What happened
On September 21, 1963, a Hiller UH-12A helicopter, registration HB-XBS, was conducting a training flight at Bern-Belp airfield. The aircraft, operated by Heliswiss Schweiz Helikopter A.G., was being flown by an instructor accompanied by a student pilot. While flying at an altitude of approximately one meter above the airfield, the helicopter suddenly tilted to the left. This sudden movement caused the rotor to strike the ground, leading to a crash. There were no injuries to the two occupants, though the aircraft sustained heavy damage, estimated at roughly 80% of its value.
The investigation
The investigation focused on the mechanical integrity of the rotor control system. The aircraft's control mechanism relies on rods connecting the swashplate to transmission levers to manage cyclic pitch. Investigators examined the condition of the bolts and safety pins at these connection points. During the post-crash inspection, the right rod connecting the transmission lever to the swashplate was found disconnected from its lever. While the rod itself was undamaged, the securing nut and the split pin were missing. The bolt used in this connection showed signs of wear, with its diameter reduced from 6.5 mm to 5.7 mm. Additionally, the front rod was found to be missing its split pin, although the bolt and nut remained in place.
Findings
- The primary cause of the accident was the failure of a critical rotor control element due to inadequate mechanical securing of a bolt connection.
- It is highly probable that the split pins had been removed at some point and not replaced, as the deficiency was present on both the right and front control rods.
- The investigation could not definitively determine when the safety pins were removed, though the worn condition of the bolt thread suggests the nut may have been loose for some time.
- While recent maintenance and pre-flight inspections were conducted, the investigation noted that the specific nature of these checks might not have focused on these particular internal linkages.