in the low cloud base (later estimated to be at around ) while over water to establish visual contact with surface landmarks and give the passengers a better view. 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 been flown at levels substantially below the route's MSA. In addition, preflight briefings for previous flights had approved descents to any altitude authorised by the US 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 . The radar equipment did not pick up the aircraft, however, and the crew also experienced difficulty establishing VHF communications. The distance-measuring equipment did not lock onto the McMurdo Tactical Air Navigation System (TACAN) for any useful period. Cockpit voice recorder (CVR) transcripts from the last minutes of the flight before impact with Mount Erebus indicated that the flight crew believed they were flying over McMurdo Sound, well to the west of Mount Erebus and with the Ross Ice Shelf visible on the horizon, when in reality they were flying directly toward the mountain. Despite most of the crew being engaged in identifying visual landmarks at the time, they never perceived the mountain directly in front of them. About 6 minutes after completing a descent in visual meteorological conditions, Flight 901 collided with the mountain at an altitude around , on the lower slopes of the 12,448-ft-tall (3,794 m) mountain. Passenger photographs taken seconds before the collision removed all doubt of a "flying in cloud" theory, showing perfectly clear visibility well beneath the cloud base, with landmarks to the left and to the right of the aircraft visible. Changes to the coordinates and departure The crew put the coordinates into the plane's computer before they departed at from Auckland International Airport. Unknown to them, the coordinates had been modified earlier that morning to correct the error introduced previously and undetected until then. The crew evidently did not