Investigators have determined that a Qantas Boeing 747-400’s stick-shaker activated when the aircraft entered a holding pattern but slowed to a speed much lower than that required.
The aircraft, which had departed Melbourne, had been descending for Hong Kong on 7 April 2017.
Its crew had expected to enter a hold at around 15,000-16,000ft over a waypoint designated BETTY, and had used the flight-management computer to calculated the best target speed to maintain the hold.
The calculation returned a speed of 223kt and the crew, having verified the figure, opted to enter a target of 225kt above 15,000ft.
But the descent profile towards BETTY was higher than expected and this meant the aircraft entered the hold at 22,000ft.
While the crew commanded the 747 to hold at this altitude, they did not adjust the target speed for the higher-than-expected level.
“The flight crew later reported that they were not aware of a higher speed requirement for holding above [20,000ft],” says the Australian Transport Safety Bureau.
As the aircraft descended, with idle thrust, its speed declined as it crossed the BETTY waypoint. The aircraft slowed to 220kt as it banked into the hold – below both the selected target speed of 225kt and the minimum manoeuvring speed of 223kt.
Investigators found that the 747 started experiencing pre-stall buffeting and, according to the crew, the stick-shaker began to activate.
The captain responded by disconnecting the autopilot and pushing the control column forward. While the captain reduced the bank angle, and advanced the thrust levers, he did not follow the recommended procedure to roll the wings level – to avoid straying out of the hold – and did not disconnect the autothrottle.
As a result of the captain’s actions, the aircraft accelerated slightly, the buffeting stopped and the aircraft continued descending.
But a few seconds later the captain pulled back on the control column in order to prevent further descent from the hold altitude. The stick-shaker activated and again the captain pushed the column forward.
This cycle of descent, followed by a pull on the column, and stick-shaker activation occurred again and the captain subsequently disengaged the autothrottle, increased thrust, and set a selected speed of 252kt.
Investigators state that the 747’s oscillations reduced and the aircraft accelerated, levelling at 21,400ft as its speed increased through 238kt.
The crew requested a lower altitude from air traffic control and the flight was cleared to descend to 21,000ft – although the aircraft momentarily attempted to climb back to 22,000ft because the crew initially did not activate the altitude selector to direct the jet to 21,000ft.
It entered the BETTY hold again at 21,000ft. The inquiry found there was no loss of separation with other traffic.
But the pilot-induced oscillations which resulted from the loss of speed and altitude had injured some occupants of the aircraft, including a cabin crew member who was thrown against the ceiling before falling onto seats.
Damage to lavatory fittings, the inquiry believes, triggered a series of smoke alarms which prompted the crew to take precautionary measures and request a priority landing at Hong Kong.
Analysis determined that, while the stick-shaker engaged, the aircraft did not enter a stall. The inquiry adds that, although relevant speeds were displayed to the crew, the pilots did not detect that the 747’s speed had fallen below the thresholds “as their attention was on other operational tasks”.
Four cabin crew members and two passengers – from among the 347 passengers and 17 crew on board – suffered minor injuries during the incident and the cabin of the jet (VH-OJU) sustained minor damage.