What happened
On 5 June 2023, a Sikorsky S-92A helicopter, registration LN-OMI, operated by Bristow Norway AS, was conducting a routine passenger flight from the Linus mobile drilling platform in the Ekoflag field to Stavanger Airport Sola. Shortly after departing the platform, while climbing between 3,000 and 4,000 feet, the crew heard a loud bang and observed that the left engine had abruptly stopped.
Following the engine failure, the pilots initiated emergency checklists and declared a mayday. After determining that the engine could not be restarted, the crew evaluated the aircraft's performance and decided to proceed to Stavanger rather than attempting an emergency landing on the offshore platform. The flight continued on a single engine and landed safely at the airport with no injuries to the two pilots or 14 passengers.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the engine, a General Electric CT7-8A, and collaborated with international agencies including the NTSB and the UK AAIB. Physical inspection of the engine revealed that the joint between the centrifugal impeller and the compressor rotor rear shaft had disconnected.
Metallurgical examinations conducted by QinetiQ found that all twelve T-bolts used to secure the impeller to the shaft had fractured. The investigation traced the engine's history back to a repair performed by ITP in Spain. While the investigators could not definitively prove the root cause, the evidence pointed toward a maintenance-related issue. The investigation also noted that the task of assembling the compressor rotor is highly complex, involving heating, cooling, and balancing procedures that increase the risk of human error.
Findings
- The left engine shutdown was caused by the fracture of all twelve T-bolts connecting the centrifugal compressor to the rear shaft.
- The failures originated as fatigue cracks in the bolts, eventually leading to an overload failure.
- The complexity of the compressor rotor assembly process increases the likelihood of human error during maintenance.
- There was a lack of specific traceability for the torque wrench used during the critical assembly task in the maintenance records.
Safety action
The NSIA issued Safety Recommendation Aviation nr. 2025/06T, advising General Electric to update the Shop Manual GEK105175. The update should include a specific safety measurement or verification step to identify potential human errors following the assembly and balancing of the compressor rotor.