Archives For Dissimilar Redundancy

Unreliable airspeed events pose a significant challenge (and safety risk) because such situations throw onto aircrew the most difficult (and error prone) of human cognitive tasks, that of ‘understanding’ a novel situation. This results in a double whammy for unreliable airspeed incidents. That is the likelihood of an error in ‘understanding’ is far greater than any other error type, and having made that sort of error it’s highly likely that it’s going to be a fatal one. Continue Reading…

Tweedle Dum and Dee (Image source: Wikipedia Commons)

Revisiting the Knight, Leveson experiments

In the through the looking glass world of high integrity systems, the use of N-version programming is often touted as a means to achieve extremely lower failure rates without extensive V&V, due to the postulated independence of failure in independently developed software. Unfortunately this is hockum, as Knight and Leveson amply demonstrated with their N version experiments, but there may actually be advantages to N versioning, although not quite what the proponents of it originally expected.

Continue Reading…

Buncefield Tank on Fire (Image Source: Royal Chiltern Air Support Unit)

Why sometimes simpler is better in safety engineering.

Continue Reading…

Tweedle Dum and Dee (Image source: Wikipedia Commons)
How do ya do and shake hands, shake hands, shake hands. How do ya do and shake hands and state your name and business…

Lewis Carrol, Through the Looking Glass

You would have thought after the Leveson and Knight experiments that the  theory that independently written software would only contain independent faults was dead and buried, another beautiful theory shot down by hard cold fact.  But unfortunately like many great errors the theory of n-versioning keeps on keeping on (1).
Continue Reading…