It was established that the flight crew either was unaware of or ignored the approved route's minimum safe altitude (MSA) of 16,000 feet (4,900 m) for the approach to Mount Erebus, and 6,000 feet (1,800 m) in the sector south of Mount Erebus (and then only when the cloud base was at 7,000 feet (2,100 m) or better). 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 2,000 to 3,000 feet (610 to 910 m)) whilst over water to establish visual contact with surface landmarks and afford the passengers a better view. During the subsequent inquiry Justice Mahon concluded that this was a deliberate attempt to conceal from the United States authorities that the flight plan had been changed, and probably because it was known that the United States Air Traffic Control would lodge an objection to the new flight path. Additionally, the computer program was altered such that the standard telex forwarded to Air Traffic Controllers at McMurdo displayed the word "McMurdo" rather than the coordinates of latitude and longitude, for the final waypoint. In contrast to the McMurdo Sound route, the updated route would result in a collision with Mount Erebus if this leg was flown at an altitude of less than 13,000 feet (4,000 m). The key difference between the routes was that the flight plan presented at 9 November briefing corresponded to a track down McMurdo Sound, giving Mount Erebus a wide berth to the east, whereas the flight plan printed on the morning of the flight corresponded to a track that coincided with Mount Erebus. The flight plan printout given to the crew on the morning of the flight, which was subsequently entered by the flight crew into the aircraft's INS, differed from the flight plan presented at 9 November briefing and from Captain Collins' map mark-ups which he had prepared the night previously.
Crucially, the flight crew of Flight 901 was not notified of the change. The Navigation section changed the McMurdo waypoint co-ordinate stored in the ground computer system at approximately 1:40 am on the morning of the flight.
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