What happened
On July 2, 2002, a Bell 206B helicopter, registration ZS-RFM, was performing a commercial transport mission from the Harmony Gold Mine to the Rand Refinery in Germiston. The flight, which included a pilot and one passenger, was carrying valuable cargo when the aircraft experienced a mechanical failure during cruise flight. Specifically, the main drive shaft assembly, situated between the engine and the main rotor gearbox, failed.
Following the initial failure, the pilot reported that the engine also ceased functioning. In response to the dual failure, the pilot executed an autorotational landing in an open grass field located south of Alberton. While the landing was successful in preventing fatalities, the uneven nature of the terrain caused one of the main rotor blades to strike the tail rotor drive shaft and its cowling during touchdown.
The investigation
Investigators examined the wreckage, which was transported by road to a maintenance facility for a detailed assessment of the damage. The investigation focused on the mechanical breakdown of the drive shaft assembly and the subsequent impact damage sustained during the landing. No injuries were reported among the two occupants, and the cargo was eventually transferred to a functional aircraft to complete the delivery.
Findings
Technical analysis revealed that the failure originated from the fracture of four fixation bolts. This breakage was caused by excessive heat generation between the mating faces of the inner and outer coupling teeth, a condition brought on by inadequate lubrication within the assembly.
Investigators identified two potential reasons for the lubrication deficiency: either the grease seal was installed incorrectly during maintenance, or the seal had suffered severe wear and tear prior to the assembly being returned to service, leading to its failure shortly after the aircraft resumed operations.