Accident Details
On 8 March 2001 at 1130Z, an Enstrom F28 helicopter registered ZS-HMC departed Wonderboom Airport (FAWB) on a training flight to the Pretoria General Flying area No. 1. The pilot-in-command, aged 43, held a commercial pilot licence with valid medical and a total flying experience of 1370 hours, including 1200 hours on type. One student was also on board. Weather at the time was fine.
Sequence of Events
At Tswaing Pan (approximately 16 nautical miles northwest of Wonderboom), the instructor took control of the helicopter and descended to about 200 feet above ground level. As the instructor opened the throttle and pulled the collective, he observed an engine overspeed condition, indicated by the needles splitting in the wrong direction—engine rpm increasing while rotor rpm decreasing. The instructor then executed an autorotational landing into the crater. During the flare, the tailrotor guard contacted the ground and was deflected into the main rotor blades, causing it to be severed. Both occupants disembarked without injuries.
Probable Cause
The accident occurred as a result of an unsuccessful autorotation due to the clutch (free wheel unit) failure. The clutch unit failed as a result of the fatigue failure of a number of the sprag teeth, causing the unit to become inoperative. The fatigue failure of the teeth was most probably caused by the misalignment of the unit in relation to the shaft. The lack of lubrication could also have played a contributing role.