THE inattention of Virgin Australia pilots led to a passenger flight losing speed after it departed Launceston Airport, the Australian Transport Safety Bureau has found.
The bureau has released a report into the Melbourne-bound flight, involving a Boeing 737-800, on January 4, 2013.
During departure, the crew selected level change as the vertical auto-flight system mode, and a climb speed of 250 knots.
The crew intended to switch modes to vertical navigation mode later during the climb, but inadvertently overlooked that selection.
In level change mode, the aircraft climbed at a constant speed of 250 knots until passing about flight level 260, when the auto-flight system sequenced automatically to continue the climb.
As climb then continued above flight level 260, airspeed gradually reduced.
The crew failed to notice the unintended vertical auto-flight mode and the gradual airspeed reduction, until the aircraft was approaching flight level 350, when a caution appeared on their control display, 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 flight level 360.
Recorded data indicates that the aircraft reached a minimum speed of 201 knots, about 6 knots below the minimum manoeuvre airspeed at that moment.
The report stated that the occurrence highlighted the importance of consistent attention to auto-flight system modes and aircraft energy state.
Since the incident, Virgin Australia has introduced a procedure requiring the announcement of flight mode annunciation changes.
Virgin also included mode awareness briefings during a 2013 recurrent training program.