Incident Details
On 13 December 1999, at 0953 UTC, a Boeing 777-200 (registration N786UA) operated by United Airlines was inbound to London Heathrow Airport on a scheduled passenger flight from Chicago, USA. The aircraft carried 15 crew and 175 passengers. During the descent, the aircraft entered an overcast cloud layer, and shortly afterwards the overspeed warning alert sounded. The autopilot was disconnected and speedbrakes were deployed manually to slow the aircraft. Approximately one minute later, the aircraft experienced severe turbulence lasting less than 30 seconds.
Following the turbulence, the purser contacted the flight deck to report that several flight attendants had been injured. One flight attendant sustained a fractured right fibula above the ankle when a passenger fell and collided with her. In total, one crew member suffered serious injuries and five crew members sustained minor injuries; no passenger injuries were reported.
Flight Data Recorder Analysis
Analysis of the Digital Flight Data Recorder (DFDR) data by the operator indicated that the aircraft experienced three significant turbulence encounters while descending between Flight Level (FL) 270 and FL160. The greatest vertical accelerations occurred during the second encounter at FL250, with a maximum positive loading of +1.75 g. At that moment, airspeed increased from 313 kt to 334 kt in three seconds, then dropped to 301 kt with a reduction to +0.51 g eight seconds later. The third encounter, at FL165, consisted of three individual bumps ranging between +1.32 g and +0.77 g.
Cabin crew reported that it was during the second encounter that the passenger fell and caused the injury to the flight attendant. The 'Fasten Seat Belt' sign had been switched on for passengers during the descent, but cabin crew were still performing their duties and were not seated.
Meteorological Conditions
The European Significant Weather Charts, covering FL100 to FL450, were provided to the AAIB by the Meteorological Office. These charts indicated that moderate Clear Air Turbulence (CAT) was forecast between FL370 and FL200, associated with a frontal system across the south of England. A northwesterly jetstream of around 150 kt was present with its core at FL310.
Recordings from a radiosonde balloon ascent, about 60 nautical miles west of London around the time of the turbulence events, indicated a vertical windshear of 10 kt per 1,000 feet between FL220 and FL240. Moderate or severe CAT typically extends for 2,000 to 3,000 feet vertically either side of a shear area.
Operator Statistics and Reporting
The operator's statistics showed that between 1993 and 1999, approximately 946 turbulence-related injuries had been documented. Currently, aircraft in service are not equipped with any accurate method to indicate the presence of turbulence relative to the actual aircraft flight path, although such systems are under development.
Turbulence encounter reports are commonly made by flight crews on the ATC radio frequency in use, which are then generally heard by or re-transmitted to other aircraft crews. In this event, turbulence encounter reports were not received by the flight crew until after the encounter had occurred.