Hmmm Hardware run by software. To illustrate the point about unpredictable errors:
On November 3rd, 1973
National Airlines DC-10-10 N60NA
"Barbara" was flying from San Francisco to Miami. The experienced crew began discussing how the auto-throttle would respond if the circuit breakers for the N1 Tachometers were pulled. They decided to try it, and see what happened.
They pulled the circuit breakers on the N1 tachs for all three engines while in auto-throttle speed mode. Nothing happened.
They then commanded a 5 kt speed reduction, and the throttles reduced slightly, and that was all.
They then disconnected the auto-throttle, in preparation for resetting the circuit breakers, disconnected the auto-throttle and reset the circuit breakers.
On doing this, the right hand engine underwent an uncontrolled overspeed, disintegrating the fan, removing the entire right cowl, severing the hydraulics, electrics and control cables, and penetrating the right hand fuel tank. The debris also impacted the fuselage and the centre engine, punctured the 6th window forward of the S3 door and the passenger seated next to this window was ejected in the subsequent explosive decompression.
