What happened
On the morning of 16 March 2025, a Robinson R44 Raven I, registration ZS-RNI, was performing a private flight from the Northern Cape to the Eastern Cape. The flight, operated under Part 91 regulations, began at Mons Farm with a pilot and one passenger on board. During the journey, the pilot performed an unscheduled landing near the Lohatla Military Base to accommodate a passenger's medical request.
After a brief ten-minute stop, the flight resumed. However, while flying at approximately 150 feet above ground level, the aircraft experienced a rapid decay in main rotor RPM, dropping below the 97 percent threshold and triggering the low RPM warning light. Despite the pilot's attempt to engage autorotation, the rotor RPM continued to fall to roughly 80 percent. The helicopter was unable to maintain a safe descent rate, resulting in a hard landing on a grass field within the military base boundaries. The impact caused the main rotor blades to strike the tail boom, leading to its separation and causing substantial damage to the aircraft. No injuries were reported.
The investigation
An investigation by the SACAA AIID examined the aircraft's mechanical state and the circumstances of the flight. Post-accident inspections by maintenance engineers found the fuel system to be functional, with sufficient and uncontaminated Avgas 100LL remaining in the tanks. The engine was found to have been operational at the time of the accident, as evidenced by the alignment of the cooling fan's wire locked nut.
While the pitch links exhibited fractures, these were consistent with overload forces sustained during the impact rather than a pre-existing failure. The investigation confirmed that the helicopter's maintenance records, including the most recent 100-hour inspection, were up to date and that the aircraft held a valid Certificate of Airworthiness. The investigation also noted that the pilot's throttle was in the full "ON" position and the collective was near the full UP position at the time of the crash.
Findings
- The primary cause of the accident was the decay of the main rotor RPM below 97 percent, which stripped the aircraft of sufficient lift and led to the uncontrolled descent.
- A contributing factor to the occurrence was poor airmanship during the critical phase of the RPM decay.
- The aircraft's low-inertia rotor system is susceptible to rapid energy depletion, requiring immediate pilot action to maintain stability.