map shift
BOAC, in #254, you imply that the FMS/Map position was correct during the approach and thus it would not shift, or if it did the discrepancy would be small.
There are many unknowns, e.g. which type of nav aids could the FMS accept and were these nav aids available for a position update during the descent and approach.
ILS is an unlikely updating aid, so too the associated DME in older FMS’s (problems of coupled systems when switching runway ends – n.a. here?).
Thus, what are the local DMEs, what was their availability – serviceability and line of sight? Does this FMS use TACAN inputs, etc, etc?
ILS displayed – maybe, but it would have to be tied to the map runway/airfield position, and it this position which is being questioned. Pre-empting another question – in some aircraft (all?), an ILS can be flown with a map shift as the beam is ground referenced. If this reference can be compared with the map, it is a good indicator of a possible map-slip. Conversely if not, it’s a hazard as it might generate a false sense of map accuracy.
The same for being 'visual' downwind, this the essence of a circling approach as 'visual contact' ties the aircraft position to the ground (no map shift), but in poor weather at low altitude the crew might be susceptible to mental map shift;- illusion, wish-think, press-on-itis, or only the first officer in contact (no crosscheck of position).
Even in speculation we require more information – and let’s speculate as it might help the investigators, or at least trigger a much needed response; but of greater importance it should enable us to think, and if applicable be shown to be wrong about these problems - thus we learn.