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At about 0600 Central Daylight-saving Time (CDT) on 7 November 2014, a Boeing 737-800, registered VH-VUR and operated by Virgin Australia, departed Adelaide, South Australia, on a scheduled service to Brisbane, Queensland. During the climb the crew selected Vertical Navigation (VNAV) mode, and continued climb in that mode until the aircraft was passing about flight level (FL) 250. At about that altitude, the crew selected level change (LVL CHG) mode, and continued climb at about 280 kt. The captain recalled that LVL CHG mode may have been selected to manage continued climb through a layer of turbulence. The crew intended to re-select VNAV mode when LVL CHG mode was no longer required, but inadvertently overlooked that selection, and climb continued in LVL CHG mode at 280 kt.

Soon after the selection of LVL CHG mode, as the aircraft climbed through about FL 265, the auto-flight system sequenced automatically from climb at a constant airspeed, to climb at a constant Mach number, consistent with normal system behaviour. Climb then continued above FL 265 at a constant Mach number of 0.69, which was the Mach number corresponding to 280 kt at the time the changeover occurred. As the aircraft continued to climb at a constant Mach number, the airspeed slowly reduced, as a function of the characteristics of the atmosphere and the relationship between Mach number and airspeed.

The slowly reducing airspeed went unnoticed by the crew until the auto-flight system was levelling the aircraft at the planned cruise altitude of FL 390. At about that time, the captain noticed that the airspeed had reduced to near the minimum manoeuvre airspeed, and a ‘buffet alert’ advisory message appeared in the scratchpad of Control Display Unit (CDU). Minimum manoeuvre speed at the time was about 216 kt. The crew responded by initiating an acceleration toward Mach 0.77, which was the FMC-programmed cruise Mach number, which corresponded to an airspeed of about 240 kt under the existing conditions. The flight continued to Brisbane without further incident.

During the operator’s investigation into the incident, the crew commented that a number of distractions may have contributed to the incident. The crew commented that sun glare directly through the windscreen for the duration of the climb was particularly problematic, and that they may also have been distracted by air traffic control and cabin-related communication requirements, and other air traffic in their vicinity. Additionally, both pilots consumed breakfast during the climb (at separate times), which may have provided a source of distraction.

This incident highlights the importance of continued auto-flight system mode and aircraft energy state awareness, and the manner in which various distractions have the potential to adversely affect such awareness. The incident also highlights the importance of robust auto-flight management procedures, supported by appropriately focused crew training and standardisation.

Aviation Short Investigations Bulletin - Issue 41


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General details
Date: 06 November 2014 Investigation status: Completed 
Time: 0631 CST  
Location   (show map):near Renmark Investigation type: Occurrence Investigation 
State: Victoria Occurrence type: Incorrect configuration 
Release date: 10 June 2015 Occurrence class: Operational 
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft details
Aircraft manufacturer: The Boeing Company 
Aircraft model: 737-8FE 
Aircraft registration: VH-VUR 
Serial number: 36606 
Operator: Virgin Australia 
Type of operation: Air Transport High Capacity 
Sector: Jet 
Damage to aircraft: Nil 
Departure point:Adelaide, SA
Destination:Brisbane, Qld
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Last update 14 November 2018