware) requires an assessment of the impact of the change on operations and maintenance personnel (Liveware-Hardware) and the possibility of the need for alterations to procedures/training programmes (to optimise Liveware-Software interactions). Unless all potential effects of a change in the aviation system are properly addressed, it is possible that even a small system modification may produce undesirable consequences. Similarly, the aviation system must be continually reviewed to adjust for changes at the Liveware-Environment