What happened
On 20 September 2011, a Robinson R44 helicopter, registration ZS-PXL, was conducting a commercial game culling operation at Graspan National Park in the Northern Cape. While flying at approximately 20 feet above the ground, the pilot and a veterinarian passenger experienced sudden, intense vibrations accompanied by a grinding noise. The aircraft immediately swayed 60 degrees to the left, and the pilot noted a loss of power and low RPM warnings.
In response to the mechanical instability, the pilot attempted an emergency landing in an open field. The landing was hard, and the left skid became embedded in soft soil, causing the helicopter to attempt a roll. During this sequence, the main rotor blades struck the tail boom, severing it from the airframe. Despite the substantial damage to the aircraft, both the pilot and the passenger evacuated the wreckage without injury.
The investigation
SACAA AIID investigators examined the wreckage and identified a failure in the tail rotor drive shaft components. Specifically, the forward intermediate flex plate had fractured. A metallurgical examination revealed that a fatigue crack had originated at a bolt hole beneath a bonded washer and had been propagating through the metal over several flights prior to the accident.
Findings
- The primary cause of the accident was a mechanical failure of the forward flex plate due to a fatigue crack.
- The fatigue crack was not detected during routine maintenance because the Aircraft Maintenance Manual (AMM) only requires a visual inspection, which was insufficient to identify a crack hidden under a washer.
- The pilot was operating within normal parameters for a culling flight, and the weather conditions were clear with good visibility.
- The investigation also noted that previous safety recommendations regarding flex plate inspections for Robinson R22 and R44 aircraft had not been implemented by the authority.
Safety action
Following the investigation, recommendations were made to the Director of Civil Aviation to implement previously approved safety measures. These include reviewing the necessity of a defined service life for forward flex plates and establishing specific inspection intervals. Additionally, the report suggested the issuance of a Mandatory Aeronautical Notice to ground similar aircraft for specialized inspections to prevent further recurrences.