What happened
On June 24, 1965, a Brantly B-2 helicopter, registration HB-XBV, was conducting a commercial photography flight near Suhr, Switzerland. The aircraft was operating at an altitude of approximately 80 meters, circling at speeds between 40 and 50 mph, when the engine suddenly failed. Witnesses observed a cloud of mist or smoke emanating from the engine during the failure.
The pilot immediately initiated an autorotation procedure, attempting to steer the aircraft toward an open field near the edge of the village. During the descent, the aircraft experienced heavy vibrations. As the pilot attempted to cushion the landing at an altitude of approximately 5 to 7 meters, the helicopter unexpectedly pitched sharply to the right, dropped, and struck the ground with significant force. While the occupants of the aircraft remained uninjured, the helicopter was destroyed.
The investigation
Investigators examined the engine components following the crash. The disassembly of the Lycoming V0-360-A 1B engine revealed that connecting rod number two had been destroyed at the crankshaft side. The fracture surfaces of the connecting rod bolts showed significant deformation and wear. One bolt remained intact but was connected to a fractured piece via a split castell nut, while the second bolt had lost its head and had broken in two places.
The investigation also reviewed the maintenance history of the engine. A major overhaul had been performed in May 1965, only weeks before the accident. While the maintenance report from the service provider mentioned the replacement of various bolts and nuts, it did not specifically document the replacement of the connecting rod bolts. Although the mechanic stated these parts were replaced, there was no written evidence to confirm this.
Findings
- The engine failure was caused by the destruction of a connecting rod, which likely resulted from the failure of a connecting rod bolt.
- The aircraft was operating at a speed and altitude that were at the limit of, or potentially outside, the safe operating parameters for a successful autorotative landing as defined in the flight manual.
- Because the engine failed under these specific flight conditions, the pilot was unable to maintain the rotor RPM necessary to cushion the landing, leading to the uncontrolled descent and hard impact.