History of the Flight
On 25 February 2011, a Robinson R44 II helicopter, registration ZS-RVA, departed from Cape Town International Airport for a flight to a site near Paardevlei. The purpose of the flight was to conduct trials for testing radar equipment. At approximately 1236Z, during one of the trials, a witness heard a loud noise coming from the helicopter just before it crashed into the ground during daytime conditions. The accident site was located at coordinates S 34° 05’ 29.98” and E 018° 48’ 26.78” near Paardevlei, Western Cape. The pilot held a valid commercial pilot licence and medical certificate, with 4,300 total flying hours including 350 hours on type. The passenger was on board; both occupants sustained serious injuries, and the helicopter was destroyed.
Aircraft and Maintenance Information
The helicopter, a Robinson R44 II (serial number 10076), was manufactured in 2005 and had accumulated 1,778.4 total airframe hours at the time of the accident. The last mandatory periodic inspection was conducted on 19 August 2010 at 1,682.5 hours. The engine was a Lycoming IO-540-AE1A5 with 1,682.5 hours since new. The aircraft was equipped with standard navigation and communication equipment as per the approved minimum equipment list, with no recorded defects prior to the flight. The operator held a valid Part 127 Air Operator Certificate, and the aircraft maintenance organization was duly certified. According to the report, the maintenance records indicated the aircraft was maintained in accordance with existing regulations and approved procedures, yet the investigation pointed to probable inferior maintenance.
Wreckage and Impact
The accident site consisted mainly of sand and grass. The wreckage area was confined to initial impact marks made by the skids and the main wreckage resting on its left side. Impact occurred in a north-eastern direction with a high rate of descent in a nose-up attitude while in a slight right-hand turn. The skid marks were approximately 15 meters from the main wreckage. The flight profile was obtained from a Differential GPS (DGPS) that was on board, accurate to within 30 cm. The flex plate on the forward coupling failed in fatigue during flight. After failure, the coupling became out of balance and caused the drive belts to come off the pulley system, which also drives the tail rotor. This resulted in the tail rotor stopping while the engine was still running. The main fuel tank, firewall, and various pipes in the vicinity of the coupling were ruptured by the coupling.
Metallurgical Analysis
Metallurgical analysis of the fracture surface of the flex plate showed definite signs of fatigue. The origin of the crack was uncertain as it started slightly off from a hole underneath the washer contact surface. No clear signs of corrosion-induced pitting were detected, nor were any other material discrepancies noted. It was considered possible that the bolt or nut might have been over-torqued, which may have resulted in crack formation, most probably due to work hardening of the base material in the area of the contact surfaces, leading to lower fatigue strength and possible micro-cracking that caused stress raisers on the surface. At the initiation point, a nick mark was present together with a reduction in thickness; the nick mark appeared to have a stress-raising effect.
Survivability
Both occupants wore safety harnesses which did not fail. The helicopter was equipped with floats that deployed due to high impact forces. The front seats collapsed downwards, absorbing much of the impact energy. There was no fire in flight or after impact.