As I understand it, the following is true: the FIFTH computer is not fully functional -- it is intended to have just enough programming to land the shuttle in the event that the four main computers all fail. Testing it safely under live conditions where the first four computers are inoperable is essentially undesirable, if not practically impossible. The fifth system has never been invoked. Worse yet, it has most likely not been maintained for compatibility with the other four. That is not what is generally thought of as N-version programming for N=2 in the realistic sense of the word, although it might be considered so for the stark subset of the functionality. It is more like a hot standby fail-safe mechanism. ------------------------------