Description

On 7 April 2017, when a Boeing 747-400 (VH-OJU) being operated by Qantas Airways on a scheduled international passenger flight from Melbourne to Hong Kong with an augmenting crew member present on the flight deck entered a holding pattern part way through descent in day VMC, a series of stall protection system activations occurred during an upset involving multiple oscillations in pitch and vertical acceleration. During this upset, four of the cabin crew and two unsecured passengers sustained minor injuries after striking the cabin ceiling and furnishings. A series of toilet smoke warnings which began immediately after the upset were confirmed to be false and were later attributed to damage to the toilet cubicle which had occurred during the event.

Investigation

An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). Relevant recorded flight data was available from the FDR and the QAR. It was noted that the Captain, who had been PF at the time of the event under investigation, had a total of 24,556 hours flying experience of which over 10,000 hours were on type. The First Officer had a total of 16,400 hours flying experience of which over 5,000 hours were on type and the Second Officer had a total of over 8,555 hours flying experience of which over 5,000 hours were on type. The Second Officer was occupying a supernumerary crew seat in the flight deck at the time of the upset and passively monitoring proceedings.

It was established that as the descent towards destination began, the crew had anticipated that they would be required to take up the hold at waypoint ‘BETTY’ at around FL 150/FL 160 and therefore input a target holding speed appropriate to that altitude of 225 KIAS into the FMC. However, as the aircraft got nearer to the ‘BETTY’ waypoint, ATC instructed it to hold there at FL220 but the crew did not then adjust the target hold speed to correspond to this higher-than-expected holding level - and subsequently stated that “they were not aware of a higher speed requirement for holding above FL 200”.

As the descent towards the hold continued, airspeed reduced below both the 225 KIAS target speed and the PFD-indicated minimum manoeuvring speed. At this point, the Captain had been reviewing the destination approach plates whilst the First Officer was looking outside and “attempting to identify aircraft traffic in the vicinity of the holding pattern”. Neither of them noticed the reducing speed and the Second Officer subsequently stated that he had seen the speed reducing towards 225 KIAS but not below it.

FDR data showed that the aircraft crossed the ‘BETTY’ waypoint at 222 KCAS whilst descending through FL 227 with idle thrust set and then began a right turn to commence the hold. As the turn continued, the aircraft descended through FL 222 and, as the AP prepared to level the aircraft at FL 220 and the speed reduced to 220 KCAS, the bank angle increased to 32° and pre-stall buffeting was followed by a brief stick shaker activation. The AP was disconnected, probably by the Captain, who then made a forward movement of the control column and slackened the bank angle. However, because of a desire to stay within the protected airspace of the holding pattern, he did not roll to wings level as per the applicable stall recovery procedure and neither did he disconnect the A/T as required by the procedure, although he did advance the thrust to just over 70% N1. The First Officer observed this response and subsequently stated that he had been “satisfied that the appropriate actions had been undertaken” although he had not identified, and therefore not called out, that the stall recovery procedure had not been completed.

The result of the Captain’s actions was a very slight airspeed increase and cessation of the buffet as the descent continued. As the aircraft descended through its cleared level of FL 220, the Captain moved the control column aft in order to prevent further descent and four seconds later, the stick shaker again activated briefly. The Captain again responded with a forward movement of the control column and a slight increase in thrust which stopped the stick shaker. The aircraft continued descending but as it passed FL218, the Captain again moved the control column aft and this caused a third stick shaker activation which he stopped with another forward movement of the control column. At about this time, it was noted that the seatbelt sign was selected on.

The FDR data then showed that over the ensuing nine seconds, the Captain had disengaged the A/T, increased thrust to above 90% N1 and changed the selected speed to 252 KIAS. This resulted in the oscillations reducing, the speed increasing and the aircraft levelling off at FL 214 as the speed continued increasing through 238 KCAS towards the new selected speed of 252 KIAS. At about this time, the First Officer alerted the Captain to the significant decent below the cleared level and on being informed, ATC issued an immediate re-clearance to FL210. The AP was then re-engaged in VNAV/LNAV modes with 21,000 feet set on the MCP - but as the altitude selector had not been activated, 22,000 feet remained as the target altitude and the aircraft began a climb back to FL 220 which resulted in ATC asking for confirmation that the aircraft was indeed descending to FL 210. The crew responded that they were and re-set the correct 21,000 ft target altitude as they “re-joined the BETTY holding pattern”. The Investigation subsequently found that hold entry had occurred with a selected speed 15 knots below that required.

