Qantas B744 near Hong Kong on Apr 7th 2017, stick shaker activation in holding pattern injures 15 passengers
Last Update: March 27, 2019 / 19:01:27 GMT/Zulu time
Australia's TSB reported 15 passengers received minor injuries when the stick shaker activated and the airframe buffeted while in a hold at FL220. The occurrence was rated a serious incident and is being investigated.
Data off the ADS-B capable transponder of the aircraft suggest the aircraft was descending to enter the hold at about 340 knots over ground on a track of 315 degrees, when descending through FL229 at 17:47L (09:47Z) the speed decayed to 290 knots over ground still on a track of 315 degrees before increasing to above 400 knots over ground in altitude fluctuations between FL214 and FL230 before levelling off at FL220 at 390 knots over ground subsequently reducing to 340 knots over ground.
On Mar 27th 2019 the ATSB released their final report concluding the probable causes of the serious incident were:
- After overwriting the hold speed in the flight monitoring computer , the flight crew did not identify the need to re-evaluate the hold speed for the higher than expected hold level.
- Prior to entering the hold, the aircraft’s speed reduced below both the selected and minimum manoeuvring speeds. The crew did not identify that the aircraft was operating below these speeds.
- The reduced speed coincided with the turn to enter the holding pattern and the level capture. These factors resulted in pre-aerodynamic stall buffeting and probable stick shaker activation.
- The pilot flying attempted to arrest the rate of descent prior to completing the approach to stall actions. The pilot monitoring did not identify and call out the incomplete approach to stall recovery actions. These combined actions led to pilot induced oscillations and further stick shaker activations.
- The operator provided flight crew with limited training and guidance in stall prevention and recovery techniques at high altitudes or with engine power above idle. (Safety issue)
- The passenger in seat 63C was not wearing a seatbelt at the time of the stick shaker activations.
Other safety factor
- The operator provided flight crew with limited training and guidance relating to the need for crew to re-evaluate their holding speed for a change in altitude (specifically above flight level 200). (Safety issue)
The ATSB reported the captain (ATPL, more than 24,000 hours total, more than 10,000 hours on type) was pilot flying, the first officer (ATPL, more tha 16,000 hours total, more than 5000 hours on type) was pilot monitoring, a second officer (CPL, more than 8000 hours total, more than 5000 hours on type) complemented the crew.
The crew anticipated to be sent into a hold at waypoint BETTY at FL150 or FL160 and prepared their FMC for this hold, selecting the hold and holding speed, the FMC computed a holding speed of 223 KIAS for FL156, the crew thus selected 225 KIAS as target hold speed. ATC "step descended" the aircraft to FL300 while nearing BETTY, which put the aircraft above the estimated descent profile. ATC subsequently cleared the aircraft to enter the hold at FL220. While capturing the altitude the aircraft's speed decreased to 222 KIAS, which was below the maneouvering speed and below the selected target speed, the autopilot rolled the aircraft into the first turn of the holding at 30 degrees bank, when according to the crew the stick shaker activated (the FDR shows no stick shaker activation at that time) and aerodynamic stall buffeting occurred.
The ATSB wrote:
After the onset of the buffeting, flight data shows the autopilot was disconnected, most likely by the captain. The captain then pushed forward on the control column to reduce the aircraft’s pitch angle and reduced the aircraft’s bank angle. Due to a desire to remain within the protected airspace of the holding pattern, the captain did not roll to wings level as recommended by the operator’s approach to stall recovery procedure. The captain also did not disconnect the autothrottle as required by the procedure, however, he manually advanced the thrust levers. Due to concerns regarding an excessive increase in pitch resulting from a large power increase he increased the engine power from about 37 per cent to about 73 per cent N1.
The first officer observed the captain’s actions and was satisfied that the appropriate actions had been undertaken. He did not identify, and therefore did not call out, that the stall recovery procedure had not been completed. As a result of the captain’s actions, the aircraft accelerated slightly, the buffeting stopped and the aircraft continued descending.
Six seconds later, at 1748:05, the aircraft descended through FL 220, the speed increased to the selected 223 kt and the thrust reduced. At the same time, the captain pulled back on the control column to increase the pitch angle to prevent further descent. Four seconds later, the stick shaker activated. In response, the captain again pushed forward on the control column to reduce the aircraft’s pitch angle and increased thrust slightly. The stick shaker deactivated and the aircraft continued descending. As the aircraft descended through FL 218, the captain pulled back on the control column to increase the pitch angle and the stick shaker again activated. In response, the captain again pushed forward on the control column to reduce pitch angle and the stick shaker deactivated. At about this time, the seatbelt sign was selected on.
Over the next nine seconds, the captain disengaged the autothrottle, increased power to greater than 90 per cent N1 and increased the selected speed to 252 kt. The oscillations reduced and the aircraft continued accelerating.
