Incident Details
During cruise flight at an altitude of 1,000 feet, a speed of approximately 130 knots, and about 60 percent torque, the pilot reported a sudden loud explosion from the right side of the helicopter. The explosion was accompanied by simultaneous No. 2 engine fire and failure warning lights and indications, along with severe vibrations. The helicopter yawed several times from left to right, and the pilot heard a "whopping" sound. At the same time, the No. 1 engine torque meter needle moved rapidly from approximately the 9 o'clock to the 3 or 4 o'clock position on the gauge.
Investigation Findings
Examination of the No. 2 engine revealed that two power turbine blades and four rear bearing support housing (RBSH) components had failed in overload. The hub had become displaced from the engine centerline, and a deflection occurred at the engine case axial midpoint. Additionally, a misalignment was found between the inlet housing and the RBSH.
The resulting damage to the gas producer (GP) system was consistent with an assembly error and mechanical failure that caused a loss of critical internal operating clearances and radial support for the rotors. As the failure progressed during continued engine operation, a compressor surge occurred, allowing combustion gases to emerge from the No. 2 engine inlet. These gases ignited the inlet cowling material.
Effects on No. 1 Engine
The fire in the No. 2 engine cowling produced smoke and combustion by-products, which were ingested by the No. 1 engine. This ingestion caused a temporary stall condition in the No. 1 engine.