What happened
On 6 April 1999, an Aerospatiale AS355F2 Ecureuil II, registration G-CAMB, was conducting police operations near St Neots, Cambridgeshire. While the pilot was reducing speed to approximately 60 kt to transition into a hover out of ground effect at 700 feet, a loud bang was heard. Simultaneously, the cockpit indicators for the right-hand engine oil pressure and generator illuminated. The pilot immediately shut down the right engine and performed a precautionary landing in a nearby field. There were no fatalities and no injuries to the crew or passengers.
The investigation
The investigation focused on the cause of the engine failure, which was identified as an uncontained failure of the first stage turbine wheel. A significant fragment of the wheel rim, representing nearly one-quarter of its circumference, had detached and was found in the aircraft's baggage compartment. The debris from the failure caused shrapnel damage to the engine deck and the fuselage skin, though the remaining engine and essential aircraft systems remained undamaged.
Investigators examined the engine for evidence of oil fires, asymmetric combustion, or overtemperature operation. While some carbon deposits were found on the No 8 bearing cap, suggesting thermal degradation of the oil, no evidence of an actual oil fire was detected. Examination of the failed wheel rim revealed numerous fine cracks on the platform, which is a typical result of thermal fatigue caused by the heating and cooling cycles of flight.
Further analysis of the fracture surface showed a pattern of interdendritic crack growth. This specific type of fracture is characteristic of rapid creep-assisted crack growth, which typically follows exposure to high turbine temperatures during engine start-up that exceed the limits specified in the maintenance manual.
Findings
- The right engine suffered an uncontained failure of the first stage turbine wheel.
- The failure originated from thermal fatigue cracks on the wheel platform.
- The fracture pattern indicated that high turbine temperatures during start-up likely caused rapid creep-assisted crack growth.
- The aircraft was not equipped with a system to record or register Turbine Outlet Temperature (TOT) excursions.
- An energy absorption ring modification, which can mitigate such failures, had not been installed on G-CAMB.