Safety Helmut,
I agree to some extent with your thoughts on the nut on the end of the starter motor on XV666. However, the failure of the nut was not the cause. The nut was/is/will never be expected, under normal conditions, to stop the turbine from coming off the end of the shaft. An automatic speed sensitive device is supposed to close off the air supply at an rpm below which the turbine would overstress the nut. Neither was it the speed sensitive device which failed. Within the same loom on the engine a live wire, operating the engine anti-icing valve (which had just been selected on) short circuited to the wire directly powering the air valve solenoid and bypassed the speed sensitive device. Therefore, the air valve opened, and remained open, and ran up the turbine, uncontrollably, to the point that the nut at the end of the shaft was stressed beyond its capabilities and the turbine came off. Its arguable whether or not a newer, or modified, nut would have retained the turbine, in that circumstance, but the fact is that the root cause was the short circuit bypassing the air valve control system.