Incident Description
At approximately 2133, an aircraft was descending toward Melbourne Airport in preparation for a night-time ILS approach to runway 27. The intercept of the ILS localiser track did not commence until after the aircraft had already crossed that track at an angle of about 36°. A southerly wind acting on the westbound aircraft resulted in it quickly deviating to the right of the localiser track.
Details of Approach Deviation
While the aircraft continued deviating to the right of the track, as long as it remained within the full-scale CDI deflection, the pilot was permitted to continue the ILS approach visually only with ATC authorisation. Since this authorisation was not requested and provided, the pilot was required to adhere to the tracking tolerances of the ILS approach. Therefore, once the aircraft tracked beyond full-scale localiser CDI deflection, the pilot was required to conduct the published missed approach procedure.
The pilot reported that they did not observe the CDI exceed full-scale deflection, instead assessing that the visual approach could be continued. Continuing the approach took the aircraft significantly beyond the localiser tracking tolerance at altitudes as low as 980 ft AMSL (2,320 ft below the minimum sector altitude).
Safety Implications
As the incident took place at night, the pilot’s ability to visually identify obstacles was limited. The continuation of the night approach outside of the localiser tolerance and below the minimum sector altitude was contrary to the required approach requirements. This in turn removed obstacle clearance assurance, increasing the collision risk to the flight.