What happened
On August 9, 2003, an Agusta Bell AB206B Jet Ranger III, registration HB-XUW, was conducting a local private flight near Zurich Airport. While flying over the Katzensee area, the pilot experienced sudden vibrations and a low rotor RPM warning. The pilot attempted to stabilize the aircraft by adjusting the collective, but the warning and vibrations recurred upon further adjustment.
Faced with engine power fluctuations and the threat of an engine failure, the pilot initiated an autorotation toward a nearby field. During the landing, the aircraft struck the ground with a high pitch angle and slid across the terrain. The impact caused the main rotor blades to strike the tail rotor drive shaft, resulting in significant damage to the tail boom and rotor components. All three occupants (the pilot and two passengers) were able to exit the aircraft without injury.
The investigation
Investigators examined the engine and the fuel control system to determine the cause of the power loss. Upon disassembly of the engine, technicians discovered that a B-nut on the pneumatic control line, which connects the accumulator to the fuel control unit, was not properly tightened. The torque paint, which should indicate a secure connection, was displaced by approximately 30 degrees toward a loose position.
Testing on an engine test stand confirmed that when artificial vibrations were introduced, the leak in this connection caused the fuel control unit to reduce engine power, mimicking the conditions experienced during the flight. The investigation also noted that the PT-governor had been replaced recently, a procedure that requires disconnecting this specific line.