John Blakeley
<<In "Ext" position, Doppler reversion should be selected to minimise navigational errors during GPS outages.>>
Are you saying that the SuperTANS would have to have been in a particular configuration to have been “blending” GPS and Doppler that day? My understanding was that this was done automatically in the SuperTANS, such that Doppler was dominant in the filters such that, say, a temporary GPS outage did not require action by the pilot – as opposed to earlier TANS when the data was available separately. If what you say was the case, was the system selected appropriately for Doppler?
.
The rest of your technical description seems to distract from the fundamental limitation I described of the system when over water – are you trying to blind the reader with science?
.
Whatever, you wrote:
<<I am not sure what relevance the navigation errors of the SuperTANS might have had on a VFR flight where it would appear that the crew had changed waypoints based on visual identification …>>
I thought I had explained the relevance clearly enough in the paragraph “ The relevance of the SuperTANS in this incident is that had it been in contradiction with another reference, the SuperTANS would have been thought to be in error and therefore could be ignored … “ of my posting # 2645 - basically, the crew were aware of its potential inaccuracy over water and would not have been alarmed if it disagreed by ˝ a mile or more with another, more trusted reference – they would not have worried about the discrepancy.
However, had they no other reference, and were using only the SuperTANS to keep them away from the Mull, then they should (and I say would) have added a safety factor (of at least ˝ a mile) and started their turn earlier than the SuperTANS directed.
.
Their action of effectively disregarding the SuperTANS by changing to the next waypoint suggests that they had either a clear visual reference or they were referring to another system.
.
As it happens, the SuperTANS was found to have been accurate near the crash site and so any visual reference they had should have reinforced the SuperTANS position and they should have started their turn as directed by it.
Confusion between the fog station and the lighthouse has been suggested but, if you refer to a map, at their angle of approach the fog station would have come up sooner and it was so little off track (to the lighthouse) that the deviation should not have been significant.
Sighting either of these structures should have served them well and it is hard to envisage that if they could not make out either of them clearly then they could have had any other physical feature clear enough to go by.
Thus I believe that they were not using a clear visual reference.
.
This only leaves another electronic system as a reference – they did not have radar. John Blakeley, you appear as a man with quite some technical background in avionics – what do you suggest could have misled them as to their distance to go that they would have trusted? Whatever other systems were there available at the time? Don’t worry as to whether they were actually fitted – just what could hypothetically have been used. Then we can discuss them and dismiss them from position of knowledge. Come on – just for interest – be the devil’s advocate – lets brainstorm all known applicable systems. I challenge you.
