What happened
On 23 July 2006, a Mi-8MTV-1 helicopter, registration ZS-RUC, was conducting fire-fighting operations near Sabie, Mpumalanga. The aircraft was tasked with transporting firefighters and water to combat a forest fire. After successfully delivering three loads of water, the crew attempted to refill the bambi bucket at the Spitskop quarry dam.
While hovering approximately 5 metres above the water level, the crew began hoisting the bucket out of the dam to transition into forward flight. During this maneuver, the aircraft experienced a loud bang and intense high-frequency vibrations, accompanied by a significant drop in main rotor RPM. As the helicopter began to yaw heavily to the left, the flight engineer shut down both engines to prevent an uncontrolled spin. The aircraft subsequently struck the water and capsized as the main rotor blades made contact with the surface. The three crew members managed to evacuate through a cockpit window and swam to safety before the helicopter sank.
The investigation
SACAA AIID investigators examined the wreckage and analyzed flight data from the aircraft's Flight Data Recorder (FDR). The investigation involved technical analysis of the engines, which required the aircraft to be transported to Russia for specialized disassembly.
Data from the FDR revealed that the right-hand engine's exhaust gas temperature (EGT) had been running significantly higher than the left-hand engine prior to the failure. At the moment of the incident, the right-hand engine experienced a surge and stall, with temperatures peaking at 1049 °C. The investigation also noted that while the left-hand engine attempted to compensate by increasing power, the low altitude of the aircraft prevented a successful recovery.
Findings
- The primary cause of the accident was the failure of the right-hand engine's turbine section, which led to back pressure in the combustion chamber, reverse gas flow, and a compressor stall.
- The overheating in the turbine section caused the destruction of the gas generator and power turbine blades.
- The aircraft was flying at an altitude too low to allow for recovery, despite the automatic increase in power from the left-hand engine.
- The crew encountered difficulty understanding audible warnings and cockpit placards because they were in Russian, whereas the crew was English-speaking.
- The impact with the water caused substantial damage to the airframe, including the severing of the tail boom, tail rotor drive shaft, and main rotor blades.
Safety action
- It is recommended that the Regulator ensures all placards on South African registered aircraft are provided in English.
- It is recommended that the Regulator ensures operators and manufacturers possess the capability to download and transfer Cockpit Voice Recorder (CVR) data into a usable format.