PJ2, defending against an identified failure path is good custom, but a slice of cheese is not necessarily a line of defence.
“Put another slice of Cheese in” is a common view of the Swiss Cheese model, however IMHO Prof Reason portrays a different aspect.
The added slice is not a steel barrier, it is just cheese, and like all of the other slices it could have holes in it. From a numerical view the more slices the better, but if there is a common origin of the holes this could apply to all slices simultaneously including the new one.
Some holes may be permanent resulting from latent factors, or they may open and close as opportunity for error or ‘the unsafe act’ arises (context / situation dependent).
The latter are predominantly the holes due human aspects (error, unsafe acts) and are always present even if not in the same place. Defences for these may involve complex solutions – its like trying to hit a moving target; however, this is still a necessary task as HF training can reduce error, e.g. situations might be better recognised and behaviours shaped to avoid the unsafe acts - an appropriate course of action is chosen - improved decision making.
Prof Reason argues that a more effective defence is to rectify the organisational / management aspects (latent issues – static holes) as these may apply to many or all of the slices. Also, because these issues might be identified more easily. Some may involve higher organisational aspects such as regulation or the wider operational system (industry norms, culture).
The solutions for these differ from the operational HF aspects which tend to focus on ‘the last slice’ and the situation in which an accident can occur (the sharp end / outside of the cheese); the management aspects require a rigorous process equivalent to financial management, and the full participation of management to enact the changes (HF - high level management behaviour, decision making, safety culture).
Overall the Swiss Cheese model shows that we need to reduce the number of permanent holes and reduce the frequency / opportunity of the variable ones. It is not so much the number of defences that are provided, but the quality of the defence that is important. ‘Depth’ can be used to describe the quality of an item such as wine or …. cheese.
Connecting SMS with this discussion; SMS is a major ‘top down’ safety tool, often associated with Reason’s organisational issues.
It is not the presence of a SMS that provides safety; it is the quality of the SMS that matters - the way in which it is implemented – practical operational use.
A major problem for the working industry is that ICAO and Regulators often promote high level initiatives – the ‘overarching Philosophy’ or ‘Policy’, without fully considering the implementation and practical use (how to …); these are the required Procedures and Practice to complete the ‘PPPP’s of SMS. Every new initiative is a problem for the industry; if you give someone a problem then you should provide guidance or an example solution.
Many of the new safety initiatives aimed at maintaining / reducing the low accident rate are depicted as Management Systems involving processes or models that loop back on themselves e.g. TEM and SMS. The important aspects in these are the points that enable a breakout from the loops; what do management do when a hazard is identified, what are the new defences, where are they and when; how do crews identify threats, errors, and what practical activities are required to defend against them – is the knowledge and thinking skills training being provided.
The Swiss Cheese model is simpler than these modern processes – its a straight line path. Originally it may have aided accident investigation and thus was a tool of hindsight. However, the way in which we learn and gain experience is by reflecting on the components of our hindsight, components that may be identified as the holes or potential holes in the cheese.
If thin cheese slices are heated they tend to stretch and leave more holes, but thicker cheese slices (or higher quality cheese) spreads out covering everything with a ‘defensive’ blanket.
The industry is under pressure to maintain its good safety record, the heat is being turned up, thus we need thicker, more resilient cheese slices, or better quality cheese in our defences.
Revisiting the Swiss Cheese model of accidents.
What do you think Grommet?