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We will look at the recent tragedy of Boeing 737 Max 8 set in risk context. Media have produced numerous fascinating articles on the tragedy, its possible causes, and risk perception. One in particular made our eyebrows raise as it seems to come to astonishing conclusions based on ill-conditioned thinking.
We reproduce verbatim part of that article:
To appreciate the psychology associated with these events, consider that in total, 346 people died on these two flights. That figure corresponds to the average number of automobile fatalities that occur in the U.S. in slightly more than three days. The average daily fatality rate on U.S. roads is about 110. Given the time between the two 737 Max 8 air crashes, the daily fatality rate from the two crashes is 2.6 per day. Moreover, in the 17 months or so since the 737 Max came into general service, there have been tens of thousands of flights of these planes, almost all without a major incident.
The author of the article seems to forget that not all risks are “born equal.”
People act differently based on the hazard scenario generating the risks (of passenger death in the case considered here). The atavistic fear of falling from the sky is perceived differently from a fatal accident on the road even thus the risk may be the same or lower.
Before coming up with “twisting statistics” it may be useful to look at the Boeing 737 Max 8 accident profile in comparison with the rest of the passenger airliners. The accident profile is a graph showing the cumulated casualties generated by the accidents of a certain plane over time. We can see from the graph below that other passenger airliners had a much gentler accident profile. (source https://aviation-safety.net/database/)
Unfortunately, we cannot look at casualties per km traveled or per trip as data is not available. That graph may yet deliver a different image. Indeed, the following risks:
would all be interesting to have a more meaningful comparison.
In our risk assessment daily practice, we look at this type of statistics, when they are available. Unfortunately, however, statistics are more often than not grossly incomplete and/or need to be used with caution. Statistical databases created for a specific purpose can be misleading for other uses.
During a recent risk assessment, we had to evaluate the probability of a failure of a pump motor in a complex process.
The pump motor was going to operate in an out-of-specs environment.
Thus, we searched the technical literature and found the following statistics in a database (COMPONENT RELIABILITY DATA FOR USE IN PROBABILISTIC SAFETY ASSESSMENT link).
We felt it necessary to check further, to gain confirmation of the values in that first database. Diving a bit further in the literature we found the above-mentioned pump motor driven failure rate probability, this time if operated in a heavy chemical environment. The rate of failure in such an environment is 2-3 orders of magnitude greater than in the previous referenced database, see below.
No data should be taken at face value as conditions and peculiarities significantly alter rates of failure. 2-3 orders of magnitude are a big gap for the same pump, in different environments.
As complete statistics are not available on many compounds/ operations/ elements (including organizational and human factor), probabilities can be estimated. That requires using analogies and experience, and “incomplete statistics” from similar subject matters. As new information becomes available probabilities updating is possible.
Indeed, risk assessment should be cyclical. They should also incorporate new information seamlessly as the risk profile of a company/ operation is certainly dynamic!
We show this ORE risk estimates flow-chart to our client and during our courses to help them understand the context of the risk framework.
“Don’t be ruled by your past. Don’t be crippled by the past. Let the past be the past and focus on what is ahead. Remember however that if you neglect the most essential lessons of the past, you shall walk into the future with one leg.”
― Ernest Agyemang Yeboah
Indeed environments change, event occurs. The tragedy of the Boeing 737 Max 8 set in risk context shows the need to update probabilities and other “statistics.” That is necessary especially in a rational risk acceptance framework That’s the only way to adopt prudent, but reasonable, stances.