What happened
On August 3, 2001, an Enstrom F28 helicopter, registration ZS-HMC, was conducting a training flight departing from Wonderboom Airport. The flight, involving an instructor and a student, was headed toward the Pretoria General Flying area No. 1. While operating near Tswaing Pan, approximately 16 nautical miles northwest of Wonderboom, the instructor descended to an altitude of roughly 200 feet above the ground.
During this maneuver, the instructor applied throttle and pulled the collective, at which point the engine began to overspeed. The flight instruments indicated a critical discrepancy where engine RPM increased while rotor RPM decreased. Recognizing the emergency, the instructor initiated an autorotational landing into the crater at Tswaiment Pan. During the flare phase of the landing, the tailrotor guard struck the ground. The impact deflected the guard into the main rotor blades, resulting in the blades being severed.
The investigation
The investigation focused on the mechanical failure that triggered the engine overspeed. Investigators examined the aircraft's clutch, specifically the freewheel unit, to determine why the engine and rotor speeds decoupled. The examination of the components revealed physical damage consistent with internal mechanical failure.
Findings
- The primary cause of the accident was an unsuccessful autorotation resulting from the failure of the clutch (freewheel) unit.
- The failure was attributed to the fatigue failure of several sprag teeth within the unit, which rendered the clutch inoperative.
- The fatigue of these teeth was likely driven by the misalignment of the unit relative to the shaft.
- A lack of adequate lubrication was identified as a potential contributing factor to the component failure.