Change of Waypoint
It is a while since I have used a TANS so my terminology may be incorrect.
A manual change of Waypoint requires a minimum of three(?) buttons to be pushed deliberately.
STR - 3 - ENT?
Less than one minute later the aircraft impacted with the TANS switched off.
The manual Waypoint change would have pointless if the switch off had been earlier. The internal workings of the computer may still have been working, but why push buttons while looking at a blank screen?
A TANS is designed not to be switched off by accident. So why did the crew choose to switch off the primary Navaid and continue towards the Mull at high speed?
And if the RACAL analysis can be used to reconstruct the final moments of flight. Why was the "off" selection never mentioned.