Archives For Safety

One of the perennial problems we face in a system safety program is how to come up with a convincing proof for the proposition that a system is safe. Because it’s hard to prove a negative (in this case the absence of future accidents) the usual approach is to pursue a proof by contradiction, that is develop the negative proposition that the system is unsafe, then prove that this is not true, normally by showing that the set of identified specific propositions of `un-safety’ have been eliminated or controlled to an acceptable level.  Enter the term `hazard’, which in this context is simply shorthand for  a specific proposition about the unsafeness of a system. Now interestingly when we parse the set of definitions of hazard we find the recurring use of terms like, ‘condition’, ‘state’, ‘situation’ and ‘events’ that should they occur will inevitably lead to an ‘accident’ or ‘mishap’. So broadly speaking a hazard is a causal explanation, based on a defined set of phenomena, that argues that if they are present and given a relevant domain `law’ there will be an accident. All of which seems to indicate that hazards belong to a class of explanatory models called covering laws. As an explanatory class Covering laws models were developed by the logical positivist philosophers Hempel and Popper because of what they saw as problems with an over reliance on inductive arguments as to causality.

As a covering law explanation of unsafeness a hazard posits phenomenological facts (system states, human errors, hardware/software failures and so on) that confer what’s called nomic expectability on the accident (the thing being explained). That is, the phenomenological facts combined with some covering law (natural and logical), require the accident to happen, and this is what we call a hazard. We can see an archetypal example in the Source-Mechanism-Outcome model of Swallom , i.e. if we have all three elements in that model then we may expect an accident (Ericson 2005). While logical positivism had the last nails driven into it’s coffin by Kuhn and others in the 1960s, and while it’s true, as Kuhn and others pointed out, that covering model explanations have their fair share of problems so to do other methods (1). The one advantage that covering models do possess over other explanatory models is that they largely avoid the problems of causal arguments, as their makers intended. Which may well be why they persist in engineering arguments about safety.

Notes

1. Such as counterfactual, statistical relevance and causal explanations.

References

Ericson, C.A. Hazard Analysis Techniques for System Safety, page 93, John Wiley and Sons, Hoboken, New Jersey, 2005.

And not quite as simple as you think…

The testimony of Michael Barr, in the recent Oklahoma Toyota court case highlighted problems with the design of Toyota’s watchdog timer for their Camry ETCS-i  throttle control system, amongst other things, which got me thinking about the pervasive role that watchdogs play in safety critical systems. The great strength of watchdogs is of course that they provide a safety mechanism which resides outside the state machine, which gives them fundamental design independence from what’s going on inside. By their nature they’re also simple and small scale beasts, thereby satisfying the economy of mechanism principle.

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Taboo transactions and the safety dilemma Again my thanks goes to Ross Anderson over on the Light Blue Touchpaper blog for the reference, this time to a paper by Alan Fiske  an anthropologist and Philip Tetlock a social psychologist, on what they terms taboo transactions. What they point out is that there are domains of sharing in society which each work on different rules; communal, versus reciprocal obligations for example, or authority versus market. And within each domain we socially ‘transact’ trade-offs between equivalent social goods.

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So what do gambling, thermodynamics and risk all have in common?

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In June of 2011 the Australian Safety Critical Systems Association (ASCSA) published a short discussion paper on what they believed to be the philosophical principles necessary to successfully guide the development of a safety critical system. The paper identified eight management and eight technical principles, but do these principles do justice to the purported purpose of the paper?

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Why something as simple as control stick design can break an aircrew’s situational awareness

One of the less often considered aspects of situational awareness in the cockpit is the element of knowing what the ‘guy in the other seat is doing’. This is a particularly important part of cockpit error management because without a shared understanding of what someone is doing it’s kind of difficult to detect errors.

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Fighter Cockpit Rear View Mirror

What the economic theory of sunk costs tells us about plan continuation bias

Plan continuation bias is a recognised and subtle cognitive bias that tends to force the continuation of an existing plan or course of action even in the face of changing conditions. In the field of aerospace it has been recognised as a significant causal factor in accidents, with a 2004 NASA study finding that in 9 out of the 19 accidents studied aircrew exhibited this behavioural bias. One explanation of this behaviour may be a version of the well known ‘sunk cost‘ economic heuristic.

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