The navigation system used by DC10 aircraft is a computerised device known as the area inertial navigation system (AINS). It enables the aircraft to be flown from one position to another with great accuracy. Prior to departure of a flight the AINS aboard the aircraft is programmed by inserting into its computers the co-ordinates of the departure and destination points (in degrees of latitude and longitude) together with those of specified waypoints en route. In the case of the Antarctic flights (which were engaged on what may be described as a return trip without touch-down) the southernmost waypoint, like each of the intermediate positions, was really a reference point to which the pilot knew the aircraft would be committed if it were left to follow the computer-directed flight track. And as mentioned the southern point for the preferred route to the McMurdo area was a ground installation at Williams Field.
During 1977 the co-ordinates for each waypoint which comprised the Antarctic routes had not been stored on magnetic tape for automatic retrieval and insertion into the navigation computer units of the aircraft. Instead the flight plan was dealt with manually and upon issue to the aircrew at the time of departure was manually typed by the pilot concerned into the aircraft computer units. When the Air New Zealand ground based computer was used in 1978 to produce computerised Antarctic flight plans they followed the same format as those that had been produced earlier. But before the ground computer could be programmed it had been necessary for an officer of the navigation section to prepare a written worksheet containing all the waypoints and their respective latitude and longitude co-ordinates which then were transcribed from the worksheet. And by reference to the original flight plan used in February 1977 this was done by Mr Hewitt, one of the four members of the navigation section at airline headquarters. He said in evidence before the Royal Commission that when he went on to take from his written worksheet the longitude co-ordinates of the McMurdo waypoint he mistakenly transcribed the correct figures of 166° 48' as 164° 48' by
inadvertently typing the figure "4" twice. This had the effect of moving the McMurdo waypoint 25 nautical miles to the west and once in the aircraft's system the navigation track which then it would follow from Cape Hallett when under automatic control would be over the[1] Sound rather than directly to Williams Field.
At this point it should be mentioned that the print-out of a flight plan shows not merely the co-ordinate waypoints but also a finely calculated statement of the direction and distance between them. This last information is obtained independently from what is called the NV90 programme of the computer which is able automatically to calculate the rhumb line track and distance between each of the respective waypoints once the co-ordinates have been fed into it. This information forms the basis for the data required to produce the computerised flight plan. So that finally when a print-out of the plan is obtained it will disclose not merely the geographical co-ordinates for each waypoint but the true track direction and the distance in nautical miles from one to the next. That last information is needed prior to a flight departure in order to calculate tonnages of fuel during the prospective journey and accordingly as a flight proceeds it enables the quantity of fuel already consumed to be checked against the anticipated consumption in the flight plan print-out. Thus the precise track and distance is used for purposes of fuel calculations and has importance as a check in navigation.
All this information is disclosed on page 96 of the Royal Commission Report where the print-out is shown for the flight plan with the co-ordinates for McMurdo showing the longitude as 164° 48' east. In the next column the track direction is given as 188.9° (grid) and the distance between Cape Hallett and McMurdo as 337 miles. On the facing page 97 there is a print-out of the flight plan actually used on the fatal flight which shows the correction made to the longitude, 166° 58' east. It will shortly be mentioned that when that correction was made the navigation section say it was thought to involve a minor movement of only 2.1 miles or 10 minutes of longitude. Despite the very small change that this could make to the track and distance between the two points a re-calculation was made and entered into the computer programme as 188.5° (grid) and the distance 336 miles. Compared with the other figures the difference seems minimal but it was still thought necessary to assess it and it was done.
The Western Waypoint
The circumstances surrounding the use of the 164° 48' E figures were in issue before the Royal Commission. It was suggested against the airline they had not been introduced accidentally: that the movement of the position 25 miles to the west had been deliberate. If that were so it would seem that a re-calculation of track and distance would have been needed and made both for the fuel plan and also as a check for purposes of navigation. However, no re-calculation of track and distance was made and entered with the 164° 48' co-ordinate. The figures which actually appear for track and distance to that point remain precisely the track and distance figures which were shown in the flight plan to the 166° 48' point for the first flight in February 1977. For purposes of comparison a calculation to the "false" waypoint was prepared and put before the Royal Commission. It showed that a direct track from Cape Hallett to that point is actually 191° and the distance 343 miles. The point is referred to in paragraph 230 of the Report within a section headed "The creation of the false McMurdo waypoint and how it came to be changed without the knowledge of Captain Collins".
In paragraph 229 it is said that submissions had been put to the Commissioner that "the shifting of the McMurdo waypoint was done deliberately so as to conform" with a track used by military aircraft proceeding to Williams Field. Then in paragraph 230 there is a summary of contrary arguments advanced by members of the navigation section to support their claim of accident. They include—
"(c) It was pointed out that if the McMurdo waypoint had been intentionally moved 25 miles to the west, then the flight plan would have a corresponding change to the track and distance information which it previously contained. Instead of a true heading from Cape Hallett to the NDB of 188.9° and a distance of 337 nautical miles, there would have been required, in respect of the changed McMurdo waypoint, a true heading of 191° and 343 nautical miles. Similar alterations would have had to be made in respect of a return journey to the true north."