What happened
On the afternoon of 16 July 2023, a Robinson R44 Raven I helicopter, registration ZS-RTN, was involved in an accident near Randfontein, Gauteng. The aircraft, operated under private flight regulations, was performing a local flight with the pilot and three passengers.
Following a successful pre-flight inspection, the pilot initiated a hover to perform a power check. As the pilot attempted to transition from a hover to forward flight, the aircraft experienced a drop in rotor RPM, triggering the warning horn. The helicopter subsequently lost altitude and struck the ground heavily on the left side, causing the left skid gear to break off. While the aircraft sustained substantial damage to the vertical tail fin and the skid, there were 0 fatalities and no injuries were reported among the occupants.
The investigation
SACAA AIID investigators examined the environmental conditions and the aircraft's performance capabilities at the time of the accident. The investigation focused on the relationship between the aircraft's weight, the ambient temperature, and the pressure altitude.
At the time of the occurrence, the temperature was 18°C at an elevation of 5,735 feet, resulting in a density altitude of approximately 7,600 feet. Although the aircraft's weight of 2,339 lbs was within the maximum allowable takeoff limit of 2,400 lbs, the investigation found that the power required for the transition phase exceeded the available power. Specifically, the pilot reported a manifold pressure of 24 In Hg during the transition, which surpassed the threshold necessary to maintain sufficient rotor RPM for lift-off.
Additionally, the investigation revealed that the pilot was operating with an invalid Private Pilot Licence, which had expired 13 months prior to the accident.
Findings
- The primary cause of the accident was the low rotor RPM experienced during the transition phase, which occurred because the aircraft was operating at a high pressure altitude.
- The pilot failed to adequately account for the effects of high-density altitude during flight planning.
- The pilot was operating the aircraft with a pilot licence that had not been renewed and was legally invalid.
- The aircraft's maintenance records, including the most recent Mandatory Periodic Inspection, were up to date and valid.