What happened
On 3-March-2000, a Bell 206B helicopter, registration ZS-HMB, was performing an international charter flight from Nelspruit to Maputo. While the pilot was maneuvering through the Crocodile River Gorge to stay below cloud cover, the aircraft experienced a sudden loud bang and a significant leftward yaw. Immediately following this, the pilot observed the engine out warning light and heard the corresponding audio alert.
Despite maintaining full throttle, the main rotor RPM began to drop, forcing the aircraft into autorotational flight. Faced with difficult terrain, the pilot identified a suitable landing area on a riverbank and initiated a descent. During this process, the pilot noted that the power turbine RPM exceeded the main rotor RPM and observed the engine chip warning light. To preserve tail rotor control, the pilot kept the engine running through the landing. Upon touchdown, a main rotor blade struck the tailboom and severed the tail rotor drive shaft. All four occupants of the aircraft disembarked without injury.
The investigation
Investigators examined the mechanical state of the engine and the drive system following the incident. The investigation established that the engine's Ng had dropped to 58% (idle) while the power turbine remained at 100% and the main rotor RPM had decreased to approximately 45%. Furthermore, the investigation found that the main rotor brake was non-functional because the input drive shaft coupling, which connects the engine to the main rotor gearbox, had failed.
Findings
- The primary cause of the mechanical failure was excessive heat generation occurring between the inner and outer coupling teeth mating faces.
- This heat was produced by the relative movement between the two couplings and was driven by inadequate lubrication.
- The lack of lubrication led to the seizure of the inner teeth mating faces, which subsequently caused the fixation bolts to fracture.