What happened
At 2019 Eastern Standard Time on 24 July 2011, a Thai Airways International Boeing Company 777-3D7 aircraft, registered HS-TKD, was conducting a runway 34 VOR approach to Melbourne Airport, Victoria. During the approach, the tower controller observed that the aircraft was lower than required and asked the flight crew to check their altitude. The tower controller subsequently instructed the crew to conduct a go-around. However, while the crew did arrest the aircraft’s descent, there was a delay of about 50 seconds before they initiated the go-around and commenced a climb to the required altitude.
What the ATSB found
The ATSB established that the pilot in command may not have fully understood some aspects of the aircraft’s automated flight control systems and probably experienced ‘automation surprise’ when the aircraft pitched up to capture the VOR approach path. As a result, the remainder of the approach was conducted using the autopilot’s flight level change mode. In that mode the aircraft’s rate of descent is unrestricted and therefore may be significantly higher than that required for an instrument approach. In addition, the flight crew inadvertently selected a lower than stipulated descent altitude, resulting in descent below the specified segment minimum safe altitude for that stage of the approach and the approach not being managed in accordance with the prescribed procedure.
What has been done as a result
In response to this occurrence, Thai Airways International issued a notice to flight crews that emphasized the importance of constant angle non-precision approaches and adherence to the segment minimum safe altitudes. Other actions included a review of the training in support of non-precision approaches and the provision of additional information relating to the use of the aircraft’s autopilot flight director system.
Safety message
This occurrence highlights the risks inherent in the conduct of non-precision approaches and reinforces the need for flight crews to closely monitor the aircraft’s flight path to ensure it complies with the prescribed procedure.
Modern air transport aircraft are equipped with ever increasing levels of automation that, when used appropriately, can greatly reduce flight crew workload. While flight crews retain the option of flying the aircraft manually, the use of automation is generally preferred and often provides increased levels of safety and efficiency. To effectively manage the aircraft and flight path, however, flight crews need to maintain a thorough understanding of the relevant automatic flight systems. Worldwide, errors associated with the use and management of automatic flight systems have been identified as causal factors in more than 20% of approach and landing accidents.