olasek, Capn Bloggs, Airbubber, et al,
Whilst the gist of my original question was to stimulate discussion as above, one aspect remains unanswered. Why have ‘BIDPE’ on the RNAV chart but not for LOC.
If ‘BIDPE’ pre-empts a VNAV path approach, then by not being on the desired path, albeit before the FAF (FAF is a more a check and not necessarily a start point?), it adds confusion and may encourage a step down procedure.
Alternatively if ‘BIDPE’ is just for info, ‘not below’ point, then it provides opportunity for an erroneous early descent – “Calculate a vertical speed to use from the FAF and make small adjustments to V/S as necessary for the proper path and step downs if applicable”, where flying the calculated descent path from ‘BIDPE’ - mistaken for FAF, would result in a 900ft error.
Re ‘lateral’ deviation. What is the primary navigation source on this aircraft how is it updated? Is this subject to ‘map slip? If so, then it might contribute to longitudinal / lateral path errors; the data link information discussed elsewhere might not represent ‘real world’.
Furthermore, we have yet to hear about EGPWS, which should have provided an alert, but if ‘the map’ - the assumed position from ‘NAV’ was closer to the airfield than reality, then EGPWS might not have alerted the crew (a good reason to have GPS embedded in EGPWS).
Also, if the crew focused on the computed RNAV solution then they similarly might not have been concerned until the ‘visual’ stage of the approach (=<MDA).