Dervish
<<I think someone mentioned the toggle switch on the Tans that AAIB said was off, which is probably quite important as MoD's case is built on their assertion that the Tans and the rest of the nav was in perfect working order and displaying information correctly. >>
Now here's a thing that you could lose your lunch over – both of the above points were true!!!
The SuperTANS was working to the end (from the recovered data, thanks to the back up battery) and yet the switch was found in the OFF position – a switch designed so that it is difficult to accidentally turn off (you have to pull it out to turn it to the off position) and given the apparent dynamics of the impact it would have been just about impossible for it to have got like that through impact.
Mr Cable (AAIB) was adamant that it had to have been in the off position, the smoke deposits indicating that it must have been off before the ground fire spread enough to effect that area of the wreckage.
If it had not been for the back up battery, very little analysis from the navigation perspective would have been possible – and we would not have had the clue of the switch being turned off after the crash.
Feeling it in your stomach yet?
Someone right there thought he was being clever by switching off the nav computer but instead left us a priceless clue – pity about the battery back-up, eh?
That Turn
Some years back I brought it up on this thread that a right turn at or near the position where they had dropped route navigation (by changing the waypoint in the SuperTANS) tied in with the handling pilot having 035 on his HoSI course selector – this suggested that the turn was deliberate with something for the HoSI track bar to work off.
This should have been enough, I thought, to get pilots amongst you thinking.
Alas, so much later someone is able to come up with a post that questions the turn without any one of you correcting him – and just in case anyone genuinely interested is left confused, I am obliged to explain how the turn can be quantified.
With respect to distance/time calculations and the actual accuracy that the Doppler system had held, the manufacturers of the nav equipment, the Board of Inquiry, and the Boeing analyst (Mitchell) all accepted that the a/c had to have travelled straight and level with very little turning (from leaving the NI coast) – however, the Board assuming it went straight in with no turn at all and Boeing that it had a small turn to the right and assumed that this occurred at the position of waypoint change – I deduced that the turn occurred after when they changed the waypoint but sometime before they crossed the shoreline.
There are two sources of data indicating a turn in the vicinity of the point where the waypoint was changed in the SuperTANS:
the GPS showed a displacement off the previous track;
the Doppler (over the ground) showed a significantly changed track angle.
First the GPS:
we are fortunate that they changed the waypoint where they did as the position was therefore memorised (let's call this WPCHX) – we therefore have, within the one system and with reference to one coordinate system, a point close to the Mull, referenced to a waypoint that the same system had used to get them there (waypoint A), and a position over the ground (shortly before impact) where a steering calculation was done;
whatever starting point you chose to work from back around Aldergrove (ATC fix, V813, Belfast radar, or the apron at RAF Aldergrove) because that's 40 miles away it doesn't make significant difference to the track to waypoint A on the Mull (work out for yourself the bearings from wpt A to those spots, all within a fraction of a degree – this is the converse of how Robin Clark dismissed the points near the Mull as showing a change of track, as I understand his argument) – however, we are now looking for a change from a known point close to the Mull and therefore a small displacement can have significant effect;
firstly, to get to WPCHX they would have been following a track from Aldergrove to waypoint A and this is without doubt what the SuperTANS had been using to establish the track – this works out at 027 mag (it follows that the handling pilot would have been expected to have had 027 on his course selector to this point, something I will come back to);
within the GPS's own world (just that system) to have got from WPCHX to the position of the last steering calculation, the a/c track would have had to have changed 4.5 degrees to the right from the track that had got all the way from NI – as the Boeing analyst pointed out, this wasinto the (strong) wind and was therefore not just a result of drift after they had stopped steering towards waypoint A;
of course we only have those two points and the a/c need not have turned exactly at the first point so 4.5 degrees is the minimum turn – a few seconds later and the turn would have to have been a few degrees more – the required turn would have got to 8 degrees (a track of 035 mag) before they got near the shoreline, a figure whose significance will soon be made apparent.
Now the Doppler:
the Doppler system may not be all that accurate on its own for position fixing but it sure is good for very accurate derivation of track angle over ground, the velocity vectors in conjunction with the a/c heading with respect to the gyro compass do the job – and they had travelled over ground for several seconds by the time that the last steering calculation was made;
at the time of the last steering calculation, according to the Racal Rpt the system had a track of 026 T (2.12.2.1 – which was equivalent to 33.5 mag) but in section 2.10.4.2 they had thoughtfully adjusted the data to reflect that it was over land at that time (the system was still set at “sea”) and they got 028 degrees true – which was equivalent to 035.5 mag back then.
035 was the setting found on the handling pilot's horizontal situation indicator course setting – as he would surely have had 027 on it until near or at the position of waypoint change, he must have changed the course selector to the course that they then followed.
Talk your way out of that.