The ATSB investigation into a serious incident at Melbourne Airport, Victoria has demonstrated the requirements and complexities that come with the use of automatic flight systems. While modern automation can greatly reduce flight crew workload, crews still need to maintain a thorough understanding of those systems.
The incident occurred on 24 July 2011 when, at 2019 EST, a Thai Airways Boeing 777-3D7 aircraft was conducting a runway approach to Melbourne Airport after a flight from Bangkok. The approach was being made using very high frequency omnidirectional radio range (VOR) — a ground-based navigation aid that emits a signal that can be received by appropriately‑equipped aircraft and represented as the aircraft’s bearing to or from that aid.
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’...
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.
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. Automation surprise occurs when an automatic system changes its operation unexpectedly.
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, which resulted 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.
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 crew 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.
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. 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. Worldwide, errors associated with the use and management of automatic flight systems have been identified as causal factors in more than 20 per cent of approach and landing accidents.
Read the report: Operational non-compliance involving Boeing 777, HS-TKD, 15 km south of Melbourne Airport, Victoria, on 24 July 2011, which provides more detail on the incident and the measures taken to remedy it.