What Do The Statistics Say, If Anything?

Statistics tell of the frequency of problems that have arisen with aircraft on which some systems are computer-controlled. These are referred to as `New-Technology Aircraft' by the Boeing Company's Annual summary of statistics. Boeing publishes the Statistical Summary of Commencial Jet Aviation Accidents each year, and displays data back to 1959 to indicate trends. It's available from Boeing Commercial Airplane Group, P.O. box 3707, Seattle, WA 98124-2207, USA. Excerpts appear below. In response to an article by Mitch Kabay in RISKS on the complexity of the modern pilot's cockpit interface, a report of a lecture to the Royal Society by Michael Bagshaw, I wrote a short commentary for RISKS-18.66 on what those statistics might show for new-technology aircraft. Mark Stalzer contributed a short reply pointing out what those statistics might not show.

Frequency or rate of accidents can tell us what the likelihood of an accident may be, if all contributing factors remain the same. Likelihood of an accident should not be confused with risk. Risk is an engineering term which attempts to combine the likelihood of an accident with the severity of the consequences. For example, you could stub your toe while entering the aircraft from the airway. The likelihood of this could be much greater than the likelihood of you sustaining severe injury on board, but the risk might be much lower, because the consequences (a sore toe) are not severe. Risk is explained by Leveson (Safeware, Addison-Wesley 1995, p179) as the hazard level combined with (1) the likelihood of the hazard leading to an accident [..] and (2) hazard exposure or duration [..]. A hazard is itself explained as a state or set of conditions of a system that, together with other conditions in the environment of the system (or object), will lead inevitably to an accident (op. cit., p177). Hazard severity is measured by assessing the severity of the worst possible accident that could result from the hazard, given the environment in its most unfavorable state (op. cit., p178, modified). Hazard level is a combination of severity with likelihood of occurrence (op. cit., p179). Everything clear?

Consider you are the punk in the Dirty Harry movie at the end of Mr Eastwood's pistol. `Do you feel lucky, today, punk? Well, do yah?'. Mr. Eastwood thus makes it clear he constitutes a hazard. If you feel lucky (the `other conditions in the environment' are that you do, and you will go for it), this will lead inevitably to an accident (for you, not for Mr. Eastwood). Furthermore, the hazard severity is high (being shot is quite severe), and the likelihood? Very close to that of how lucky you feel (Mr. E could in real life suffer a stroke at the very moment you move, but this is the movies so the likelihood is zero).

See the section The Measurement of Risk, below, for some more comments on risk and perception of risk (which are known to differ, according to research by social psychologists).

There follows a short synopsis of 1996 accident statistics, with a list of significant fatal airline accidents. Boeing has for many years produced an annual statistical summary of aircraft accidents. Some excerpts from the 1959-1995 summaries show:

The complete report, Statistical Summary of Commercial Jet Aircraft Accidents, is a standard work in aircraft safety, published every year. It may be obtained from Boeing Commercial Airplane Group, P.O.Box 3707, Seattle, WA 98124-2207, USA.

People seem to like to make comparisons between the risk to life of driving a car and the risk to life of flying on commercial carriers. But exactly what are these figures, where did they come from, and who did the comparisons? There were some articles in the journal Risk Analysis from 1989-91, using figures from the late 70's to late 80's, which I summarise in the essay To Drive or To Fly - Is That Really The Question?. The answer is that it depends on who you are. But one thing is pretty certain. If you're a drunken teenager, better take the bus.

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