What happened
On 28 June 2004, a Boeing 777-200, registration 9V-SRC, was operating a scheduled passenger flight from Singapore Changi Airport to Nagoya, Japan. During the early morning hours, while flying near waypoint ARESI on airway L625, the aircraft encountered moderate to severe turbulence. The event lasted approximately 10 to 15 seconds and included abrupt vertical movements, with flight data recording a maximum vertical G of +1.75.
Prior to the encounter, the flight crew had observed weather cells on radar and had requested a deviation from the planned route to avoid convective activity. In anticipation of potential turbulence, the pilot flying attempted to alert the cabin crew by cycling the fasten seat belt switch. However, the flight crew did not provide a verbal announcement via the passenger address system.
During the turbulence, a cabin crew member (a leading steward) was in the aft galley stowing a duty-free merchandise cart. The suddenness of the movement prevented the crew member from securing themselves. The impact resulted in one injury to the crew, specifically a cut above the left eye and two broken wrists. The aircraft continued to Nagoya without further incident.
The investigation
The investigation focused on why the cabin crew was not adequately alerted to the impending turbulence. Investigators examined the operation of the cockpit seat belt switch and the interpretation of the resulting chime signals. Testing on similar aircraft and simulators revealed that if the switch is operated too rapidly (in less than two seconds), the cabin only hears a single chime rather than the multiple chimes intended to signal a need for the crew to sit down.
Findings
- The primary cause of the injury was the sudden onset of turbulence while the crew member was performing galley duties.
- The pilot cycled the fasten seat belt switch too quickly, which generated only a single chime in the cabin.
- Because no verbal announcement was made, the cabin crew interpreted the single chime as a precautionary measure for light turbulence, which did not require them to cease service or take seats immediately.
- The practice of using the number of chimes to communicate safety instructions is not a robust method, as the signal depends on the speed of the switch operation.
Safety action
Following the investigation, the operator was advised to review procedures regarding the use of chime signals for communication. The operator has since issued instructions to alert flight crews that rapid switching can result in insufficient audible signals. Additionally, a review of the Safety and Emergency Procedures Manual was suggested to clarify crew actions during sudden turbulence.