What happened
On 17 October 2011, a Singapore Airlines Boeing 777-300, registration 9V-SYH, was operating a scheduled passenger flight from Shanghai to Singapore. While cruising at 35,000 feet, the flight crew identified a weather cell on their radar and requested clearance from Air Traffic Control to deviate from their original track to avoid the clouds. The crew executed a deviation of approximately 4.5 nautical miles to the left of the original path.
As the aircraft passed the weather cell, the crew observed light radar returns and initiated a further turn to increase separation. During this maneuver, the aircraft entered a zone of severe turbulence that lasted for approximately eight seconds. The encounter caused the aircraft to lose about 100 feet in altitude, with vertical accelerations fluctuating between 1.66 g and -0.27 g. Because the flight had been smooth during the initial deviation, the crew had not yet activated the seat belt sign.
The investigation
The investigation focused on the effectiveness of the aircraft's weather radar and the crew's decision-making regarding weather avoidance. Investigators examined the Honeywell RDR 4B radar system installed on 9V-SYH. It was determined that this specific radar variant required manual antenna tilt adjustments to assess weather intensity. At the time of the encounter, the radar was unable to detect the portion of the weather cell below 20,000 feet, leading the crew to believe the cell was less severe than it actually was.
In contrast, the investigation noted that other aircraft in the same fleet equipped with the more advanced RDR 4000 system were able to identify a much more severe weather cluster ahead. The investigation also reviewed the injuries, noting that 21 persons were injured, including 3 flight crew, 5 cabin crew, and 13 passengers. One passenger sustained a simple rib fracture. Most injuries occurred in the economy class section, where several passengers were unseated or moving through the aisle at the time of the turbulence.
Findings
- The primary cause of the encounter was the inability of the RDR 4B radar system to provide an accurate vertical profile of the weather cell, as it could not detect precipitation below 20,000 feet.
- The flight crew's assessment of the weather severity was based on incomplete radar data, leading to an insufficient deviation distance.
- The seat belt sign was not activated prior to the encounter because the flight had been smooth during the initial deviation.
- The crew did not issue a Pilot Report (PIREP) to Air Traffic Control regarding the turbulence encounter, though they later acknowledged this should have been done to alert other aircraft.
Safety action
Following the investigation, the operator implemented several safety measures, including:
- Mandatory questionnaires during recurrent training to ensure pilots fully understand weather radar operations.
- Enhanced weather avoidance and turbulence management presentations during flight crew training.
- A technical circular reminding all crews to issue PIREPs when encountering significant weather or turbulence.