A Comparison of the Role of Degraded Modes of Operation in the Causes of Accidents in Rail and Air Traffic Management

Degraded modes of operation occur when technological systems fail to meet the levels of service that are expected by staff and managers. Over time, operators develop 'work arounds' that help them to cope with these degraded modes. This has led to a culture of 'making do' where co-workers try their best to maintain service provision in spite of system failures. The extent to which operators will adapt to degraded modes illustrates the flexibility and resilience of socio-technical systems. However, these adaptations and 'work arounds' undermine safety. A central aim of this paper is to begin to identify why teams of co-workers continue to operate safety critical systems when key elements of their infrastructure have been compromised, for example during routine maintenance. The following pages build on four case study accidents from the rail and air traffic management domains.