It was at this point the wind suddenly changed direction, resulting in the aircraft accelerating past 325 knots and towards its maximum allowable speed of 340 knots.
The pilot pulled the control stick back to override the autopilot, which was expected to then revert to a different autopilot mode – called “control wheel steering-pitch” (CWS-P) – that would raise the aircraft’s nose and slow its descent.
However, the aircraft had been modified to remove that autopilot reversion, and so the autopilot disengaged all together, the ATSB found.
“The autopilot disengagement produced a sharper elevator response than reversion to CWS‑P,” the report says, with the aircraft accelerating to 339.5 knots and its angle of attack changing 3.87 degrees in one second.
“A cabin crew member who had not yet reached her seat felt the cabin floor drop and then quickly come up,” the ATSB said.
“The force of the aircraft’s movement resulted in her fracturing her leg, while a second cabin crew member fell and hit her head on a trolley. She also received injuries to her knees, back and neck.”
First aid was administered but the cabin crew member with the broken leg remained lying on the cabin floor in the rear galley for the remainder of the flight, which touched down at Canberra Airport about 20 minutes later.
Both injured crew members were transferred to hospital, where the crew member with the broken leg was admitted and the other member was treated then discharged.
None of the 177 passengers on board were injured.
The ATSB said the pilot’s actions to slow the plane were “understandable” and common practice at Qantas, with the pilot in question saying it formed part of the initial training to fly the aircraft.
“Modification of the autopilot, however, had inadvertently left the aircraft vulnerable to this type of scenario,” the report says.
The ATSB noted that Boeing prefers the use of speedbrakes to manage increasing airspeed, which removes the hazards.
Qantas has issued a notice warning pilots that it was preferable to keep autopilot engaged when a plane is in risk of going too fast, and that disengaging or overriding autopilot could lead to a worse outcome than the overspeeding itself.
“The report stated that the pilot in command performed his duties consistent with his training and expertise,” a Qantas spokeswoman said.
Boeing has told the ATSB it was considering changing its 737 training manual after reviewing the draft report.
Reporter for The Age
Morning & Afternoon Newsletter