What happened
On 21 August 2019, an Airbus A304-642, registration G-VFIT, was operating a commercial passenger flight from New Delhi, India, to London Heathrow. While cruising at FL360 over northern Turkey, the aircraft encountered moderate to severe turbulence.
At the time of the event, the flight crew had observed a single, non-threatening cloud on their flight path that did not appear on the weather radar. As the aircraft passed abeam this cloud, it experienced a sudden and significant wind shift, transitioning from a 40 kt tailwind to a 20 kt headwind within seconds. This caused vertical accelerations between +0.64 g and +1.56 g and a rapid increase in airspeed to Mach 0.86. To manage the overspeed, the pilot flying selected a lower Mach number and deployed speed brakes.
During the period of instability, a passenger in the rear galley was thrown to the floor. The passenger sustained a serious injury involving a dislocated and broken ankle. A cabin crew member also sustained a minor shoulder injury after striking a bulkhead. Following the encounter, the crew turned on the seatbelt signs and provided medical assistance to the passenger via ground-based medical services. The aircraft declared a medical emergency and requested a priority approach into London Heathrow, where paramedics met the aircraft.
The investigation
The AAIB examined flight data, meteorological charts, and cockpit observations. The investigation reviewed the weather radar, pre-flight documentation, and the flight crew's use of electronic flight bag applications. Investigators also analyzed satellite imagery of cloud top heights and the wind charts in effect at the time of the incident.
Findings
- The turbulence was caused by a rapid wind shift.
- There were no visual or radar indications of approaching turbulence, and the weather forecast did not predict clear air turbulence in the area.
- While the flight plan indicated potential vertical shear, the values were not high enough to suggest significant turbulence to the crew.
- The seatbelt signs were switched off at the time of the encounter because there was no prior warning of atmospheric instability.
- At the time of the accident, the flight crew's charting application could not be updated in-flight, meaning it only contained pre-departure weather data.