Incident Overview
On 3 July 2003 at 0300 UTC, a Boeing 777-236, registration G-YMME, was cruising at FL370 approximately 10 miles north of KINDU, Central Africa, on a scheduled passenger flight from London to Harare. The aircraft was operated by two first officers while the captain was on a rest break. The flight had encountered intermittent light turbulence about an hour earlier while navigating around isolated storm activity associated with the Inter Tropical Convergence Zone, but conditions had since become smooth.
Sequence of Events
The operating pilots reported clear skies, with stars visible and no weather activity indicated on the weather radar. The seat belt signs were off, and cabin lights were dimmed as most passengers slept. Some cabin crew in the rear galley were preparing trolleys for breakfast. Suddenly, the aircraft experienced a small positive 'g' movement followed by a slight airframe shake. The seat belt signs were immediately switched on, at which point the aircraft underwent a severe downward movement with associated negative 'g'. The pilots guarded the thrust levers and speedbrake to control airspeed, which remained constant. The disturbance ceased quickly, and the aircraft resumed normal flight. Over the next hour, minor clear air turbulence was encountered, though none as severe as the initial event.
Injuries and Damage
The sudden downward movement caused one seated passenger to strike her head, resulting in a cut. Two cabin crew members in the rear galley, along with catering trolleys, became airborne, sustaining bruising and shock; one also suffered cuts and abrasions. The passenger was treated with the aircraft's first aid kit and declined medical assistance upon arrival. The injured cabin crew were taken off duty for the remainder of the flight and advised to remain off duty until further assessment in the UK. No damage to the aircraft occurred.
Analysis and Possible Cause
Following the incident, the captain and first officers discussed possible causes. They ruled out wake vortex due to no aircraft appearing on TCAS or broadcasting via In Flight Broadcast Procedure. Wind shear was considered unlikely given no marked changes in temperature or wind speed. Continued starlight indicated the aircraft remained clear of cloud. The most likely cause, as assessed by the crew, was turbulence associated with cloud buildup occurring some distance below the aircraft's flight path. This hypothesis was supported by subsequent clear air turbulence encountered over the next hour. The weather forecast had not predicted clear air turbulence in the area, though the captain noted regional forecasts were not always accurate.
Seat Belt Policy
The captain stated that company policy advises passengers to keep seat belts fastened while seated even when the seat belt signs are off. He believed this policy helped prevent further injuries during the incident.