altitude when the crew was unsure of the plane's position. The customary minimum altitude prohibited descent below even under good weather conditions, but a combination of factors led the captain to believe the plane was over the sea (the middle of McMurdo Sound and few small low islands), and previous Flight 901 pilots had regularly flown low over the area to give passengers a better view, as evidenced by photographs in Air New Zealand's own travel magazine and by first-hand accounts of personnel based on the ground at NZ's Scott Base. Mahon inquiry In response to public demand, the New Zealand government announced a further one-man Royal Commission of Inquiry into the accident, to be performed by Justice Peter Mahon. This Royal Commission was initially handicapped in that the deadline was extremely short; originally set for 31 October 1980, it was subsequently extended four times. Mahon's report, released on 27 April 1981, cleared the crew of blame for the disaster. Mahon said the single, dominant, and effective cause of the crash was Air New Zealand's alteration of the flight plan waypoint coordinates in the ground navigation computer without advising the crew. The new flight plan took the aircraft directly over the mountain, rather than along its flank. Due to whiteout conditions, "a malevolent trick of the polar light", the crew were unable to visually identify the mountain in front of them. Furthermore, they may have experienced a rare meteorological phenomenon called sector whiteout, which creates the visual illusion of a flat horizon far in the distance. A very broad gap between cloud layers appeared to allow a view of the distant Ross Ice Shelf and beyond. Mahon noted that the flight crew, with many thousands of hours of flight time between them, had considerable experience with the extreme accuracy of the aircraft's inertial navigation system. Mahon also found that the preflight briefings for previous flights had approved descents to any altitude authorised by the US ATC at McMurdo Station, and that the radio communications centre at McMurdo Station had indeed authorised Collins to descend to , below the minimum safe level of . In his report, Mahon found that airline executives and senior pilots had engaged in a conspiracy to whitewash the inquiry, accusing them of "an orchestrated litany of lies" by covering up evidence and lying to investigators. Mahon found that, in the original report, Chippindale had a poor