Ark, hi
The Racal 'Supertans Investigation Report, is very clear on the valid points you make:
"2.4.3.2
During the flight over the sea, the accuracy of maintaining a doppler position is reduced because of the movement of the water due to tide and wind. The system shows the Doppler position about 327 metres to the east of the GPS position. This is equivalent to less than 0.5% of distance flown, which is exceptionally good performance from Doppler over water."
As an aside, the correction for wind, or 'Surface Motion Compensation' as you will remember it, was operating in the automatic mode, since the operator had correctly selected SEA, on the SEA/LAND switch, but had not entered a surface wind through the keyboard.
It was this 'background' SMC calculation to estimate surface wind from system wind at low altitude, that has provided the estimated altitude of ZD576, at approximately 15 secs prior to powerdown.
The timing of this background activity is controlled by a simple counter 'LCOUNT' which increments by one, every half second between 0 to 31. This parameter LCOUNT was recorded at the value 30, which means that the last calculation of surface wind from system wind, and memory of the associated altitude used in the calculation, was performed at least 15 seconds before power down.
Interestingly, the same altimeter's (a Gilham encoding capsule within the RH pilot's altimeter) baro-compensated altitude at the time of power down is quoted as 665ft +/- 50ft.
Yet we know ZD576 struck the Mull at 810ft AMSL.
Perhaps this is why the RNS252 Supertans is "not designed to provide 'historic' data."
The Board had confidence in the calculated altitude of 468ft +/- 50ft, recorded at 15-18 secs prior to impact, but cast doubt on the accuracy of the same parameter at power down. The Board considered that the accuracy of this latter value was probably affected by the mechanics of the crash, and developing fireball, as the altitude is derived from a static pressure source, and suffers all the inherent inaccuracies associated with this method.
Any comments guys?
Supertans was selected to Tactical steer (ie, no automatic waypoint change) at powerdown, displaying:
Tac:B
<<025'M
Dis: 86.7Nm
vTTG: 32.0
If only they'd had a Flight Data Recorder!