History of Flight
The pilot, accompanied by a veterinarian passenger, operated the Robinson R44 helicopter (registration ZS-PXL) on a commercial game or livestock culling flight at Graspan National Park in the Northern Cape Province. The flight was intended to dart buffalo belonging to South African National Parks (SANParks). While flying at approximately 20 feet (7 meters) above ground level, the pilot reported a sudden severe vibration accompanied by a grinding or growling noise from the helicopter. The helicopter swayed left about 60 degrees, lost power, and an audible low RPM warning activated. The pilot applied collective control just before touchdown to cushion the landing, but the helicopter landed hard. The left skid became stuck in soft soil, prompting the pilot to apply aft cyclic to prevent rolling. A loud bang came from the rear as the main rotor blades struck and severed the tail boom. The helicopter swung violently left until the pilot closed the engine throttle; it then settled on the ground and was shut down completely. Both occupants evacuated without injury.
Damage to Aircraft
The helicopter sustained substantial damage. The main rotor blades struck the tail boom, severing it.
Personnel Information
The pilot held a valid commercial pilot license, was type endorsed on the Robinson R44, and had 1,808.3 total flight hours, including 760 hours on type. The pilot was 31 years old and had no previous accidents.
Aircraft Information
The Robinson R44 II (serial number 11828) was manufactured in 2007 and had 1,003.7 total airframe hours at the time of the accident. It was powered by a Lycoming IO-540-AE1A5 engine (serial number L32037-48E) with 1,003.7 hours since new. The helicopter was maintained by an approved maintenance organization, and the last mandatory periodic inspection (MPI) was performed on 16 September 2011 at 999.8 hours, with 3.9 hours flown since. Review of logbooks and flight folio revealed no anomalies or open defects.
Mechanical Failure and Probable Cause
Investigation revealed that the forward intermediate flex plate (part number A947-2G, batch Lot 292, inspection stamp RHC 28 USA) in the tail rotor driveshaft had failed. The flex plate was severely deformed and fractured adjacent to one of its four fixing holes, causing the tail rotor driveshaft to separate. The probable cause was determined to be a mechanical failure experienced during flight, resulting in an emergency landing followed by a hard landing. Contributory factors included the forward flex plate failing due to a fatigue crack developing at a bolt hole underneath the bonded washer, and the fatigue crack not being detected during maintenance inspection, as the Aircraft Maintenance Manual (AMM) limits flex plate crack detection to visual inspection.