A lot of comment here about detection of incorrect data from multiple sensors; voting algorithms and the like. This is a serious and involved technical topic involving considerable insight and expertise in algorithm design. It is not easy.
Can you algorithmically detect two out of three incorrect sensors? Well, it depends on what fault detection and tolerance algorithms you have decided to implement, and it has a lot to do with whether you think the HW and SW needed for implementing those algorithms is more reliable than a simple system which doesn't detect such anomalies but is rarely subject to such failures.
SPA83 thinks that failure to detect the sensor-anomaly situation on the accident aircraft is a
Originally Posted by SPA83
serious breach of Airbus in the certification standards
No, it's not. You can't condemn a manufacturer for not solving a problem that is generally insoluble without considerable trade-offs (and even then only part-soluble). Maybe SPA83 would like to propose his own solution to the problem and write it out here. That would at least ensure that heshe understands the problem before waggin the finger at someone for not solving it.
BOAC said:
Originally Posted by BOAC
I would have expected an aircraft, which is supposed to be all things to all pilots, to know when 2 of its 3 PRIMARY sensors have 'failed' and therefore disagreed with the third.
No one who works in this area knows of any reliable way of accomplishing this feat in general, the way it is stated here. Specific sorts of failures can be detected and accomodated: in this particular case, of course, putting a water-detector in each instrument would have sufficed, but it is (I hope, obviously) impractical in general to think of every specific possible anomaly and put in a circuit to detect exactly that condition.
None of the general methods are oriented towards common-cause failures such as happened to the accident aircraft.
Just to be clear, the sensor failures on the accident aircraft were not Byzantine faults. I also find the Wikipedia article on Byzantine faults to be confusing and generally poorly written. There will shortly appear a set of slides from a brilliant keynote talk last Thursday by Kevin Driscoll at SAFECOMP 2010 on instances of Byzantine failures in aerospace.
PBL