What happened
On December 8, 1971, a Hughes 269B helicopter, registration HB-XCR, was conducting a training flight at Geneva-Cointrin Airport. The flight involved a student pilot under the supervision of a flight instructor, intended for mountain landing practice. After performing standard checks and a period of hovering to allow ground crew to remove snow pads, the pilot transitioned from a hover to a climb.
At an altitude of approximately 7 to 10 meters and a speed of 25-40 mph, a loud noise was heard, followed by an immediate engine overspeed. The helicopter lost altitude rapidly. The instructor took control and attempted an autorotative landing on an access road near the airfield. However, the area was obstructed by construction machinery and earth piles. The helicopter struck the ground with its tail and skids, eventually toppling onto its left side and causing the main rotor blades to strike the terrain. Both occupants sustained minor injuries.
The investigation
Technical investigators examined the wreckage and the failed component, specifically the lower drive shaft. Metallurgical analysis by multiple laboratories, including the manufacturer, revealed that the shaft had suffered a fatigue failure under torsional load. The examination showed evidence of oxidation and temperature increases (ranging from 3-550°C) on the fracture surfaces, indicating that the shaft had been rotating while already cracked, causing friction-induced heat.
Investigators also reviewed the engine's recent maintenance history. The engine had undergone a total overhaul and was reinstalled in the aircraft on November 30, 1971. During the run-in process, the engine was operated with the rotor disengaged. Evidence from the engine overhaul report suggested that the connecting rods had shown signs of scuffing, which is indicative of an overspeed event.
Findings
- The primary cause of the accident was the fatigue failure of the rotor drive shaft.
- The failure was likely triggered or exacerbated by the engine exceeding the maximum allowable RPM while the rotor was disengaged during the recent engine run-in procedure.
- The FAA had previously noted that the specific shaft part number (269A5504-3) was susceptible to increased torsional vibrations and potential failure when the engine exceeded 1600 RPM with the rotor disengaged.
- The low altitude at the time of the failure significantly limited the available area for a safe autorotative landing.
- The presence of construction obstacles on the landing site made a successful emergency landing highly unlikely.