### Archives For QF 72

When good voting algorithms go bad

Thinking about the QF72 incident, it struck me that average value based voting methods are based on the calculation of a population statistic. Now population statistics work well when the population is normally distributed, or otherwise clustered around some value. But if the distribution has heavy tails, we can expect that extreme values will occur fairly regularly and therefor the ‘average’ value means much less. In fact for some distributions we may not be able to put a cap on the upper value that an ‘average’ could be, e.g. it could have an infinite value and the idea of an average is therefore meaningless.

In an earlier post I commented that in the QF72 incident the use of a geometric mean (1) instead of the arithmetic mean when calculating the aircrafts angle of attack would have reduced the severity of the subsequent pitch over. Which leads into the more general subject of what to do when the real world departs from our assumption about the statistical ‘well formededness’ of data. The problem, in the case of measuring angle of attack on commercial aircraft, is that the left and right alpha sensors are not truly independent measures of the same parameter (2). With sideslip we cannot directly obtain a true angle of attack (AoA) from any single sensor (3) so need to take the average (mean) of the measured AoA on either side of the fuselage (Gracey 1958) to determine the true AoA. Because of this variance between left and right we cannot use a median voting approach, as we can expect the two sensors the right side to differ from the one sensor on the left. As a result we end up having to use the mean of two sensor values (one from each side) as an estimate of the resultant central tendency.

The QF 72 accident illustrates the significant effects that ‘small field’ decisions can have on overall system safety Continue Reading…

Over the last couple of months I’ve posted on various incidents involving the Airbus A330 aircraft from the perspective of system safety. As these posts are scattered through my blog I thought I’d pull them together, the earliest post is at the bottom.