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air safety

Feb 19, 2013

Thai 777 disobeyed pull up orders for 50 seconds in Melbourne incident

A Thai International 777-300 which flew too low approaching Melbourne Airport in 2011 twice disobeyed tower instructions to pull up and go around according to a

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A Thai International 777-300 which flew too low approaching Melbourne Airport in 2011 twice disobeyed tower instructions to pull up and go around according to a report by the Australian Transport Safety Bureau.

The ATSB says the 50 second delay in response by the big Boeing’s pilots could have proven ‘more hazardous’ had air traffic control been trying to warn it of nearby aircraft or an immediate risk of collision with the ground.

The report is not a good read from the point of view of anyone flying Thai International, which had a similar yet different in its technical details incident with another of its 777s approaching Melbourne airport in 2007.

In this incident the flight was making a winter night time approach in rain showers to the airport at the end of a flight from Bangkok when the tower noticed that it was too low at a point abreast of Essendon having turned to approach the runway from the south.

According to the route advisories in place at the time of the incident pilots were warned of construction cranes below the approach path the Thai 777 was following in the rain and dark that night.

The ATSB report says:

Shortly after clearing the aircraft to conduct a visual approach, the tower controller observed that the aircraft was low on the approach and asked the flight crew to check their altitude. At that time, the aircraft was descending through about 1,100 ft and was about 7 NM (13 km) from the runway threshold. This placed the aircraft significantly below the standard approach path height of about 2,500 ft at that point in the approach and below the relevant SMSA of 1,950 ft. As a result, the tower controller instructed the crew to conduct a go-around, but with no effect. The controller issued additional instructions to go-around about 35 seconds and 47 seconds after issuing the initial instruction.  

While the flight crew did arrest the aircraft’s descent, the time delay between the tower controller’s initial go-around instruction and selection of go-around thrust was about 50 seconds. On this occasion, the tower controller issued the go-around instruction because the aircraft’s was unusually low on the approach. Equally, however, that instruction may have been issued to ensure separation from other traffic or terrain. In that case, a delay of 50 seconds could have resulted in a more hazardous situation.

Before the Thai jet began to climb away for the twice directed go around for a new approach it had descended to 948 feet when 12 kilometres from the perimeter of the airport when it should have been at 2400 feet. (International convention uses imperial measures for the vertical and metric for the horizontal. Yes, it is silly. )

The report notes that Thai International said it would try not to do this again. Actually it said:

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.

In its findings  the ATSB says the crew may have experienced ‘automation shock’ when the 777 surprised them with  sudden pitch up during the approach. It suggests that the captain of the Thai jet was unfamiliar with the automated systems on the 777, which is a somewhat damning thing to say about the airline’s training standards however politely expressed.

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.

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