What happened
On 3 June 2000, a BAe 1246-300, registration G-UKAC, was operating a passenger flight from Marseille to Stansted. While cruising at FL280, the crew encountered a light layer of cloud and requested a climb to FL310 to avoid potential engine icing. Upon reaching the higher altitude, the crew identified a line of cumulonimbus clouds approximately 50 nm ahead. To navigate between two storm cells, the crew descended to FL260 and activated the engine anti-ice systems.
As the aircraft cleared the weather, the commander noticed a gradual decrease in the fan speed (N1) of the number four engine, accompanied by a rising Turbine Gas Temperature (TGT). The crew initiated the Rollback Emergency Check List; however, the TGT continued to rise, reaching 890°C. Consequently, the number four engine was shut down. The crew declared a PAN call and diverted the aircraft to Paris, Charles De Gaulle, where the flight concluded without further incident. There were no fatalities and no injuries to the 5 crew members or 109 passengers.
The investigation
An analysis of the Flight Data Recorder by the aircraft manufacturer determined that the uncommanded thrust reduction was caused by an ice-induced engine 'Rollback' on the number four engine. The investigation found that ice had accumulated on the engine core supercharger exit guide vanes, effectively choking the engine core.
Data indicated that the Total Air Temperature (TAT) probe likely iced up several minutes before the N1 decay. The investigation also examined the validity of the flight limitations in place at the time. While the European Significant Weather Chart indicated occasional cumulonimbus activity in the area, a discrepancy was identified regarding the interpretation of flight restrictions. The manufacturer's intent was that the entire flight should remain below 26,000 feet if any cloud was forecast, whereas the operator's interpretation allowed for higher altitudes in areas not directly affected by known icing.
Findings
- The primary cause of the thrust reduction was the accumulation of ice on the engine's exit guide vanes.
- The aircraft was operating in meteorological conditions conducive to the rollback event.
- There was a difference in interpretation between the manufacturer and the operator regarding the altitude restrictions for aircraft with unmodified engines during forecast cloud activity.
- The engine failed to respond to thrust lever inputs due to the internal ice buildup.