Operational event - Darwin Aerodrome, Northern Territory, 17 December 2008, PK-GZJ, Boeing 737-4MO

On 17 December 2008, a Boeing Company 737-4MO aircraft, registered PK-GZJ, made a significant diversion around weather at night while en route to Darwin, Northern Territory from Denpasar, Indonesia. The aircraft was cleared to conduct the runway 11 very high frequency omnidirectional radio range (VOR) approach via the initial approach fix NASUX. After the weather diversion, it was more convenient for the flight crew to make a pilot intercept of the 285 radial from the VOR but there was a period of misunderstanding as a result of a breakdown in the application of standard radiotelephony readbacks.

The flight crew left the previously-cleared altitude of 3,000 ft on descent although they had not been cleared to do so. When this became apparent, no updated clearance for a pilot intercept of the 285 radial was issued by the controller. The aircraft continued to descend on the basis of the runway 11 VOR descent profile, even though it was not conducting the runway 11 VOR approach.

The flight crew used the position calculated by the aircraft's inertial reference system (IRS) to intercept the 285 radial, instead of using the signal from the VOR. The IRS position was not accurate enough for this, and the aircraft tracked to outside of the stipulated 5 degrees tolerance either side of the 285 radial. From then on, the aircraft was no longer 'established' on the 285 radial even though it was below the minimum sector altitude in cloud. When it broke through the cloud, the aircraft was clearly not aligned with the runway and a missed approach was carried out.

The investigation did not identify any organisational or systemic issues that might adversely affect the future safety of aviation operations. However, in response to the occurrence, the aircraft operator developed a corrective action plan that addressed a number of crew resource management and non-precision approach training aspects to the event.