What happened
On April 24, 1968, a Bell 47 G-3 B 1 helicopter, registration HB-XBT, was performing a transition training exercise at Bern Airport. The flight was part of a mountain pilot instructor course. While the crew was executing a low-altitude maneuver approximately 1.5 meters above the ground, a loud noise, similar to a detonation, was heard. The aircraft immediately began to vibrate and pitch backward, causing the tail rotor to strike the ground and shatter.
In an immediate response, the instructor reduced the collective pitch and cut the engine power to force the aircraft into the grass, which caused the right skid to collapse. Despite the impact, there were no injuries to the two occupants.
The investigation
Investigators examined the flight control linkage, specifically the assembly connecting the control rod to the cyclic control lever. The investigation, conducted in collaboration with the Federal Laboratory for Materials Testing, focused on the mechanical integrity of the components.
Upon inspection, the investigation revealed that the assembly was not installed according to the manufacturer's maintenance manuals. The control rod had been mounted upside down, and the bolt used was 3.5 mm shorter than the required specification (an AN 174-25 bolt was used instead of the required AN 174-26).
Findings
- The primary cause of the accident was the fatigue failure of a bolt within the cyclic control linkage, which rendered the flight controls inoperable.
- The broken bolt had been in service for 581 hours since its installation during a previous general overhaul.
- Metallurgical analysis showed that while the bolt met standard mechanical strength requirements, the fracture originated at the base of the threads due to surface irregularities (grooves) present from the manufacturing process. These irregularities likely acted as stress concentrators.
- The investigation could not definitively prove that the incorrect installation of the shorter bolt or the inverted rod directly caused the failure, but the manufacturing defects in the bolt's surface were a contributing factor to the fatigue crack initiation.