At 1748:31, the aircraft levelled off at FL 214 and the speed was increasing through 238 kt toward the selected speed. At about this time, the first officer alerted the captain that the aircraft had descended below the cleared level. In response, the captain asked the first officer to request a lower level from ATC, who immediately cleared the flight crew to descend to FL 210. The autopilot was then re-engaged in VNAV and LNAV modes with 21,000 ft in the altitude window of the mode control panel. However, as the altitude selector was not activated, 22,000 ft remained as the commanded altitude and the aircraft commenced climbing to FL 220.
As the aircraft climbed, ATC contacted the flight crew to confirm that they were descending to FL 210. The flight crew confirmed that they were descending and activated the 21,000 ft altitude selection. The aircraft then descended to FL 210 and re-joined the BETTY holding pattern. During the event, there was no loss of separation with any aircraft.
During the pilot-induced oscillations, the CSM, who was standing in the left aisle in the vicinity of rows 63 and 64, struck the cabin ceiling before falling on a seat armrest, sustaining injuries. Five other cabin crewmembers also struck the ceiling, with three sustaining injuries. A passenger located in an L5 lavatory struck the cabin ceiling landing on the lavatory seat, resulting in minor injuries and damage to the lavatory fittings. A passenger in seat 63C who did not have her seatbelt fastened, was also injured.
After the aircraft stabilised, the CSM was alerted to the injured passenger in the L5 lavatory. The CSM provided assistance to this passenger and then conducted the call back procedure. During the call back procedure, cabin crew advised the CSM of further injured passengers and cabin crewmembers.
As the aircraft tracked on the outbound leg of the holding pattern, the lavatory smoke alarm activated and the flight crew received a lavatory smoke alarm warning. The captain asked the first officer to request a priority landing from ATC. ATC immediately cleared the flight directly to Hong Kong International Airport.
The cabin crewmembers established that the smoke alarm originated at the R5 lavatories and that there was no evidence of smoke, fire or fumes. The CSM reported to the flight deck that they believed the smoke alarm to be a false alarm caused by the lavatory damage.
While approaching Hong Kong International Airport, the smoke alarm activated a further six times. The cabin crewmembers determined that these alarms were also false. The aircraft landed without further incident.
Four cabin crewmembers and two passengers received minor injuries during the incident and the aircraft cabin sustained minor damage.
The ATSB analysed:
Prior to arriving at BETTY, the VNAV profile in the flight management computer (FMC) calculated holding at between FL 150 and FL 160. When selecting a target speed, the flight crew verified the FMC provided speed by comparing the speed to the flaps up manoeuvre speed calculation. However, the flight crew training manual advised that the flaps up manoeuvring speed guaranteed at least full manoeuvre capability, to stall warning activation at low altitudes. The flight crew were not aware of the requirement to use a different speed calculation verification for altitudes above FL 200.
Had the crew recalculated the hold speed for FL 220 using the flight management computer, it would have provided a target hold speed of 240 kt. In this case, the flight crew likely did not have an adequate understanding of how the FMC calculated the target hold speed. They also did not understand the use of the best speed provided on the hold page in the FMC. Using best speed would have provided the crew with a hold speed for the actual aircraft weight, altitude and configuration at that time. Orasanu (2010) outlines that decision errors in aviation are often a result of a lack of knowledge:
"[They] typically are not slips or lapses in carrying out an intention, but errors of intention itself (Norman, 1981). The decision maker acts according to his/her understanding of the situation, and the source of error is in the decision maker’s knowledge base or in the process of reaching a decision."
The selection of an incorrect hold speed resulted in the aircraft entering the hold with a selected speed 15 kt below the required speed. Using the best speed in the FMC hold page or recalculating the hold speed using the FMC for the higher level would have resulted in the use of a speed which provided sufficient margin to prevent a stick shaker activation.
Neither the operator or aircraft manufacturer provided procedures or guidance which stated that a hold speed was required to be re-evaluated for a change in hold level when a speed was selected in the FMC during the planning stage of a descent. Therefore, the flight crew did not have the requisite knowledge to identify the need to re-evaluate the selected speed.
The ATSB analysed with respect to the aerodynamic stall buffeting and probable stick shaker activation:
As the aircraft turned to enter the holding pattern, the bank angle increased and the aircraft began to transition from descent to level flight. The effects of these manoeuvres combined to increase the vertical acceleration and wing angle of attack. The increasing angle of attack initiated preaerodynamic stall buffeting.
At this time, although it was not recorded on the flight data, all flight crewmembers reported a stick shaker activation. The recorded angle of attack also indicated that a stick shaker likely occurred. The manufacturer advised that an activation of less than one second in duration may occur without being captured in the flight recorder data.
This article is published under license from Avherald.com. © of text by Avherald.com.
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