Walters final question is a very good one. I doubt that there is anyone on this thread with the technical expertise to answer it. I think it would take a RACAL expert to tell us.
I never knew that once the TANS display was switched off, it carried on computing.
But if you think about it it is logical that it did. For instance TANS off, you can still look at your raw doppler drift and groundspeed. You almost certainly have not powered down your GPS receiver. The information has to go somewhere. Why not into the TANS computer?
As an example if your VOR display unit fails,if you know where to look on the avionics rack you can still obtain accurate readings.
So the fact that RACAL could extract all the data at final powerdown comes as no surprise. What beats my poor level of knowledge is how the hell they could work backwards.
For instance on a low level route, I would frequently go Bearing / Distance, Along / Across and back, it took about one second, depending on dexterity with buttons.
After every turning point, whilst doing my Heading, Airspeed, Timing checks I left all navigation modes to scan all important data. (heading, TAS, L/L, doppler inputs etc,etc). This seemingly complex process took about five seconds. Then I would return to Navigation mode of choice, in my case nearly always A/A.
So what I can easily understand is how RACAL could extract data at powerdown. What remains a mystery to me is how they could possibly interpret where the rotary switches were, and what buttons were pressed just leading up to the accident?
It is important. Is there an expert out there who knows sufficient to answer?