What happened
On October 26, 1976, an Agusta Bell 204 B helicopter, registration HB-XCQ, was performing aerial work involving the transport of concrete buckets between Domat/Ems and the Emser Mayens. During the third lifting operation of the day, the pilot was hovering approximately five to six meters above a truck to secure a new load.
As the pilot lifted the load to verify its weight, a loud bang occurred, followed by an immediate loss of engine power. The pilot attempted to maneuver the aircraft laterally while simultaneously releasing the load. During this maneuver, the helicopter performed a quarter-turn to the left and struck the rear edge of a loading platform on a second stationary truck. This impact caused the aircraft to tilt right and strike the ground with its rotor blades and landing gear. The pilot escaped the aircraft uninjured, though the helicopter sustained substantial damage.
The investigation
Investigators examined the wreckage and the Rolls Royce GNOME H 1200 Mk6l0 engine. The investigation revealed that the impact with the truck caused a large hole in the fuselage and severed electrical connections. Notably, the mechanical connection between the turbine and the main rotor was broken during the ground impact.
Detailed analysis of the engine, conducted under the supervision of British aviation authorities, identified significant damage within the gas generator compressor. A blade from the fourth compressor stage had fractured, and a second blade was found to have an existing crack at its root. Laboratory testing of the fracture surface indicated a high-cycle fatigue failure originating at the edge of the blade's dovetail section. The engine had only 759 total operating hours, with the replaced compressor blades having completed only 509 cycles since their last overhaul.
Findings
- The primary cause of the engine failure was a compressor stall resulting from the fatigue failure of a fourth-stage compressor blade.
- The sudden loss of power was triggered by the blade fracture, which led to an immediate drop in performance and a rapid increase in exhaust gas temperature.
- The engine's automated control system responded to the temperature spike by throttling fuel delivery to flight idle, causing the sudden loss of altitude.
- The investigation concluded that the fatigue crack likely developed due to aerodynamic instability (flutter) combined with high axial loads and insufficient damping between the blade platforms.