What happened
On 25 February 2011, a Robinson R44 Ravin II, registration ZS-RVA, departed from Cape Town International Airport to conduct radar equipment testing near Paardevlei. During the flight, while the pilot was performing orbital maneuvers around a radar site, a witness reported hearing a loud noise. Immediately following this sound, the helicopter entered a high rate of descent and crashed into the ground. The impact destroyed the aircraft and caused serious injuries to both the pilot and the passenger.
The investigation
SACAA AIID investigators examined the wreckage and the flight profile using DGPS data. The investigation established that the aircraft was operating under clear weather conditions and was properly licensed and equipped for the mission. Metallurgical analysis focused on the forward coupling, which serves to transfer power from the engine to the main rotor gearbox. The study revealed that the flex plate on this coupling had failed due to fatigue. Investigators noted a nick mark and a reduction in material thickness at the initiation point. While maintenance records suggested the aircraft was maintained according to regulations, the physical evidence suggested the crack may have originated under a washer, making it difficult to detect during standard inspections. There was also a possibility that the bolt or nut had been over-torqued, leading to work hardening and subsequent micro-cracking.
Findings
- The primary cause of the accident was the uncontrollability of the helicopter following the failure of the flex plate on the forward flex coupling.
- The failure of the coupling caused the drive belts to detach from the pulley system, which subsequently stopped the tail rotor from functioning while the engine remained operational.
- A contributing factor was identified as probable inferior maintenance practices.
- The maintenance manual was found to be unclear regarding specific procedures for this component.
Safety action
Following the investigation, an urgent Mandatory Aeronautical Notice (MAN) was issued to ground all South African registered Robinson R44 and R22 helicopters. This grounding was intended to allow for thorough inspections of the forward flex plates. Additionally, the SACAA Airworthiness section was tasked with developing a standardized inspection procedure, utilizing non-destructive testing (NDT) or specific inspection intervals, to prevent similar fatigue-related failures.