Pilots need to ensure that they have a thorough understanding of how automated flight systems work, says Australia's air accident investigator, following an incident in 2011 where a Boeing 777-300 flew below the required altitude while on approach to Melbourne airport.
The incident took place on 24 July 2011 and involved a Thai Airways International 777-300 with registration HS-TKD, according to the Australian Transport Safety Bureau (ATSB) report.
The aircraft was conducting a very high frequency omnidirectional radio range (VOR) approach to runway 34 at Melbourne airport with the autopilot, lateral navigation (LNAV) and vertical navigation (VNAV) modes engaged.
During the approach, the automated flight director changed VNAV modes from speed to path to ensure compliance with the VOR approach path, causing the aircraft to pitch up to intercept the calculated flight path. This led the first officer, who was performing the approach, to select flight level change mode to control the aircraft's descent.
Shortly afterwards, a tower controller at the airport advised the flightcrew to "check altitude" after noticing that the aircraft was low on the approach. Seconds later, the controller instructed the crew to climb and initiate a go-around, to which they responded "climbing", but there was a 50s delay before the go-around was initiated, during which the aircraft continued to descend.
The lowest altitude recorded was 984ft (300m) when the aircraft was 6.4nm (12km) from the runway threshold.
The ATSB found in its investigation that the first officer may not have understood some aspects of the automated flight control system and was surprised by the aircraft pitching up earlier in the descent. This led him to conduct the rest of the descent using the flight level change mode, in which the aircraft's rate of descent is unrestricted and may be higher than required on instrument approaches.
"In addition, the flightcrew 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," says the report.
As a result of the incident, Thai issued a notice to flightcrews emphasising the importance of constant angle non-precision approaches and the adherence to minimum safe altitudes. It has also reviewed training for non-provision approaches, providing additional information on the autopilot flight director.
In addition, the ATSB says that the investigation highlights the need for crew to maintain thorough understandings of automated 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," says the report.