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heading information provided by the ADIRUs were resolved into earth-related
acceleration rates to provide horizontal and vertical navigation information.
The IRS-derived wind speed and direction that was displayed to the crew was
computed by the aircraft’s ADIRUs as follows:
• Wind speed. The wind speed was computed from the difference between the
aircraft’s GS and TAS. For a computed wind speed of greater than 50 kts, the
accuracy of those parameters was:
– wind speed, ± 12 kts
– wind direction, ± 10 degrees.
• Wind direction. The wind direction was computed from the difference in the
aircraft’s track (TRK) and heading (HDG).
That wind was computed about 10 times per second, and was displayed on the
crew’s NDs.
– 9 –
In addition, the aircraft manufacturer advised that the true track accuracy was ±2.3
degrees with a GS of 200 kts, and that the true heading accuracy was ±0.4 degrees.
The aircraft manufacturer cautioned that, as a result of the accuracy of the various
parameters used to compute the wind speed and direction, crews should use the
displayed wind speed and direction ‘with care’.
The ADIRUs were located in the aircraft electrical and electronic compartment
beneath the cockpit. Because of their location forward of the aircraft centre of gravity
(CG), transient yaw accelerations could affect the derived wind speed and direction
data presented to the crew. However, once any transient yaw stabilised, the value of
the derived wind speed and direction would equal that experienced at the aircraft’s
CG.
Communications
The internal communications between the ATS controllers and transmissions
between ATS and the crew during the aircraft’s descent and approach to Melbourne
Airport were recorded by ground-based automatic voice-recording equipment. The
quality of those recorded transmissions was good. A chronology of the relevant ATS
communications is included at table 2.
Table 2: Chronology of relevant ATS communications
Time Event
1128:00
1128:52
The crew established contact with the Melbourne Centre CANTY Sector
Controller (CANTY Sector Controller). The controller informed the crew
that the duty runway at Melbourne was runway 27 and that the wind
direction and speed at Melbourne was 290 (M), between 8 and 18 kts.
The controller asked the crew if runway 27 would be suitable for the
landing and, in response, the crew requested runway 34.
The CANTY Sector Controller discussed the crew’s request for runway 34
with the controller acting as the Melbourne Flow Controller , who advised
‘There is up to 23 knots of crosswind.’ The CANTY Sector Controller then
advised the crew that there was 23 kts crosswind on that runway. The
crew informed the controller that runway 34 would be acceptable under
those conditions.4
1131:04
1131:16
The acting Flow Controller advised the Melbourne Tower Co-ordinator that
the crew required runway 34 for landing.
The Tower Co-ordinator advised the acting Flow Controller that runway 16
would be the preferred runway because ‘…it’s getting up to 9 kts
downwind on 34…’
1132:20 The acting Flow Controller advised the CANTY Sector Controller ‘There’s
9 knots of crosswind now on runway 34 so if Thai requires the long
runway it’ll be 16.’ 5
4 During the investigation, Airservices Australia reported that it was common practice for crews of
wide-body international aircraft to request the longer of the two Melbourne runways (runway 16/34)
when crosswind on that runway was not a significant factor.
5 The acting Flow Controller inadvertently transposed the 9 kts reported downwind component on
runway 34 as a 9 kts crosswind on runway 34. The controller supervising the Acting Flow controller
did not identify that transposition.
– 10 –
1132:31 The CANTY Sector Controller advised the crew ‘…from the Flow, there is
9 knots of crosswind on runway 34, if you still require a long runway it will
be runway 16, advise.’ The crew responded that they would accept
runway 16.
1134:48 The CANTY Sector Controller advised the crew that Melbourne ATIS
‘Yankee’ was current, and that the duty runway at Melbourne was runway
27. The controller advised the crew that the surface wind at the
aerodrome was ‘…230 degrees to 280 degrees 18 to 30 knots…’
1149:17 On instruction from the CANTY Sector Controller, the crew contacted the
Melbourne Departures Controller. They did not inform the controller, and
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