What happened
On 29 November 2003, an ATR72-202, registration G-BWDA, was conducting a passenger flight from Guernsey to Southampton. While descending from FL150 to FL130, the crew observed a significant decay in airspeed and noted substantial ice accumulation on the icing probe and forward windows.
Following established checklist procedures for severe icing, the crew requested an immediate descent and disconnected the autopilot. Upon disconnection, the aircraft exhibited several uncommanded movements, including a 40-degree left roll, a left yaw, and a 10-degree nose-up pitch. The handling pilot applied corrective inputs, including full right rudder, to stabilize the aircraft. During the descent, the aircraft experienced further pitch oscillations before control was eventually regained as airspeed increased. At FL60, the aircraft experienced a brief, significant yaw as ice cleared asymmetrically from the wings. The flight proceeded to land at Stansted without further incident.
The investigation
The investigation examined the flight sequence, the aircraft's technical state, and the subsequent reporting processes. Investigators found that the aircraft had been flown into meteorological conditions that exceeded the certified flight envelope for the type.
Crucially, the investigation also scrutinized the post-flight actions. After landing, the commander performed a visual inspection and, finding no obvious damage, did not record the incident in the aircraft's technical log. The aircraft subsequently flew a scheduled service back to Guernsey. While a Mandatory Occurrence Report (MOR) was eventually filed, the delay in reporting and the lack of immediate management notification meant that the Digital Flight Data Recorder (DFDR) was not preserved. Consequently, the data was overwritten, preventing a detailed analysis of the aerodynamic disruptions caused by the ice.
Findings
- The aircraft encountered severe airframe icing that was outside the certified flight envelope.
- The uncommanded roll, yaw, and pitch oscillations were likely caused by the disruption of airflow over control surfaces due to ice accumulation.
- The commander failed to enter the incident in the technical log or ensure a thorough engineering inspection was performed.
- Inadequate post-flight reporting procedures and a lack of timely management awareness resulted in the loss of critical DFDR data.
Safety action
Following the incident, the airline's engineering contractor issued an alert regarding the necessity of reviewing FDR data following MOR actions. The airline also updated its Operations Manual to require that MORs be reported immediately and implemented procedures to disable the FDR/CVR after shutdown if a serious incident is suspected, ensuring data preservation.