The Australian Transport Safety Bureau (ATSB) is investigating an approach by an Virgin Australia ATR 72-600 that resulted in several terrain alert warning system (TAWS) alarms.
The alarms occurred as the aircraft, bearing registration VH-FVR, was on the downwind position of the circuit on approach to runway 16 at Moranbah, Queensland, on 15 May 2013, says the ATSB. The aircraft was on a scheduled service from Brisbane.
“During the approach to the downwind position of the circuit, the crew observed a band of low scattered cloud and fog along the flight path,” says the ATSB. “The crew reported that the cloud base appeared to be about 1,500ft [457m] above ground level, which was their planned downwind altitude for the circuit.”
When approaching 1,500ft above ground level (AGL), however, the crew observed that the clouds were denser than previously thought. To remain in visual contact with the runway, the captain, who was the pilot flying, increased the rate of descent to about 1,900ft per minute.
This brought the aircraft to 450ft AGL. According to the flight data recorder, the crew heard a number of TAWS alarms.
“The crew reported that, as they were visual, the alerts were acknowledged and flight continued below the cloud base,” says the ATSB. “They also reported that the height of the cloud base was difficult to judge due to the combination of the scattered cloud and the underlying fog.”
After clearing the cloud band, the aircraft climbed to 950ft AGL before turning on to the circuits base leg. During the base leg two, TAWS “Don’t Sink” alerts were sounded. These alerts were also acknowledged by the crew.
The ATSB report will look at areas such as Virgin’s standard operating procedures, the TAWS system and Virgin’s management of TAWS alerts. The final report is due to be completed no later than April.