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The actions of the captain of a Boeing 737 in response to a sudden speed increase during a high speed descent resulted in sudden pitch changes and injuries to two cabin crew members, a new ATSB report details.
The Virgin Australia-operated Boeing 737-800 was at the top of the descent 136 nautical miles south-east of Adelaide Airport conducting a scheduled passenger flight from Melbourne on 13 September 2017, with the first officer acting as pilot flying.
Air traffic control instructed the crew to perform a high speed descent, which they commenced with the autopilot engaged with a target descent speed of 320 knots, which was higher than the normal descent speed of 280 knots.
During the descent, the first officer attempted to manage airspeed by using changes in the autopilot modes and reductions in the target airspeed. As the aircraft descended through around 17,000 feet, the tailwind affecting the aircraft decreased suddenly and significantly, which caused the indicated airspeed to increase and approach the maximum operating speed limit of 340 knots.
Highly concerned about overspeeding the aircraft, the captain responded by abruptly pulling back on the control column, causing the autopilot to disconnect. The resulting control forces caused sudden changes to the aircraft’s pitch attitude and vertical acceleration.
Two cabin crew members who were standing in the aircraft’s rear galley eating a meal received injuries during the upset, with one sustaining a broken leg.
ATSB Director Transport Safety Dr Stuart Godley said that during the occurrence, the aircraft only exceeded its maximum operating speed by one knot, which did not require any structural inspections to ensure the ongoing airworthiness of the aircraft. Instead, the safety implications associated with this event related to the actions of the captain in response to the sudden change in airspeed.
“Even though the autopilot was operating correctly, when the aircraft was approaching and exceeding the maximum operating speed, the captain’s perception was that the autopilot was not controlling the aircraft and that urgent intervention was necessary,” Dr Godley said.
“However, the captain did not follow the normal procedure for taking over control of pilot flying duties, and the large pitch control inputs made by the captain were probably influenced by a perception of urgency.”
Dr Godley said the investigation serves to remind pilots that they are entitled to decline air traffic control instructions where they do not perceive they can safely comply.
“Due to increased kinetic energy and reduced margins to placard speed limits, high-speed descents involve a higher level of risk, including increased risk of harm due to abrupt control input.”
The investigation also highlights the challenges pilots face when responding to sudden or unexpected situations, Dr Godley said.
“There will often be a reduction in safety when pilots perceive a situation is urgent and when they make decisions rapidly and reflexively. In these situations, pilots may not be able to effectively process information or make good decisions.”
The investigation also notes that it took over 90 minutes for the injured cabin crew member, who had sustained a badly-broken leg, to be removed from the aircraft after its arrival at Adelaide, while the operator’s ground operations supervisor, Aviation Rescue Fire Fighting Service (ARFFS) officers and SA Ambulence officers co-ordinated the extraction.
The injured cabin crew member was not able to walk or to sit in a wheelchair, and an ambulance stretcher would not fit down the aircraft aisle, however, the ground operations supervisor declined an ARFFS request to use a catering truck, the report notes.
Instead, emergency services personnel used a slide sheet to drag the injured cabin crew member along the aisle to the front of the aircraft, then placed the injured cabin crew member on a stretcher.
Virgin Australia has subsequently updated the training and information provided to pilots about overspeed and overspeed recovery. The airline has also changed procedures for ground handling staff when responding to requests from emergency services.