Description

On 13 October 2010, a Boeing 717-200 being operated by Cobham Aviation Services Australia for QantasLink on a scheduled passenger flight from Perth to Kalgoorlie Western Australia carried out two consecutive approaches at destination in day Visual Meteorological Conditions (VMC) which resulted in stick shaker activations and subsequent go arounds. A third approach at a higher indicated airspeed was uneventful and continued to a landing. There were no abrupt manoeuvres and none of the 102 occupants were injured.

Investigation

An Investigation was carried out by the Australian Transport Safety Bureau (ATSB) to establish the cause and consequences of the stall protection system activations.

It was noted that the aircraft commander had been PM for the incident flight until the third successful approach when he took over as PF and flew the approach approximately 15 knots faster than the previous approaches. ‘Control difficulties’ were reported to have been experienced on both of the first two approaches and attributed to turbulence. The First Officer stated that during the initial approach, the pitch limit indicator on the PFD was “bouncing down” and the red tape on the PFD speed scale which indicates the speed at which the stick shaker activates was “bouncing up", “constantly overriding the autothrottles”. The second approach was flown with an additional 5 knots added to the calculated approach speed and with bank angle limited to 20° but there had still been similar control difficulties. At no time during or after completion of the flight did the crew apparently realise the source of the handling difficulties they had encountered.

It was found that the stick shaker activations had in each case been generated by correct activation of the stall protection system and at indicated airspeeds above the approach reference speed being used. The reason for this was that the estimated landing weight in the FMS was 9.5 tonnes less than the actual aircraft weight. This erroneous figure was found to have been a direct consequence of the pre flight actions of the flight crew when the aircraft commander had read out the APS (aircraft prepared for service including operating crew but not fuel or payload) weight as the figure for the First Officer to enter into the FMS as the ZFW. This error was not detected during subsequent procedural cross checking and so all performance weights generated by the FMS were consequently wrong. These erroneous weights included the TOW used, but it was reported by the crew that there had been no resulting operational consequences until the approach at destination.

It was calculated during the course of the Investigation that the effect of the data entry error had been to bring the approach reference speed used to within 15 knots of the straight and level stall speed and to within 10 knots of the corresponding stick shaker activation speed.

The similarity between both the systemic and human factors origins of the investigated event and those of many previous incidents detailed in the ATSBs January 2011 Research Report entitled: Take-off performance calculation and entry errors: A global perspective was noted by the Investigation.

An examination of DFDR data showed that the flight crew had not followed the prescribed stall recovery procedure on either approach and neither did they perform the immediate go-around which is prescribed following recovery. It was considered that “not following the prescribed stall recovery procedure increased the risk of the aircraft becoming aerodynamically stalled”. The Investigation found that although stickshaker and stall recovery training had been provided as part of the initial licence endorsement of the aircraft type, there was no provision for corresponding recurrency training to validate their ongoing competency.

The Findings of the Investigation included the identification of four ‘Safety Issues’ as follows:

• The presentation on the aircraft load sheet of the zero fuel weight immediately below the operating weight increased the risk of flight crew selecting the inappropriate figure for flight management system data entry.

• The operator's procedure for confirming the validity of the flight management system generated take-off weight did not place sufficient emphasis on the check against the load sheet.

• The operator’s procedures did not include a validation check of the landing weight generated by the flight management system, which resulted in a lack of assurance that the approach and landing speeds were valid.

• The operator’s recurrent training programs did not address the recovery from a stall or stickshaker activation such that the ongoing competency of their flight crew was not assured.

The Final Report of the Investigation: Aviation Occurrence AO-2010-081 was published on 9 February 2012. No Safety Recommendations were made but safety action intended or taken by the Operator was noted.

Further Reading

• Aerodynamic Stall Awareness and Avoidance
• Takeoff Weight Entry Error and Fatigue (OGHFA SE)
• Aircraft Load and Trim
• Cross-checking Process
• Human Error Types