Originally Posted by
david340r
There's a significant assumption here that bit flip events are what are referred to by statisticians as IID (independent and identically distributed), but if they are bits within a single word of data in memory or a single register within the processor they could be physically extremely close together. The 286 is based on 1.5um technology so those bits may be only tens of um apart. For memory devices they could be closer. I don't know enough about cosmic ray events to know whether a single event could affect multiple bits within a small area like that.
The Seattle Times article (quoted earlier) specifically states that the FAA regarded the 5 bits flipping as a "single event", however improbable this may seem.
AFAIK this is equivalent to a "single point of failure", and since there are no redundant software checks, it cannot be permitted in an automated flight control.
Edit: Cosmic rays are not the only way to trigger bit-flips, merely one of the more colourful possibilities that a lay audience can understand. Electrical spikes, and various hardware factors, may also play a role. Some computers are fitted with parity checks, to detect specific kinds of bit-errors, but these are not foolproof.
P.S. I previously pointed out that the probabilities suggested by
Notanatp are misleading.