Event Description
A helicopter equipped with a FADEC-controlled turboshaft engine experienced a loss of engine power after taking off from an offshore platform in the Gulf of Mexico. The pilot, a commercial-rated helicopter pilot with 10,000 hours of flight experience, reported that after the helicopter cleared the platform deck and entered translational lift, it "lurched violently to the left and started a rapid descent." The pilot lowered the collective and dropped the nose to avoid striking the platform. He recalled hearing a "screaming or screeching type of sound" as the helicopter descended and was unable to maintain main rotor RPM. The helicopter landed into the water and rolled to the right. After the occupants evacuated, the helicopter submerged 30 feet under the surface.
The skid-mounted float system was not activated, although the pilot stated that the floats were "armed" and that he attempted to deploy them during the autorotation via a float activation button mounted on the collective. The button was located within a circular ring on the pilot's collective.
Investigation Findings
Download of the Incident Recorder (IR) of the Electronic Control Unit (ECU) showed that the IR was first triggered by a droop in rotor RPM, followed by a MGT exceedance, power surge, engine flameout, and Ng underspeed over a span of 3.38 seconds. Download of the MGT instrument showed a recorded peak temperature of 953 degrees Celsius for 1 second.
Teardown examination of the engine revealed a catastrophic failure of the power turbine assembly. Metallurgical examinations by the NTSB and Rolls-Royce revealed evidence that the 3rd stage turbine wheel airfoil(s) had failed. A root cause of the failure was not determined, and further testing was conducted at Rolls-Royce.
Examinations of the airframe (fuel system, rotor systems, flight control systems, drive systems, and electronic control systems) found no anomalies other than within the power turbine assembly.
Subsequent Actions
As a result of some dynamic test results, Rolls-Royce issued several Commercial Engine Bulletins (CEBs) regarding inspection of in-service turbine assemblies. Additionally, Bell Helicopter issued an Alert Service Bulletin (ASB), which introduced flight manual revisions to avoid power turbine RPM (Np) steady-state operation between 68% and 97%.