On 27 June 2000 an Airbus A300-600 being operated by American Airlines on a scheduled passenger service from London Heathrow to New York JFK was being flown manually in the day Visual Meteorological Conditions (VMC) climb and approaching FL220 when a loud bang was heard and there was a simultaneous abrupt disturbance to the flight path. The event appeared to the flight crew to have been a disturbance in yaw with no obvious concurrent lateral motion. Although following the disturbance, the aircraft appeared to behave normally, the aircraft commander decided to return to London Heathrow rather than commence a transatlantic flight following what was suspected to have been an un-commanded flight control input. An uneventful return was made followed by an overweight landing 50 minutes after take off.
An Investigation was carried out by the UK AAIB. Extensive ground testing was followed by a test flight which replicated the incident flight. No anomalies were found in the way the flight controls functioned when the aircraft was manoeuvred vigorously whilst being operated in the same manner as on the incident flight.
The ways in which a localised atmospheric turbulence event might have been caused were examined. Clear Air Turbulence (Clear Air Turbulence) was considered unlikely. The nearest jet stream was over the North Sea and the horizontal wind gradients in the area in which the disturbance was encountered were very small. Noting that CAT can also be associated with breaking gravity waves generated by surface topography or convection which then require suitable stability conditions to allow the waves to break, this possibility was also considered. However, the surface winds at the time were extremely light and the atmosphere very stable, so that gravity waves were unlikely to have been generated and there was no evidence from satellite photographs of their actual presence. There was also no evidence of convective cloud tops having been in the vicinity of the aircraft.
The only remaining possibility was that a wake vortex encounter had been the cause. Since in the prevailing conditions of stability and light winds, wake vortices from heavy aircraft can be relatively long-lived as they sink, an encounter with a wake vortex generated by another aircraft appeared to be the most likely cause. As the aircraft climbed on a westerly track, radar recordings showed that it was directly behind a Boeing 777 that had passed through exactly the same airspace some 4 minutes and 18 seconds earlier. When it passed through the incident position, Boeing had been at FL229 and at an estimated mass of 243 tonnes. It was found from the radar data that no other aircraft had recently passed the incident position close to the incident level either along or across track.
The Investigation concluded that:
“The aircraft was established in the climb, in seemingly quiescent air, with no aircraft apparently in the vicinity when it encountered a sudden disturbance, which the flight crew perceived as a yaw excursion. The value of the lateral acceleration, recorded from the accelerometer mounted at the aircraft's centre of gravity, showed a small disturbance but it is probable that the flight crew, being seated in the cockpit, perceived a higher level of lateral acceleration, which they interpreted as uncommanded rudder input. Extensive engineering investigation did not find any reason for the disturbance to have occurred and no anomalies in the operation of the aircraft were found during the test flight. Furthermore, there was nothing from the engineering investigation that could explain the loud noise reported by both the flight crew and the cabin attendants. A number of meteorological phenomena were considered. It is most probable that the reason for any localized turbulence was an encounter with the wake vortex generated by a B 777 aircraft which had passed through the same airspace some four minutes and 18 seconds earlier. Research into wake vortex encounters indicates that a loud noise can be associated with entry into the core of the vortex if the geometry is appropriate.”
The aircraft commander was noted as having been experienced on the aircraft type. The experience of the First Officer, who had been acting as PF at the time, was not recorded.
The Final Report of the Investigation can be seen in full at SKYbrary bookshelf: AAIB Bulletin No: 2/2001 EW/C2000/6/10
No Safety Recommendations were made.