What happened
On March 24, 2017, an AS 350 B2 helicopter, registration PT-YOL, took off from a non-certified landing site located within the Goiânia Autodrome in Goiás, Brazil. The flight was a private operation intended to transport one pilot and two passengers to Goianésia.
Approximately 100 feet above the ground and at a speed of 50 to 60 knots, the pilot heard a loud bang coming from the rear of the aircraft, immediately followed by a loss of engine power. The pilot executed the prescribed autorotation procedure, performing a forced landing approximately 120 meters from the takeoff point. The aircraft slid across a combination of asphalt and grass, resulting in light damage to the tail rotor blades, the tail cone, and the engine. All three occupants escaped the aircraft without injury.
The investigation
CENIPA investigators examined the Arriel 1D1 engine, which was sent to the manufacturer, Turbomeca, for detailed analysis. The investigation focused on the mechanical state of the engine modules and the maintenance history of the aircraft.
Technical inspections of the engine revealed metal shavings trapped in the oil filter, although the bypass system had not been activated. In module 2, investigators found significant rubbing marks caused by contact between the compressor's axial stage blades and the housing. In module 3, similar rubbing marks were found between the centrifugal impeller and the housing. The investigation established that the damage to the compressor turbine blades was caused by premature elongation of the components due to an abnormal increase in exhaust gas temperature, which led to a sudden rotor imbalance and the subsequent failure of the G3 bearing.
Findings
- The primary cause of the engine failure was the premature elongation of the 2nd stage compressor turbine blades due to an unrecorded increase in exhaust gas temperature.
- This elongation caused the blades to rub against the compressor housing, leading to blade breakage, rotor imbalance, and the failure of the G3 bearing.
- The engine's scheduled calendar inspection, which was due in May 2016, had not been performed. The aircraft had been operating for ten months past the expiration of the engine'related inspection extension.
- The failure to perform the scheduled maintenance meant that the developing condition of the turbine blades could not have been identified and corrected through routine inspection.