What happened
On July 26, 2024, at 10:40 Central Daylight Time, a Bell 407 helicopter, operated by Viking Aviation LLC, experienced an uncontained engine failure while en route from Oklahoma City, Oklahoma, to Okmulgee, Oklahoma. The aircraft, registration N793SF, was performing its third flight of the day, transporting medical personnel to refuel after a patient drop-off. The pilot was utilizing night vision goggles (NVG) and flying in level flight at 2,000 feet MSL.
During the flight, the pilot reported hearing a loud boom and seeing a bright flash of light from the left side of the aircraft, followed by a loss of engine power. The helicopter yawed and rolled to the left, prompting the pilot to enter an autorotation and maneuver toward a nearby field. During the descent, the pilot adjusted the pitch to avoid wires crossing the flight path. The aircraft landed in a field, and all 3 occupants were not injured.
The investigation
Investigators examined the wreckage at the accident site and at a facility in Arkansas, where Rolls-Royce personnel participated in the engine analysis. The engine, a Rolls-Royce M250-C47B turboshaft, showed significant damage. The engine exhaust duct was pock-marked and featured two exit holes, and debris was found in the engine bay.
Detailed examination of the engine revealed that the gas producer stage 1 turbine wheel had experienced a disk burst. The investigation found that the stage 3 turbine wheel was heavily damaged, with one airfoil showing a high cycle fatigue (HCF) fracture. This failure created an imbalance that caused the power turbine rotor to rotate eccentrically, leading to contact and rub within the bearing support. This imbalance transferred high radial and axial loads to the gas producer rotor, causing a speed mismatch between the stage 1 and stage 2 wheels. This mismatch created a torsional load that fractured the tie-bolt, allowing the stage 1 wheel to decouple and overspeed, ultimately resulting in the disk burst.
Engine control unit (ECU) data confirmed that the initial indications of the failure were a reduction in power turbine speed and torque, followed several seconds later by a drop in fuel flow and gas producer speed.