Originally Posted by
Tagron
The visual misidentification was a fatal error. That was an opportunity to verify INS accuracy beyond doubt. Had the visual position been correctly identified the crew would surely have realised there was something seriously amiss with their INS position and maintained altitude to await a corroborative fix. As it was, confirmation bias seemed to set in. The INS could not be so wrong. A 27 nm error in their INS performance was quite contrary to their experience.
Regarding visual misidentification - I highly recommend watching the complete "Impact Erebus" video if you have not already seen it, as it demonstrates the optical illusions involved in a very useful and understandable way. The phenomenon of whiteout as it was briefed to the ANZ pilots was actually the wrong kind of whiteout - the type caused by refractions from powdery snow low to the ground, as opposed to the type which trapped Captain Collins and his crew, which can affect almost any altitude below overcast cloud. It was an error, no question about it, but it was an error capable of affecting even pilots who know the area well - with the fog bank (well below them) obscuring the end of the bay and the foot of Erebus, the only visible difference between the cliffs of Lewis Bay and those of McMurdo Sound would be the way the cliffs in the latter fan out in the distance - at 1,500ft that would have been extremely difficult to spot.
Originally Posted by
chris lz
But isn't it also a basic requirement than in VMC, a visual letdown below MSA first requires a positive visual confirmation of one's exact position? I was under the impression the crew was only able to form a visual confirmation (in their minds at least) after they had descended below the cloud layer and could see the terrain to the left of their track, most of which (including Erebus) was not visible at 16,000 ft. (My memory could be wrong here.)
The full overcast affected only the immediate area of Ross Island itself. North of Ross Island, the cloud was patchy and there were more than enough gaps to make a visual let-down viable (as you can see from the passenger photos). The problem was not that the crew could only form visual confirmation once under the cloud at the point of let-down, it was that once down, Lewis Bay and McMurdo Sound were almost indistinguishable from one another from the point-of-view of the flight deck in the conditions they encountered. Once they had reached the level Mac Central had invited them to descend to, the only clue that something was wrong was that they could no longer communicate with Mac Central via radio.
The question that bothers me is that if the radar let-down was completely above board from Mac Central's end, why did they erase the radar tapes?