The flight had earlier paused during the approach to McMurdo Sound to carry out a descent, via a figure-eight manoeuvre, through a gap in the low cloud base (later estimated to be at approximately ) while over water to establish visual contact with surface landmarks and give the passengers a better view. It was established that the flight crew either was unaware of or ignored the approved route's minimum safe altitude (MSA) of for the approach to Mount Erebus, and in the sector south of Mount Erebus (and then only when the cloud base was at or better). Photographs and news stories from previous flights showed that many of these had also been flown at levels substantially below the route's MSA. In addition, pre-flight briefings for previous flights had approved descents to any altitude authorised by the US Air Traffic Controller (ATC) at McMurdo Station. As the US ATC expected Flight 901 to follow the same route as previous flights down McMurdo Sound, and in accordance with the route waypoints previously advised by Air New Zealand to them, the ATC advised Flight 901 that it had a radar that could let them down to . However, the radar equipment did not pick up the aircraft, and the crew also experienced difficulty establishing VHF communications. The distance measuring equipment (DME) did not lock onto the McMurdo Tactical Air Navigation System (TACAN) for any useful period.