Qantas B738 near Canberra on Mar 13th 2017, turbulence and changed autopilot response injures two flight attendants

Last Update: March 14, 2018 / 14:36:01 GMT/Zulu time

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Photo of Qantas VH-VZZ, Boeing 737-800 — Qantas VH-VZZ, Boeing 737-800 (Photo credit: Peter_Mackey thanks for 800,000 views / Flickr / License: CC by-nd)

Incident Facts

Date of incident
Mar 13, 2017

Classification
Accident

Cause
Turbulence

Airline
Qantas

Flight number
QF-718

Departure
Perth, Australia

Destination
Canberra, Australia

Aircraft Registration
VH-VZZ

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Qantas Boeing 737-800, registration VH-VZZ performing flight QF-718 from Perth,WA to Canberra,AC (Australia), was descending through FL240 towards Canberra at about 22:00L (11:00Z) when the aircraft encountered severe turbulence prompting the flight crew to illuminate the fasten seat belt signs. While returning to their seats two flight attendants received injuries. The aircraft continued for a safe landing on runway 35 about 28 minutes later.

The Australian TSB reported one flight attendant received a serious injury, the other a minor injury. The occurrence was rated an accident and is being investigated by the ATSB.

The airline reported one flight attendant broke her ankle during the turbulence encounter.

On Mar 14th 2018 the ATSB released their final report concluding the probable causes of the accident were:

- The increase in headwind while VH-VZZ was making a routine high speed descent at 320 kt 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 ATSB added following safety message:

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 ATSB reported that the crew had been instructed to maintain maximum speed during the descent towards Canberra. The crew therefore maintained 320 KIAS (instead of the usual 280 KIAS) for the descent with Vmo being 340 KIAS. Shortly after leaving FL310 light turbulence was noticed. Descending through FL280 the crew announced that cabin crew should prepare for landing (which is a 10 minutes warning to fasten all loose items until the fast seat belt sign would be turned on). Descending through FL270 the aircraft was maintaining 320 KIAS. The turbulence began to increase, the flight crew illuminted the fasten seat belt signs in response expecting any cabin crew would be seated within one minute. A flight attendant was on her way to the back of the cabin when she needed to hold on due to turbulence. She then reached the aft galley, closed a stowage latch and secured a rubbish bin. The aircraft descended through FL220 when wind changed direction by about 80 degrees resulting in a head wind speed increase from 15 to 32 knots, as result the IAS increased to 325 KIAS. The first officer, pilot flying, applied back pressure on the yoke to avoid exceeding the maximum operating speed, about one second after he had applied his maximum control force the autopilot disconnected and the G-Load peaked at +2.2G, the pitch angle changed by 3.87 degrees within one second and the airspeed increased to 339.5 KIAS. The flight attendant in the aft galley felt the floor drop away under her feet and then come up again. She received a leg fracture. Another flight attendant in the forward galley fell due to the aircraft movement and hit her head against a trolley. Both injured crew members were taken to a hospital after landing.

The ATSB analysed:

The Qantas internal investigation of this occurrence found that in the past 6 years there had been 47 previous overspeed occurrences involving its Boeing 737-800 aircraft, of which 20 had been managed via manual intervention (that is, overriding the autopilot into CWS-P). The internal investigation also found that it was common practice among its Boeing B737-800 flight crews to descend at 320 kt (20 kt below the VMO) when cleared by ATC to conduct a high-speed descent.

The common practice of flight crews to prevent an overspeed was not a documented Qantas or Boeing procedure. As a result, the potential consequence of this practice was not considered when the autopilot was modified.

Qantas also advised that the flight crew of VZZ had no information that indicated that there would be a risk of turbulence. The wind information provided in the flight plan did not show there would be a significant change in the wind direction during descent. As such, descending at 320 kt was not considered to present an increased risk.

The first officer of VZZ responded to the increase in speed towards the aircraft’s VMO consistent with his training, experience, and observations of other crew members. As VZZ had a modified autopilot, it was not possible to override it into CWS-P. Consequently, when sufficient force was applied to the control column, the autopilot disengaged. That resulted in abrupt elevator deflection and pitch change, which in turn led to the cabin crew injuries.

Metars:
YSCB 131300Z AUTO 16004KT 9999 // ////// 18/14 Q1020=
YSCB 131230Z AUTO 13008KT 9999 VCTS ////// 18/15 Q1020=
YSCB 131200Z AUTO 11008KT 9999 // ////// 18/15 Q1020=
YSCB 131130Z AUTO 09007KT 9999 // ////// 19/15 Q1020=
YSCB 131100Z AUTO 09006KT 9999 // ////// 19/14 Q1019=
YSCB 131030Z AUTO 10008KT 9999 // ////// 20/15 Q1018=
YSCB 131000Z AUTO 12007KT 090V150 9999 // ////// 20/15 Q1018=
YSCB 130930Z 10005KT 060V140 9999 FEW020 SCT040 20/15 Q1018 1730Z=
YSCB 130900Z 12014KT 9999 FEW040 SCT200 20/14 Q1016 NOSIG=