What happened
On 1 February 2007, an MBB BO-105-CBS-4 helicopter, registration ZS-RBB, departed from Nelspruit Aerodrome on a law enforcement mission. The crew, including a pilot and a law enforcement officer, were en route to the Roossenekal area to support South African Police ground units during a stock theft operation.
Roughly 15 minutes into the flight, the pilot noticed a rise in the turbine outlet temperature on the first engine, followed by two consecutive compressor stalls. In response to the engine abnormality, the pilot performed a precautionary landing in an open area near the Kulna dam. Upon touchdown, a "CHIP" warning light activated and smoke emerged from the cockpit, prompting the pilot to immediately shut down both engines. There were 0 fatalities and 0 injuries during the incident.
The investigation
SACAA AIID investigators examined the aircraft and the engine, a Rolls-Royce 2/50-C20B. Post-landing inspections revealed that the number one engine's oil reservoir was empty and the engine was coated in oil. The investigation found that the front bearing of the engine had been significantly disrupted.
A detailed metallurgical teardown conducted at a specialized facility revealed that the third-stage compressor wheel airfoil had fractured due to fatigue. This failure was preceded by intergranular corrosion and erosion on the leading edge of the blades. The investigation also uncovered that the maintenance organization had been using standard tap water for the 50-hour compressor rinsing procedure, rather than the distilled water required by the manufacturer's maintenance manual. The chemicals in the tap water were found to have accelerated the corrosion process.
Findings
- The primary cause of the incident was the fatigue fracture of a 3rd stage compressor wheel airfoil.
- This fracture was driven by erosion and corrosion on the leading edge of the blades.
- The failure of the airfoil created excessive radial loads, which subsequently damaged the number one bearing.
- A significant contributing factor was the failure to follow prescribed maintenance protocols, specifically the use of tap water instead of distilled water during compressor rinsing, which exacerbated metal degradation.