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.