What happened
On 13 September 2006, a Robinson R22 Beta, registration ZS-RBZ, was conducting a game counting mission over the Inyala Game Ranch in the Ventersdorp district. The flight, which involved the pilot and a passenger, was monitoring antelope populations when the aircraft entered the lee side of elevated terrain. During this maneuver, the helicopter encountered a sudden and intense downdraft. In an effort to maintain altitude and avoid the ground, the pilot applied maximum power; however, this action led to an over-pitching of the rotor system. The resulting decay in main rotor RPM became unrecoverable, leading to a hard landing on sloping ground and a subsequent right-side rollover. There were 0 fatalities and 0 injuries resulting from the event.
The investigation
Investigators examined the environmental and performance factors present at the time of the accident. The aircraft was operating at an elevation of 5,200 feet AMSL, which translated to a density altitude of approximately 8,000 feet. While the aircraft's weight of 1,295.8 pounds was below its maximum certified takeoff mass, performance analysis indicated that while a hover in ground effect (IGE) was possible, an out of ground effect (OGE) hover would not have been achievable under those conditions. The investigation also reviewed the maintenance history, noting that the aircraft had undergone a mandatory periodic inspection approximately one month prior to the accident and was maintained by a certified organization.
Findings
- The primary cause of the accident was pilot error involving the failure to properly assess prevailing wind conditions while flying near high terrain.
- The encounter with a severe downdraft on the lee side of the hills necessitated immediate corrective action.
- The pilot's attempt to compensate for the downdraft caused an over-pitch of the rotor blades, triggering a critical loss of main rotor RPM.