@ FrequentSLF:
Accepted catastrophic failure-rates are actually much higher:
The 10E-9 - figure is the failure-rate of _one_ system leading to a catastrophic failure.
Given an error-(failure-)chain, the accepted probability is 10E-6. This accident is most probably in this category.
The source is AMC 25.1309. ("Acceptable means of compliance to Certification Specification 25")
I cite the explanation:
In assessing the acceptability of a design it was recognised that rational probability values would have to be established. Historical evidence indicated that the probability of a serious accident due to operational and airframe-related causes was approximately one per million hours of flight. Furthermore, about 10 percent of the total were attributed to Failure Conditions caused by the aeroplane's systems. It seems reasonable that serious accidents caused by systems should not be allowed a higher probability than this in new aeroplane designs. It is reasonable to expect that the probability of a serious accident from all such Failure Conditions be not greater than one per ten million flight hours or 1 x 10-7 per flight hour for a newly designed aeroplane. The difficulty with this is that it is not possible to say whether the target has been met until all the systems on the aeroplane are collectively analysed numerically. For this reason it was assumed, arbitrarily, that there are about one hundred potential Failure Conditions in an aeroplane, which could be Catastrophic. The target allowable Average Probability per Flight Hour of 1 x 10-7 was thus apportioned equally among these Failure Conditions, resulting in an allocation of not greater than 1 x 10-9 to each. The upper limit for the Average Probability per Flight Hour for Catastrophic Failure Conditions would be 1 x 10-9 , which establishes an approximate probability value for the term "Extremely Improbable". Failure Conditions having less severe effects could be relatively more likely to occur.
Thomas