What happened
On 20 October 2013, an ATR 72-212 A, registration G-COBO, was operating a commercial passenger flight from Guernsey Airport to London Gatwick. While cruising at FL120 near the Isle of Wight, the aircraft encountered significant weather cells on its radar. The crew attempted to navigate through a gap in the thunderstorm activity.
During this maneuver, the aircraft entered a zone of intense turbulence characterized by a powerful downdraught followed by a sudden updraught. This vertical movement caused the aircraft to climb approximately 780 ft. The force of the encounter caused the autopilot and flight director to disengage, requiring the co-pilot to take manual control. During the upset, a transient low oil pressure warning appeared on the left engine due to negative g-forces.
Inside the cabin, the turbulence dislodged various items and overturned a galley trolley. The encounter resulted in two minor injuries to the cabin crew, including a cut to a leg and a head injury from striking the ceiling. The flight crew eventually stabilized the aircraft back at FL120 and continued the flight to Gatwick, where passengers disembarked without further incident.
The investigation
The AAIB examined flight data recorder information and interviewed the crew. The investigation confirmed that the aircraft's structural loads remained within safe limits. The manufacturer noted that the disengagement of the autopilot and the momentary oil pressure warning were consistent with the aircraft's design response to the vertical accelerations experienced.
Investigators also reviewed the airline's operating procedures and the UK Aeronautical Information Publication (AIP) standards for turbulence reporting. It was noted that the turbulence occurred at a distance of at least 10 nm from the visible weather cells, demonstrating that associated turbulence can exist outside of radar-detected cells.
Findings
- The primary cause of the incident was the encounter with severe turbulence involving significant vertical accelerations.
- The sudden upward and downward movements caused the autopilot to disengage and led to an unauthorized altitude deviation.
- The low oil pressure warning was a transient event caused by the negative g-load during the transition out of the updraught.
- The cabin crew sustained minor injuries because the turbulence occurred while the cabin service was still being conducted.