Fred Lewis Posted July 5, 2012 Report Share Posted July 5, 2012 Reference is made to a typical risk matrix such as the one proposed by Transport Canada. The claim is made that this risk matrix is objective. Nothing could be further from the truth. The degrees of both probability and severity are arbitrary and therefore subjective. In order for the measure of a thing to be objective, some sort of number must be attached to it. Sometimes objectifying reality is a difficult but risk matrices are wrong and can be dangerously misleading. The discussion of risk is more thoroughly described by approaching it from the statistical perspective which includes an application of probability. The TC risk matrix attempts to classify probability into five levels. What is the difference between the probability of Level 1 Risk, described to be “Mishap almost impossible” and Level 2 Risk described to be “Postulated event. (Has been planned for, and may be possible, but not known to have occurred)”? One could argue that “almost impossible” is in fact a more frequent occurrence than “may be possible, but not known to have occurred”. The same is true of a comparison of Level 3 Risk and Levels 1 and 2. The best that can be said about these loosely defined levels is that they all describe the same level of probability. Level 4 and Level 5 Risk are likewise similar. They really describe the same level of risk. In the grand scheme of things when the nature of helicopter accidents is considered on a global scale, just about every conceivable calamity befalls someone somewhere on an annual basis, effectively eliminating Level 4 risk as a consideration. The criteria for severity of the levels of risk are similarly subjective. How could any rational person chose between courses of action which could result in “Personnel - Disability/Severe injury. “ or “Personnel - Fatal, life threatening.”? Under what conceivable circumstances could one option be acceptable and the other not in light of the fact that no objective probabilities for these events are provided with which the matrix user can work? The values of probability and the values of severity are then multiplied together to quantify risk. Are we to believe that probability 5 and severity 5 is 5 times riskier than probability 1 severity 5? What is the statistical basis for this? Why are the risk and severity multiplied together? Why are they not added? There is no reason. Objectifying risk in aviation is difficult. The probability of occurrences must be determined empirically as opposed to mathematically and can therefore be inaccurate. To illustrate, consider the tossing of a coin or the rolling of a die. In the former, the probability of a toss resulting in a head can be immediately computed to be 0.50 and the probability of not tossing a head to be 0.50. In the latter case, the probability of rolling any particular number is one in six, or 0.17 and the probability of not rolling a particular number is 0.83. Suppose one plays a game in which one wagers $1 that the result of the roll of a die can be correctly guessed. Success results in a payoff of $5. If the game is played 600 times, on average success will be achieved 100 times. The winnings will be $500 but it will have cost the player $600 in wagers for a net loss of $100. When the odds of winning are greater than the ratio of the wager to the payoff, the game is a winning proposition or Probability of success >= Wager/Payoff. In the case of the die game, the probability of success 0.17 and the Wager/Payoff is 0.20. It is not a good bet. This applies to empirically determined probabilities as well as mathematically determined ones. The Transportation Safety Board of Canada has published the number of fatal helicopter accidents (66) and the total number of hours flown (6,167,000) for the years 1998 to 2007 inclusive. From these numbers, the average fatal accident rate per hour for these years can be estimated. It is 0.0000107 and the probability of no fatal accident is 0.9999893. Another legitimate way of looking at this is that if 10,000,000 helicopters each flew for one hour, 107 of them would be involved in a fatal accident. Conversely, 9,999,893 would not. Suppose a helicopter with a hull value of $1,000,000 does a one hour job and that the operator is on the hook for it if there is total loss. The wager is $1,000,000 and the payoff is the return of the machine undamaged and any profit realized, say $500. If the probability of success is greater than the wager divided by the payoff, then in the long run, the operation is a winning proposition. In this hypothetical case, the wager divided by the payoff ($1,000,000/$1,000,500) is 0.9995. The probability of success (0.9999893) is greater than the ratio of the wager to the payoff (0.9995). It is a winning proposition. This scenario is simplified. In reality the passengers have value and so does any cargo. The range of loss can be from total to perhaps as little as a few thousand dollars but almost all the time there is a small gain. Each sort of loss will have its own probability of occurrence. However, the point that there is a real way of quantifying probability and relating it to helicopter operations is illustrated. Increasing the probability of success and the payoff, and reducing the wager will favour the operation. The wager can, for instance, be reduced by not taking more passengers than completion of the job requires. The payoff can be increased by maximizing the profit on each hour flown, which some operators do very poorly, but the most effective way of reducing the risk in to increase the probability of success, which is almost entirely in the control of the pilot. The pilot can increase the probability of success by: always loading the aircraft so that it will have sufficient reserves of thrust to maneuver nimbly out of ground effect without exceeding engine or transmission limits; considering relative humidity when computing density altitude; using the utmost discretion when flying in marginal weather; not operating in the shaded area of the height velocity diagram longer than is necessary; closely monitoring his physical and psychological health and not flying when either of those are compromised; not flying when he is fatigued; not pressing the limits of his abilities; not attempting to match the performance of pilots of superior skill; cultivating skills for firmly but politely dealing with aggressive customers; cultivating skills for firmly but politely dealing with aggressive employers. The pilot is the single most important part of a safe and successful helicopter operation. The responsibility for human lives and expensive machinery is in his hands. No known laws equip the pilot with the authority to deal with this responsibility. It is something every pilot must develop for himself. None of this is rocket science. The math has been around for hundreds of years. It is very sad when the wheel is reinvented disguised, in this case as a risk matrix, as some new and innovative thing. Risk matrices are wrong. They will never give the right answer. The common sense approach to safety will produce better results. For another perspective on risk matrices have a look at What’s Wrong with Risk Matrices? 4 Quote Link to comment Share on other sites More sharing options...
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