Section 21 (2) of the Transport Safety Investigation Act 2003 (TSI Act) empowers the Australian Transport Safety Bureau (ATSB) to discontinue an investigation into a transport safety matter at any time. Section 21 (3) of the TSI Act requires the ATSB to publish a statement setting out the reasons for discontinuing an investigation.
The ATSB commenced an investigation into a flight control system event that occurred on 25 May 2017 involving a Boeing Company 737-800, registered VH-YIJ and operated by Virgin Australia International, on a flight from Brisbane, Queensland to Wellington, New Zealand.
The captain was the pilot flying, and he was conducting the night arrival into Wellington. The weather conditions were fine, and the descent (below flight level 250) and approach were briefed to be flown with the autopilot and autothrottle disengaged for practice.
During the approach to runway 34, the flight crew progressively selected flaps 1 then flaps 15. The landing gear was selected down and then flaps 25. The right flap moved to flap 25 but the left flap initially remained at flap 15, before moving very slowly to flap 25, which was not initially detected by the flight crew. Flap 40 was then selected, however, the flaps remained in the 25 and 15+ positions.
While carrying out the landing checklist, the flight crew detected the flap asymmetry. The flight crew attempted to rectify the problem with various flap lever selections, which were unsuccessful. Approaching 1,000 ft, the aircraft did not meet the operator’s stable approach criteria, so the flight crew initiated a missed approach.
During the missed approach, the left flap slowly extended to flap 25, correcting the initial ‘Trailing Edge Flap Asymmetry’ to a ‘Trailing Edge Flap Disagree’ condition. The aircraft, still being manually flown, subsequently climbed above the cleared altitude of 5,000 ft (reaching 5,340 ft) and the flap limit speed was marginally exceeded on two occasions. The flight crew positioned the aircraft into a holding pattern, completed the ‘After Takeoff’ checklist and ‘Trailing Edge Flap Disagree’ non-normal checklist and briefed for a second approach to runway 34 with flaps 25.
Prior to leaving the holding pattern, the captain briefed the cabin supervisor about the situation. However, the format of that briefing was the same as what the cabin supervisor would expect for an emergency. As a result, the cabin supervisor perceived that the cabin needed to be prepared for an emergency landing, which was not the captain’s intention.
A second approach was conducted with the autopilot and autothrottle engaged to 136 ft. The aircraft landed without further incident.
Engineers later performed the required aircraft inspections. They could not reproduce the flap fault, however, replaced the left-hand trailing edge flap position transmitter as a precaution.
The ATSB obtained the operator’s investigation report, and interviewed the flight crew and the cabin supervisor. The ATSB also obtained data from the aircraft’s flight data recorder, aircraft maintenance records and relevant sections of the operator’s operations manual. Based on its review of this information, the ATSB concluded that the operator had conducted a detailed investigation and it was unlikely that further ATSB investigation would identify any systemic safety issues.
The ATSB noted that, although there were flight crew errors made during the approach and the subsequent missed approach, the missed approach was conducted at an appropriate time. In addition, while the cabin crew conducted the cabin preparation drill for a non-normal landing when it was not required, doing so was an example of the operation ‘failing safe’ rather than increasing risk.
The operator has subsequently used this incident as a basis for some recurrent training for its flight crew and cabin crew. Consequently, the ATSB has discontinued this investigation.