In my last post, I commenced a whinge about the PIG or as it is more commonly known, the likelihood-consequence matrix. I signed off that post with a promise to further the discussion on the risk matrix within an aviation safety context. Here goes...
Consequence is an inappropriate dimension to consider in aviation safety. For two reasons which I call vulnerability and proximity. Let's take them in turn.
Aviation is a perilous undertaking. Every time you take to the sky you tend to inject enough potential energy into the situation that no amount risk mitigation can stand between you and catastrophe1.
In other fields, a graduated scale of consequence may be appropriate. Finance, for example, can easily delineate between monetary outcomes when limits can be set by how much you put into an uncertain situation. In aviation, you are all in.
Okay, there may be a few readers wishing to interject at this stage. I'm going to take a guess at two counter-arguments to the above position2. The first being that aircraft sizes/occupancies vary. The second is that many, many, many occurrences do not result in total annihilation of hundreds of passengers.
Let's take the second one first. The "but" that I would like to throw in after that sentence is that in everyone one of those near-misses, minor incidents, major incidents or even non-fatal accidents, catastrophe could have occurred. There was no inherent limit within the situation that meant complete loss of life was not a possibility.
Back to the first point now. Yes, you could limit the amount of life lost by limiting the number of passengers. This method of segregating risk decisions appears throughout the aviation safety sphere - the certification of aerodromes is a good example, the requirements kicks in at aircraft with more than thirty (30) seats. If you were to insert this into a PIG with "death of x number of people" along the consequence dimension, all you would end up with is a 2-D matrix of accident frequency acceptability/unacceptability.
And this leads into proximity...
The "risks" we tend to look at within the aviation safety realm are quite varied. One second we might be considering the chance of an engine failure and its impact during Extended Diversion Time Operations, then we'll be looking at the impact of a poorly maintained operations manual and following that up with an assessment of an ineffective hazard reporting system. Each of these conditions falls in a completely different area of the accident causation chain.
I've started to think about this problem as proximity. How close is this condition to the ultimate outcome? Obviously, conditions closer to the end result are more important and things further upstream are less so, right? I think we start to hit another issue here and its one I'm working through at the moment and hope to write about next week.
But before I go, I do want to sum up the above rant.
I believe that the traditional likelihood-consequence matrix is not suited to risk management (assessment/evaluation) within the aviation safety realm. A graduated consequence scale with anything less that complete loss of life fails to recognise the persistent potential for catastrophe and a graduated scale based loss of life limited by aircraft size cannot be applied to conditions ("risks") which exist upstream of the final stop of the accident causation chain.
I think there is an answer to these problems. In fact, I think there are a couple. Stay tuned.
1. Until that is, Q unveils his inflatable aircraft cocoon - something like this.
2. If you have any more please feel free to comment.