It was found that although no actual loss of prescribed separation had occurred during the upset, the sequence of “pilot-induced oscillations” had resulted in six of the cabin crew, including the CSM, who were routinely preparing the cabin for arrival, striking the cabin ceiling before falling back into seats or onto the floor, with three of them sustaining minor injury as a result. A passenger in one of the rear lavatory compartments also struck the cabin ceiling which caused minor injury and damage to the compartment fittings and another unsecured passenger towards the rear of the passenger cabin was also injured. Vertical acceleration during the upset was recorded as varying within the range 1.45g to 0.09g.

Once the aircraft trajectory had stabilised on the (first) outbound leg of the holding pattern and the CSM was coordinating the cabin response, a lavatory smoke warning was activated on the flight deck and the crew asked ATC for a “priority landing”. The cabin crew were able to confirm that this warning had originated from one of the rear lavatory compartments and the CSM then reported to the flight deck on the basis of an inspection of this compartment, the warning appeared to be false and was suspected to be a result of damage to the compartment during the upset. During the remainder of the flight, there were six more lavatory smoke warnings which the cabin crew determined were also false and the remainder of the flight was completed without further event.

The analysis made by the Investigation focused on the Captain’s repeated failure to follow the applicable stall recovery procedure - which resulted in recovery not being achieved until the fourth oscillation and after an upset which was sufficient to cause occupant injuries. It was also concerned at the First Officer’s failure to identify and call out the Captain’s incomplete actions as required by the stall recovery procedure. However, it also noted that pilot simulator training for stall recovery at Qantas did not include any scenarios that approximated the conditions experienced in the investigated event i.e. stick shaker activation whilst manoeuvring at altitude. It was noted that Qantas’ own investigation after the event had drawn similar conclusions, having concluded that stick shaker recovery training was being conducted using unrealistic scenarios which provided “limited exposure to the complexity of the required recovery actions at altitude in real life scenarios” and that adopting more realistic training scenarios was likely to provide increased exposure to “the relationship between control column movement and true airspeed” thereby reducing the risk of “over-controlling events brought about by what was described as startle effect.

The Investigation also noted a 2013 ATSB Research Report ‘Stall Warnings in high capacity aircraft - The Australian context’ which whilst finding that stall warnings occurring in normal operations were normally low risk events which are effectively managed, the avoidance of higher risk stall warning events requires that pilots remain aware of angle of attack and airspeed at all times.

In respect of the crew’s failure to re-evaluate the appropriate speed for a hold conducted at a higher level than initially programmed, the flight crew stated that “in practice they used the flaps up manoeuvring speed and then added an arbitrary buffer when selecting a hold speed”. This was not only contrary to available FCTM guidance but was also not recommended by any other operator or included in any manufacturer guidance on the procedure or how to calculate the appropriate buffer size. The FCTM content provided a way to approximate holding speeds pending the look up of more accurate ones in the QRH which in the case of holding speeds above FL200, stated that “recommended holding speeds can be approximated by adding 100 knots to the calculated flaps 30 landing reference speed”. This method would have produced an approximate holding speed of 243 knots. It was also found that neither pilot was “aware of the function or use of the best speed in the hold page of the FMC” and that Qantas had no documented training exercises of a holding pattern conducted above FL 200 and holds were usually linked to the lower levels likely prior to commencement of an approach.

The Investigation Findings included the formal documentation of six Contributing Factors as follows:

• 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.

One other Safety Factor was also identified as:

• 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]

Safety Action taken by Qantas Airways whilst the Investigation was in progress was noted to have included:

• commencing the retraining of all their pilots in stall warning recovery scenarios using amended recurrent training scenarios which introduced more complex stall warning recovery events.
• amending the content of their Boeing 747-400 FCTM relating to hold speed selection to provide enhanced holding pattern information and updating ground school training to ensure it adequately covered holding pattern training.
• reviewing crew training and guidance provided for pilots of other Boeing aircraft types in its fleet (the 787 and the 737) and making changes to incorporate more complex stall warning recovery events as well as amending the content of the 787 and 737 FCTMs on hold speed selection.

These actions were collectively considered to have effectively addressed the Safety Issues identified by the Investigation.

The Final Report was released on 27 March 2019. No Safety Recommendations were made.

Related Articles

• Stall
• Aerodynamic Stall Awareness and Avoidance
• Stall Warning Systems
• Recovery from Unusual Aircraft Attitudes
• Holding Pattern
• Cross-checking Process
• Augmented Flight Crew

Further Reading

• Stall warnings in high capacity aircraft: The Australian context, ATSB Australia, Nov 2013