What happened
On 21 December 2008, a Bell 206B helicopter, registration ZS-HWM, was conducting a private flight from Bergsig Game Farm near Mossel Bay toward a farm in the Riversdale area of the Western Cape. During the descent toward the intended landing site, the pilot observed the "Engine Out" warning light activate twice, accompanied by an audible alarm.
As the pilot attempted to manage the situation, the aircraft encountered a downdraught caused by a prevailing southerly wind blowing over a nearby hill. While attempting to arrest the sink rate and flare for landing, the helicopter experienced a hard landing on an upward slope. The impact caused the main skids to fracture and the tail boom and tail rotor to separate from the airframe. The aircraft subsequently rolled onto its left side. The pilot escaped the wreckage without any injuries.
The investigation
An investigation into the mechanical failure focused on the engine's fuel control system. Technicians examined the Pc air supply line, which transmits compressor discharge pressure to the power turbine fuel governor. The inspection revealed that the line had fractured underneath its securing clamp.
Evidence showed that the protective lining within the clamp had worn away, leading to metal-to-metal contact. This caused fretting and corrosion on the air supply line. Furthermore, the investigation noted tool marks and evidence of improper bending of the line, which did not meet design dimensions. This fracture led to a loss of pneumatic signal to the N1 governor, which in turn reduced fuel flow and caused the loss of engine power.
Additionally, an inspection of the engine bearings revealed heat damage and metallic contamination. While the primary cause of the power loss was the air line fracture, the investigation determined that the damage to the No. 6, 7, and 8 bearings was consistent with oil starvation that occurred after the helicopter rolled onto its side, uncovering the oil pump's scavenge port.
Findings
- The primary cause of the accident was the fracture of the Pc air supply line due to fretting at the securing clamp.
- The loss of pneumatic pressure to the fuel control unit resulted in reduced fuel flow and a subsequent loss of engine power.
- The hard landing was exacerbated by a downdraught encountered during the approach.
- The structural failure of the tail boom and skids was a direct result of the high impact forces during the unsuccessful autorotational landing.