Australian authorities have highlighted the importance of crews being aware of autoflight system modes, following inquiry into an incident in which a Boeing 737 flew close to the minimum manoeuvre airspeed.

The incident occurred on 7 November 2014 and involved a Virgin Australia-operated 737-800 (VH-VUR) which was flying from Adelaide to Brisbane, the Australian Transport Safety Bureau says in its report.

As the aircraft was climbing through 25,000ft, the crew put the autoflight system into level-change mode, to avoid turbulence, and continued to climb at 280kt. The crew had intended to switch it into vertical-navigation mode after passing through the turbulence, but it remained in the previous mode.

When the aircraft passed around 25,000ft, the system automatically transitioned from a climb at constant speed to a climb at constant Mach – then set at M0.69, which corresponded to the 280kt speed programmed.

"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," says the ATSB.

However, the crew did not notice that the speed was reducing until it reached the planned cruising altitude of 39,000ft, at which point a "buffet alert" advisory appeared on the control display unit, warning that the aircraft was nearing its minimum manoeuvring speed of 216kt.

Following that advisory, the crew accelerated the aircraft to M0.77, taking it to around 240kt, and continued the flight without further incident.

During Virgin Australia's investigation of the incident, the crew reported that they may have become distracted by sun glare and air traffic control communications.

"For flightcrew, this incident highlights the importance of continued autoflight system mode and aircraft energy-state awareness," the ATSB says. "The incident also highlights the manner in which various distractions have the potential to adversely affect such awareness."

Virgin has told the ATSB that, as a result of the incident, it will highlight potential distractions associated with glare and communication in future training programmes.

Source: Cirium Dashboard