What happened
On 13 December 1999, a Boeing 777-20 and registration N786UA was conducting a scheduled passenger service from Chicago to London Heathrow. During the descent phase, the aircraft entered an overcast cloud layer, at which point an overspeed warning was triggered. In response, the autopilot was disengaged, and the crew manually deployed speedbrakes to manage the aircraft's velocity.
Shortly after this, the aircraft encountered intense turbulence lasting approximately 30 seconds. Following this period of instability, the purser notified the flight deck that several flight attendants had sustained injuries. Specifically, one crew member suffered a fractured fibula after being struck by a passenger who had fallen during the disturbance.
The investigation
Investigators analyzed Digital Flight Data Recorder (DFDR) information, which revealed that the aircraft encountered three distinct periods of significant turbulence while descending between FL270 and FL160. The most intense event occurred at FL250, where the aircraft experienced a maximum positive loading of +1.75 'g'. During this specific encounter, the airspeed fluctuated rapidly, rising from 313 kt to 334 kt in just three seconds before dropping to 301 kt eight seconds later. A third encounter was noted at FL165, characterized by three separate vertical accelerations.
Meteorological data from the European Significant Weather Charts indicated that moderate Clear Air Turbulence (CAT) had been forecast for the region between FL370 and FL200, driven by a frontal system over southern England and a 150 kt northwesterly jetstream. Additionally, a radio sonde balloon ascent near London confirmed the presence of vertical windshear of 10 kt per 1,000 feet between FL220 and FL240.
While the 'Fasten Seat Belt' sign had been activated for passengers, the cabin crew were still moving through the cabin performing duties and were not seated at the time of the second encounter. The investigation also noted that no prior turbulence reports from other aircraft had been received by the crew before the event occurred.
Findings
- The primary cause of the injuries was the severe turbulence encounter at FL250.
- The aircraft encountered three separate significant turbulence events during its descent.
- 1 serious injury and 5 minor injuries were sustained by the crew.
- Meteorological conditions included forecast moderate CAT and confirmed vertical windshear in the vicinity of the flight path.
- The cabin crew were not seated during the period of highest vertical acceleration.