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B738, en-route, west of Canberra Australia, 2017
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|On 13 March 2017, the crew of a Boeing 737-800 responded to an increase in indicated airspeed towards Vmo after changing the FMS mode during a high speed descent in a way that more abruptly disconnected the autopilot than they were anticipating which resulted in significant injuries to two of the cabin crew. The Investigation found that the operator’s customary crew response to an overspeed risk at the airline concerned was undocumented in either airline or aircraft manufacturer procedures and had not been considered when an autopilot modification had been designed and implemented.|
|Actual or Potential
|Human Factors, Loss of Control|
|Type of Flight||Public Transport (Passenger)|
|Origin||Perth International Airport|
|Intended Destination||Canberra International Airport|
|Take off Commenced||Yes|
|ENR / APR|
|Origin||Perth International Airport|
|Destination||Canberra International Airport|
|Approx.||50nm west of Canberra|
|Tag(s)||Inappropriate crew response (automatics)|
|Tag(s)||Uncommanded AP disconnect,|
Aircraft Flight Path Control Error
|Tag(s)||Cabin Crew Incapacitation|
|Damage or injury||Yes|
|Causal Factor Group(s)|
On 13 March 2017, a Boeing 737-800 (VH-VZZ) being operated by Qantas on a scheduled domestic passenger flight from Perth to Canberra was subjected to an upset which resulted in significant injury to two of the cabin crew after the flight crew responded to an increase in indicated airspeed during descent at night through FL 220. None of the 177 passengers were injured.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB) with recorded flight data from the aircraft involved available. It was noted that the First Officer had been acting as PF for the incident sector.
It was established that with the aircraft level at FL 310, ATC had cleared the aircraft to descend and asked the crew to maintain maximum speed and expect runway 17 at Canberra. Descent was commenced and instead of the standard descent speed of 280 KIAS, the speed was increased to 320 KIAS, twenty knots below Vmo. Soon after beginning this descent, a runway change to 35 was notified and passing FL 280, the cabin crew were advised over the PA to prepare for landing. This meant that it could be expected that the fasten seat belt signs would be switched on after a further ten minutes or so and that by this time all loose items in the cabin would need to have been secured. Approaching FL 230, just 5½ minutes after the “prepare for landing” PA had been given and with just “minor” turbulence evident, the seat belt signs were switched on and passengers and crew advised by PA to be seated with their seat belts fastened.
It was noted that Qantas’ procedures required cabin crew members to be seated within one minute of such an announcement. At this time, one of the cabin crew was on her way back to the rear of the cabin and reported that “the turbulence was such that she needed to hold onto the overhead lockers as she walked along the aisle”. She reported reaching the rear galley, closing a stowage latch and securing a rubbish bin prior to securing herself in her crew seat. Then, about 30 seconds after the ‘fasten seat belt’ signs had been switched on, recorded data shows that the wind direction had changed by about 80° and approximately doubled the head wind component. This led to the indicated airspeed beginning to increase and in response the PF applied control column backpressure to avoid exceeding Vmo which, although not a documented Qantas response, was found to be a widely-used one amongst Qantas pilots on type.
Almost immediately, the back pressure input was followed by an abrupt AP disconnection, upon which the aircraft nose suddenly dropped and was then increased with a maximum of +2.2g being recorded during the recovery. The Captain subsequently stated that as both pilots were checking the instrument approach for the new runway, they had heard the AP disconnect tone and had looked up and seen the airspeed continuing to increase towards Vmo prompting the First Officer’s manual pitch up. FDR data showed that indicated airspeed peaked at 339.5 KIAS within a second before the manually-commanded increase in pitch took effect.
The member of cabin crew who was about to take her seat in the rear galley reported feeling the floor suddenly drop and then quickly come up again and she fell, breaking her leg. Another member of the cabin crew was still standing in the forward galley and about to take her seat and the sudden vertical disturbance to the aircraft flight path led to her falling too and hitting her head on a galley cart resulting in injury to her knees, back and neck.
The flight crew were advised of the injuries by interphone and as “the turbulence had subsided”, the flight crew turned off the seat belt signs and first aid was administered. Within less than 2 minutes, the indicated speed had returned to normal and the AP was re-engaged. A landing at Canberra followed just over 20 minutes later with the member of cabin crew with the broken leg lying on the cabin floor. Both injured cabin crew were transported to hospital for treatment and one was admitted. There was no damage to the aircraft and no passengers or any of the other 5 cabin crew were injured. ATC were alerted to the turbulence experienced and advised that there had been no other reports of turbulence in the area.
Qantas and the pilots involved advised the Investigation that it was “common practice” for its 737 flight crews to respond to an impending overspeed by applying rearward control column force to override the autopilot in order to de-clutch (but not disconnect) the AP and allow the pilot to temporarily reset the pitch manually by using “Control Wheel Steering - CWS”. However, after being appraised of the event under investigation, Boeing advised that they were considering making a revision to the 737 FCTM to add a preferred response to impending overspeed after having recognised that in some situations, A/T response would not be sufficient to prevent short term overspeeds. This procedure would involve leaving the AP engaged and slowly, partially and briefly selecting the speedbrakes to achieve the required reduction in airspeed before retracting them, again slowly, once below Vmo/Mmo.
It was then found that Qantas 737 APs were progressively being modified under guidance from Boeing to remove the automatic reversion to CWS in response to column back pressure because of a concern that “flight crews may not recognise or correctly interpret the autoflight system automatic transition to the Control Wheel Steering mode”. A consequence of this modification, which had been actioned on the aircraft under investigation, had been to produce a sharper elevator response to permanent AP disengagement by control column movement than that which had characterised the previous temporary reversion to CWS since the latter had “provided a smoothed resistance as a function of the pitch rate”.
Further review of the FDR data showed that the indicated airspeed had begun to increase after ‘Level Change’ FMS mode had been selected at the same time as the seat belt signs had been switched on - which had also coincided with the end of the 30 second period of “minor turbulence” which appeared to have been the reason they were selected on earlier than originally anticipated.
It was noted that since the widely adopted and trained CWS response to any overspeed risk was not a documented Qantas or Boeing procedure, the potential effect on this practice was not considered when the AP modification being implemented at the time of the investigated event was developed.
The Findings of the Investigation were formally documented as follows:
- The increase in headwind while VH-VZZ was making a routine high speed descent at 320 knots resulted in the airspeed increasing towards the aircraft’s maximum allowable speed.
- The pilot flying applied a control column input to prevent an overspeed, which resulted in the autopilot unexpectedly disengaging. The consequent change of pitch and g-loading led to two cabin crew suffering injuries.
- The aircraft’s autopilot had been modified such that, if sufficient control column back pressure was applied, the autopilot would disengage rather than revert to the Control Wheel Steering (CWS) mode. Autopilot disengagement resulted in larger elevator and pitch responses than those associated with reversion to CWS mode.
The Board concluded that the Investigation had provided the opportunity to define a Safety Message which was as follows:
- “Although there was no expectation of varying wind conditions during the descent on this occasion, this occurrence highlights the increased risk of overspeed when operating with a reduced margin below Vmo. The intervention by the pilot flying to prevent the impending overspeed was understandable, and consistent with previous responses of other flight crew in similar situations. However, as detailed in the Qantas Safety Information Notice, when faced with an impending overspeed, abrupt pitch changes may have more adverse consequences than an overspeed event. The manufacturer’s preferred use of speedbrakes to manage increasing airspeed, removes the hazard associated with abrupt pitch changes.”
The Final Report was released on 14 March 2018. No Safety Recommendations were issued.