Probably because the captain thought he wouldn't be getting the radar service. Even if he was within range with his transponder being interrogated by the radar, that was of no use to him without VHF contact with the Tower. Of the ten minutes that elapsed between the offer of the radar assist and the decision to descend, most of it was spent trying to make VHF contact, with Mac Station and with the Tower. Radio communications in the Antarctic were notoriously unreliable, so the captain probably thought that the VHF was playing up, hence the decision to go scud running while he still had the chance.
PS - The Air Marshall didn't let AirNZ off the hook:
Sir Rochford’s criticism of the crew is almost mild in comparison with his scathing denunciation of Air New Zealand’s flight planning, which he says deserves 80 to 90 percent of the blame for the disaster. “Air New Zealand treated the flights as a picnic for senior captains. They asked to fly them and they went in order of seniority. The air force insists that potential Antarctic captains do at least one or two trips in the copilot’s seat, which is eminently sensible. But only the flight engineer (Brooks) had been there before and from the voice recording he was the only one concerned. The briefings for the captains were pathetic. I was horrified throughout the inquiry to realise that, in preparation for the flights, the airline had no contact with the RNZAF with their great fund of Antarctic flying experience. This manifested in the ridiculous situation that the airline could not even give Collins a proper topographical map of the area. At Whenuapai (the air force base at Auckland) I saw for myself drawers full of maps. An adequate briefing was available at Whenuapai, if the airline had just pocketed its pride and asked.”
Hughes adds: “That (navigation coordinate change) was an added and very sinister factor which just again weighed the odds against Captain Collins, who was after all following the procedure adopted by captains of the previous flights. Without doubt he should have been told of the change. One can have no sympathy whatsoever with the poor organisation which Judge Mahon revealed was prevalent. It was a chapter in Air New Zealand’s history I’m sure they wish had never been written. (North & South magazine, November 1989, pages 86 & 89)
Last edited by ampan; 3rd Jan 2012 at 17:53.