Introduction
On 5 June 2023, a Sikorsky S92A helicopter, registration LN-OMI, operated by Bristow Norway AS, experienced a serious incident shortly after departure from the mobile drilling platform Linus, located approximately 10 NM west of Ekofisk L in the North Sea. The helicopter was on a passenger flight with two pilots and 14 passengers on board. No personal injuries were reported, and the incident occurred during daylight conditions.
Sequence of Events
The helicopter had landed on the Linus platform during a routine flight. Passengers boarded while the engines were running and the rotor system was rotating. After departure, as the helicopter climbed to an altitude between 3,000 and 4,000 feet, the crew heard a loud bang, and instruments indicated that the left engine had stopped abruptly. The pilots executed the relevant checklists and issued a mayday call. After troubleshooting, they determined that the engine could not be restarted and that the flight would need to continue on one engine.
Evaluating that the helicopter's performance was insufficient for an emergency landing on the platform, the pilots elected to fly directly to Stavanger Airport Sola. A rescue service helicopter met them en route. The helicopter landed on one engine without complications, and no one was injured.
Engine Examination
The left engine, a General Electric CT7-8A with serial number 947246, was sent to H+S Aviation Ltd in England for inspection. In July 2023, the NSIA examined the engine together with Sikorsky, General Electric, the NTSB, and the UK AAIB. Dismantling revealed that the joint between the centrifugal impeller and the compressor rotor rear shaft had disconnected. All twelve T-head rotor bolts that normally join the impeller and rear shaft had fractured, causing the uncommanded in-flight shutdown. No visual damage to the engine had been identified initially, except for small metal pieces found in the turbine area.
Maintenance History
Bristow Norway held approval only for line maintenance on the engines. The rear compressor shaft could not be inspected without engine removal and module dismantling. The engine had been removed from another helicopter on 30 May 2020 and sent to ITP in Albacete for repair due to compressor rub against the casing and high oil pressure. In 2021, during a test run after repair, the engine suffered a flameout and stopped abruptly, attributed to a fault in the newly installed fuel control. The engine sustained extensive damage and was repaired per work order 42355. It was tested and approved in 2022, with an EASA Form 1 issued on 22 June 2022.
Flight Operations Review
The NSIA conducted interviews with both pilots, which confirmed that the crew followed company procedures. The pilots' report of hearing a bang was consistent with the fracture of the twelve bolts and subsequent engine stop.
Engine Description
The Sikorsky S-92 is powered by two General Electric CT7-8A free-turbine engines, each delivering 2,498 SHP (1,863 kW). The engine comprises four main modules: accessory section, cold section, hot section, and power turbine module. The cold section contains a combined axial-flow and centrifugal compressor with five axial stages and one centrifugal impeller stage. The centrifugal impeller connects to the rear shaft via a mounting flange with interference fit and twelve T-head bolts.
Compressor Rotor Assembly
Assembly of the centrifugal impeller and rear shaft follows a detailed procedure in the General Electric Shop Manual (GEK 105175). Steps include measuring flange thickness, heating the rear shaft to 260°C for 20 minutes, joining flanges, installing and tightening T-bolts in stages, and final thickness measurements. The assembly is then balanced, with balance weights allowed only on two of the twelve T-bolts. After balancing, nuts are torqued to 70–75 in-lb (7.9–8.5 Nm).
Conclusion
The incident involved a fracture of all twelve T-bolts in the compressor rotor joint, leading to an uncommanded left engine shutdown. The helicopter landed safely on one engine with no injuries.