The one-in-a-billion figure quoted in the news article is standard for certification under FAA rules. The certification requirements state that there should be no single point failure that would cause death or loss of the aircraft. If such failures are not able to be designed out then the manufacturer must show that statistically, the failure should occur no more frequently that 1 10-9 or one time in a billion hours of fleet operation. First of all, it must be understood that the ultimate number is arrived at by manipulating percieved or demonstrated failure rates that may or may not relate to the element being analyzed.
This number is then incorporated into a safety hazards analysis (Fault Tree) and it is further manipulated through Boolean Algebra so that at the top level the ultimate loss of the horizontal stabilizer can be shown to be 1 10-12 or higher. It's all numbers and they do not necessarily reflect what the real failure rate is.
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The Cat
[This message has been edited by Lu Zuckerman (edited 20 November 2000).]