What aircraft was involved and where did it happen?

The accident on 2007-01-18 involved a MBB Bo 105 CBS-4, registration ZS-RNO, at Approximately 5nm offshore during cruise flight over the Atlantic Ocean, ZA.

What was the probable cause of the accident?

The failure of the number 2 engine was caused by the separation of several stator vanes from the upper compressor case, which subsequently damaged the downstream compressor wheel airfoils.

Engine Failure on MBB Bo 105 During Offshore Flight

A commercial helicopter flight from an offshore mining vessel to Namibia experienced an in-flight engine shutdown due to internal compressor component failure.

What happened

On 18 January 2007, an MBB Bo 105 CBS-4 helicopter, registration ZS-RNO, was performing a commercial flight from the Sakawe mining vessel off the Namibian coast toward Lüderitt Aerodrome. While cruising approximately 5nm offshore over the Atlantic Ocean, the crew heard a loud bang originating from the rear cabin area.

Following the noise, the pilot observed that the turbine outlet temperature for the number 2 engine had spiked to its maximum limit, while engine torque dropped to roughly 20%. As the low rotor RPM warning horn sounded, the pilot reacted by lowering the collective pitch to unload the blades and stabilize the rotor speed. The pilot successfully transitioned to single-engine flight, secured the inoperative engine according to the Pilot’s Operating Handbook, and performed a controlled descent. The aircraft landed safely at Lüderitz Aerodrome with no injuries to the pilot or the four passengers on board.

The investigation

An investigation by the SACAA AIID examined the engine's compressor assembly, which was sent to a specialized laboratory in the United States for metallurgical analysis. The examination revealed that several stator vanes from the third, fourth, and fifth stages had detached from the upper compressor case. This separation caused significant impact damage to the downstream compressor wheel airfoils, specifically from the third through the sixth stages.

Investigators also identified evidence of erosion on the compressor's plastic coating and the leading edges of the vanes. The presence of intergranular corrosion suggested that once the protective coatings were compromised by erosion, moisture from the sea air had penetrated the metal surfaces. The investigation noted that the aircraft was operating in a harsh environment characterized by both desert dust and salty sea air, and that the pilot was responsible for daily maintenance tasks, including compressor water washes.

Findings

The primary cause of the engine failure was the separation of multiple stator vanes from the upper compressor case, which led to extensive downstream damage.
The erosion of the compressor vanes was likely caused by the ingestion of sand or dust during operations.
The presence of corrosion was a result of sea air penetrating the surfaces where the protective coating had been eroded.
The lack of a full-time maintenance engineer at the Lüderitz base acted as a significant contributory factor.
The challenging operating environment, involving both maritime and desert conditions, contributed to the degradation of the engine components.

Safety action

It is recommended that the Airworthiness Department review maintenance requirements for aircraft operating in combined desert and offshore environments, specifically regarding the frequency of compressor water washes and the cleaning of sand filters.
The investigation highlighted the need for increased oversight by the SACAA regarding South African-registered aircraft and operators performing contracts in foreign territories to ensure maintenance and operational standards are strictly upheld.