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Section 21 (2) of the Transport Safety Investigation Act 2003 (TSI Act) empowers the 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 statement is published as a report in accordance with section 25 of the TSI Act, capturing information from the investigation up to the time of discontinuance. |
Overview of the investigation
The ATSB commenced an investigation into an engine pod strike involving a Singapore Airlines Cargo Boeing 747-412F freighter, registered 9V-SFO, which occurred at Sydney Airport on 28 November 2019.
The flight crew were conducting an approach to runway 34L at night. The automatic terminal information service (ATIS) stated the wind was 040° at 15 kt (crosswind of 12 kt), and the latest weather observation reported the wind as 040° at 22 kt with gusts up to 32 kt. The flight crew stated they briefed for the threat of strong, gusty crosswinds. The captain was the pilot handling for the landing.
During the approach, the flight crew recalled experiencing moderate turbulence and undershoot shear below 300 ft. When passing 200 ft, the airspeed rapidly trended below the minimum approach speed, the first officer called ‘speed’ and the captain recovered the target speed.
As the aircraft entered the flare it drifted right of centreline, followed by a sudden uncommanded roll to the left. In response, the flight crew initiated a rejected landing manoeuvre, during which the number 1 engine pod struck the ground. After a second approach, the aircraft landed without further incident.
Engineering inspections identified damage to the number 1 engine fan cowl and thrust reverser skin panel. Following maintenance action, the aircraft returned to Singapore where it underwent further inspections. As a result, damage to the number 1 engine main gear box was identified as being beyond the serviceable limit and the engine was removed from service.
As part of its investigation, the ATSB:
- interviewed the flight crew and examined their training records
- analysed recorded data from the aircraft’s flight data recorder (FDR) and cockpit voice recorder (CVR)
- reviewed Boeing’s flight data analysis and other documents
- reviewed air traffic control recordings
- examined engineering reports, documents, manuals and correspondence relating to the operator’s Boeing 747 operation
- reviewed Bureau of Meteorology (BoM) weather forecasts and analysis
- conducted significant analysis of potential wake turbulence
- reviewed data related to the 2020 low-level windshear alert system trial at Sydney Airport
- reviewed other investigations and references where similar themes had been explored.
Analysis of recorded data indicated that the aircraft was responding correctly to flight control inputs made by the pilot flying and that the inputs were appropriate for the environmental conditions. Research and analysis undertaken by the ATSB demonstrated that wake turbulence was not a factor.
Prior to the occurrence no windshear was forecast, broadcast by air traffic controllers, or detected by aircraft or ground systems. However, during the approach, the aircraft was affected by moderate undershoot shear and wind gusts during the landing. About 4 minutes after the occurrence, a Boeing 737 landed on runway 34R. That flight crew advised air traffic controllers they lost about 15 kt of airspeed during the flare due to moderate undershoot shear. Controllers subsequently broadcast to all aircraft there was moderate undershoot shear on runway 34R.
Following the incident, Singapore Airlines made minor procedural changes and enhanced its simulator training program by requiring demonstrated competency in all crosswind landing techniques.
Low-level windshear alerting systems
At low altitudes (below 1,000 ft) during critical stages of landing and take-off, windshear can present a significant hazard to aircraft. Low-level turbulence and windshear can be caused by many processes; however, regardless of the cause, a well-designed low-level windshear alerting system is capable of detecting operationally-significant windshear and turbulence in near real time.
In 2001, there was a windshear event involving a Boeing 737 at Brisbane Airport (ATSB investigation 200100213). The ATSB recommended that BoM expedite the development, testing, and installation of advanced weather radar systems to detect hazardous windshear in high-risk airport terminal areas. In response, the BoM advised that it would continue to derive the maximum operational utility from existing and future Doppler radar systems, as the cost of specialised radar systems was high and they were not fully suitable for general weather work. Additionally, BoM advised it would maintain knowledge of international research and development of experimental low-level windshear alert systems.
In 2007, there was a micro-burst event involving a Boeing 747 at Sydney Airport (ATSB investigation AO-2007-001). The investigation raised a safety issue, which stated that there was no ground-based automatic low-level windshear warning system at Sydney Airport. At that time, BoM and Airservices Australia initiated scoping activities and a proposed system was identified. A risk assessment identified that, with existing controls (including onboard windshear detection systems), the risk level was within the broadly acceptable range.
In 2013, meetings were held with industry to discuss the proposed solution. The airline participants agreed that unexpectedly encountering windshear did not pose a significant threat to aviation at Sydney Airport. The substantial expense of the proposed system, and environmental challenges associated with siting additional anemometers required by that system, were noted. Overall, it was concluded that the benefits of continuing with the proposed system were not sufficient to justify the cost at that time.
In 2020, Sydney Airport identified that the cost of a light detection and ranging (LIDAR) system had reduced significantly and did not have the same environmental impact. A LIDAR working group was established involving Sydney Airport, BoM, Airservices Australia, airlines and a pilot’s union to consider the implementation of the system, particularly given the benefits over other systems. In general, a ground-based LIDAR system is much better at detecting turbulence in clear air, whereas onboard systems are more effective for turbulence within detectable precipitation, such as rain. BoM advised the ATSB:
Scanning doppler lidar is able to observe turbulence in the atmosphere at smaller scales than is possible with existing anemometer and wind profiler assets at Sydney. As a result, automated alerts derived from the lidar data are able to capture transient and small scale turbulent features that are, by design, not typically represented in current aeronautical meteorology products such as the TAF and WS Warning. Integrating lidar observations into the turbulence forecast process for Sydney airport is therefore likely to result in improved turbulence and low level wind shear forecast quality.
A trial of a low-level windshear alerting system using a scanning doppler LIDAR system was conducted at Sydney Airport. The trial proved the effectiveness of the system to enhance awareness of turbulence and low-level wind shear when it occurs, and the working group is now considering implementation issues.
The ATSB strongly encourages the use of LIDAR systems at airports such as Sydney Airport where turbulence-related events have been known to occur.
Reasons for the discontinuation
Based on a review of the available evidence, the ATSB considered it was unlikely that further investigation would identify any new systemic safety issues or important safety lessons. Consequently, the ATSB has discontinued this investigation.
The evidence collected during this investigation remains available to be used in future investigations or safety studies. The ATSB will also monitor for any similar occurrences that may indicate a need to undertake a further safety investigation.