What happened
On 17 January 2015, a Robinson R44 Raven II helicopter, registration ZS-RAX, departed George aerodrome for a scenic flight to Cradock peak. The flight, conducted for private purposes, included the pilot and two passengers. After climbing to 6,000 feet, the aircraft approached the mountain peak from the south-east.
As the pilot transitioned into a hover to land on the rocky terrain, the aircraft encountered downdraft conditions. This caused the low rotor RPM/blade stall warning light and the annunciator horn to activate. In an attempt to recover the rotor speed, the pilot lowered the collective lever; however, the aircraft could not maintain sufficient RPM. The helicopter drifted and struck a rocky slope with significant force, resulting in damage to the skids and the lower fuselage. During the impact, the main rotor struck the tail boom, severing its top section. All three occupants disembarked from the upright aircraft without injury.
The investigation
SACAA AIID investigators examined the wreckage and the pilot's flight history. The investigation confirmed that the aircraft was well-maintained, with the last periodic inspection completed only 26.1 hours prior to the event. The helicopter was operating within its certified weight and balance limits, and the density altitude at the time of the accident was approximately 6,567 feet.
Technical analysis focused on the mechanics of the rotor stall. Investigators determined that the downdraft increased the angle of attack on the rotor blades, leading to a decay in rotor RPM. The investigation also noted that while the pilot had significant experience on the type, they had not completed a specific mountain flying course.
Findings
- The primary cause of the accident was poor technique during the landing phase.
- The aircraft encountered unexpected downdraft conditions while initiating the landing on the peak.
- The pilot's attempt to maintain altitude by increasing rotor pitch likely demanded power exceeding the engine's available capability, leading to the rotor stall.
- The pilot lacked specialized training in mountain flying operations.