On 4 January 2013, a Boeing 737-800 registered VH-VUZ, departed Launceston, Tasmania on a scheduled passenger service to Melbourne, Victoria. During departure, the crew selected level change (LVL CHG) as the vertical auto-flight system mode, and a climb speed of 250 kts. The crew intended to switch from LVL CHG mode to vertical navigation (VNAV) mode later during the climb, but inadvertently overlooked that selection.
Had the crew switched to VNAV mode as intended, the aircraft would have accelerated during the climb in accordance with a programmed speed schedule. In LVL CHG mode however, the aircraft climbed at a constant speed of 250 kts until passing about flight level (FL) 260, when the auto-flight system sequenced automatically to continue the climb at a constant Mach 0.62 (which was the Mach Number corresponding to 250 kts when the changeover occurred).
As climb then continued above FL 260 at a constant Mach 0.62, airspeed gradually reduced. The unintended vertical auto-flight mode and the gradual airspeed reduction went unnoticed by the crew until the aircraft was approaching FL 350, when a ‘buffet alert’ caution appeared on the Control Display Unit and the auto-flight system made a small reduction in aircraft pitch attitude. At that moment, the crew noted that the airspeed had reduced to near the top of the amber bar on the airspeed indicator, representing the aircraft minimum manoeuvre airspeed.
In responding to recognition of the minimum manoeuvre airspeed condition, the crew reduced the aircraft pitch attitude to the point that the aircraft entered a shallow descent. Soon after, the crew was able to establish an accelerated climb to the intended cruising level of FL 360. Recorded data indicates that that aircraft reached a minimum speed of 201 kts, about 6 kts below the minimum manoeuvre airspeed at that moment.
The occurrence highlights the importance of consistent attention to auto-flight system modes and aircraft energy state. Since this occurrence, the operator has introduced a procedure requiring the announcement of flight mode annunciation changes. The operator also included mode awareness briefings during a 2013 recurrent training program.