What happened
On November 21, 1968, an Agusta Bell 47 G2 helicopter, registration HB-XCO, was performing a series of circuit training maneuvers at Bern Airport. The flight was being conducted by a student pilot under the direct supervision of an instructor, who was observing from the ground. During the fifth circuit, while the pilot was flying solo, the aircraft experienced a sudden and violent mechanical failure.
A tail rotor blade snapped, creating a severe imbalance that caused the remaining portion of the tail rotor and the transmission box to detach from the tail boom. The loss of the anti-torque system also tore away the tail rotor guard. The pilot immediately attempted to enter autorotation to manage the emergency, but the sudden loss of the tail rotor assembly caused the aircraft to pitch forward and lose control. The helicopter struck the ground at an angle of approximately 30 degrees in a field near the airport, resulting in the death of the pilot and the total destruction of the aircraft.
The investigation
Investigators examined the wreckage and the debris from the tail rotor, which had been scattered across a 120-meter area. Laboratory analysis of the broken blade, conducted by the Federal Laboratory for Testing of Materials, focused on the fracture site located 21.5 cm from the blade root. The investigation also reviewed the maintenance history of the aircraft, noting that a 100-hour inspection had been performed just the day before the accident.
Technical examinations of the metal components revealed that while the alloy composition was generally correct, there were specific metallurgical discrepancies. Furthermore, the investigation looked into the history of the tail rotor blades, which had been manufactured in Italy and previously sent back to the manufacturer for balancing.
Findings
- The primary cause of the accident was the in-flight failure of a tail rotor blade.
- The failure was triggered by manufacturing defects within the blade's reinforcement sleeve, which created localized stress points.
- Metallographic analysis confirmed that fatigue cracks originated from these manufacturing flaws, which were present even before the surface treatment was applied.
- The sudden loss of the tail rotor assembly caused an immediate loss of directional control and a significant pitch-down moment that the pilot could not recover from.
- Evidence of buckling was also found on both blades, likely caused by vibrational stresses, though the exact cause of this buckling could not be determined.