Incident Summary
On July 26, 2024, at 10:40 Central Daylight Time, a Bell 407 helicopter powered by a single Rolls-Royce M250-C47B turboshaft engine experienced an uncontained engine failure while en route to refuel after dropping off a patient. The helicopter was operated by Viking Aviation LLC, doing business as Survival Flight, as a Part 135 on-demand air ambulance. The pilot reported that the incident occurred during the third flight of the day while using night vision goggles, traveling from Oklahoma City, Oklahoma, to Okmulgee, Oklahoma. The autopilot was engaged, and the helicopter was in level flight at 2,000 feet MSL (1,400 feet AGL). Engine parameters indicated torque 78%, NGT 707–710°C, and Ng 98.6%. Shortly after a fuel check, the pilot heard a loud boom and saw a bright flash from the left side, followed by a loss of engine power. The helicopter yawed and rolled left; the pilot corrected and entered an autorotation, turning toward a field. During descent, wires were observed, and the pilot pitched the nose down to avoid them. No abnormal cockpit indications or warning lights preceded the failure. The pilot stated that an engine power assurance check completed at the start of the shift was satisfactory, and oil levels were normal with no bypass indications. Paramedics reported that this helicopter had a high-pitched whine since it arrived at the base, which was not present on other aircraft. None of the three occupants were injured.
Investigation and Damage
Personnel from Survival Flight and the Federal Aviation Administration documented the wreckage at the site. The helicopter was then transported to the Viking Aviation facility in Batesville, Arkansas, where Rolls-Royce joined the investigation. The NTSB did not attend the on-scene or follow-on examinations. The airframe was intact but sustained damage: main landing gear skids slightly splayed (helicopter remained on skids), tops of both tail rotor finlets damaged, and the engine exhaust duct showed pock-marking and two exit penetrations. The exhaust collector support had impact marks and multiple penetrations, and debris marks were on the horizontal firewall shield. Bolts securing the gas producer support case to the power turbine support case were fractured, creating a gap where the energy-absorbing ring was visible. The containment ring around the gas producer nozzle assembly was distorted but intact. Multiple turbine engine debris pieces were found on the firewall, including a piece consistent with the gas producer stage 1 turbine wheel.
Engine Examination
The engine was shipped to Rolls-Royce in Indianapolis, Indiana, where it was examined and disassembled with representatives from the FAA, Rolls-Royce, Arrow Aviation LLC, Viking Aviation, and the NTSB. The gas producer stage 1 turbine wheel had burst into multiple fragments; about 80% (by weight) were recovered. The curvic coupling teeth on stage 1 and stage 2 disks were rotationally smeared, with little tooth height remaining. The stage 2 disk was intact with all blades. The tie bolt securing the stage 1 and stage 2 wheels was fractured in line with the stage 1 wheel. All fragments were sent for metallurgical evaluation. The stage 2 turbine nozzle inner stationary air seal showed rotational damage, outward distortion, and flattened knife edges. All eight anti-rotation lugs engaging with the gas producer turbine support were fractured. In the power turbine section, the Nos. 6 and 7 roller bearing housing was attached to the stage 3 turbine nozzle support by four struts; all were fractured, and the bearing support was loose. The bearings were intact but dry. The power turbine stage 3 wheel was completely corn-cobbed, with almost all airfoils fractured across the platform. Stage 4 blades were present but showed hard body impact damage and transverse fractures within the outer 25% of the blade span. Both stage 3 and 4 nozzle assemblies had vane impact damage, circumferential scoring, and material transfer.
Metallurgical Findings
Metallurgical examination of the gas producer stage 1 turbine wheel fracture surfaces using binocular and scanning electron microscopes revealed dendritic and ductile dimple features consistent with overload. A cross-section of the rim and a partial blade showed a coarsened microstructure and partially solutioned gamma prime region in the airfoil, consistent with thermal distress; the rim did not. Composition matched the manufacturing print. The tie bolt fracture surfaces showed ductile dimples consistent with tensile and torsional overload; composition also matched. During the last engine overhaul, the gas producer stage 1 wheel and tie bolt were installed as new, zero-time components. For the power turbine stage 3 wheel, one airfoil exhibited features consistent with high-cycle fatigue fracture, with initiation on the trailing edge and propagation approximately half an inch before overload. No chemical or material anomalies were found near the fatigue crack origin, but oxidation was present, consistent with exposure to engine gas path operating temperatures. Composition matched the print, and grain size was as specified; no thermal distress or microstructural anomalies were observed. The stage 3 turbine wheel is susceptible to high-cycle fatigue blade fractures due to vibratory responses at certain power turbine operational speed ranges. The FAA issued Airworthiness Directive 2006-20-07 to address this, requiring tracking of power turbine speed excursions. Stage 3 and 4 wheels are allowed a maximum of six such excursions. Review of Viking Aviation's records showed that as of May 2023 (after the last overhaul), the stage 3 and 4 wheels had all six allowable excursions remaining. The stage 3 wheel had been visually and fluorescent penetrant inspected at the last overhaul, 45.5 hours and 99 cycles before the accident.
Engine Control Unit Data
The engine electronic control unit data showed first indications of a problem: loss of torque with corresponding decreases in power turbine and rotor speeds, and an increase in gas turbine temperature, all occurring simultaneously. Faults for power turbine speed and engine torque limit exceedances were recorded at the same time. These parameters were consistent with a failure in the power turbine section first. Seconds later, gas producer speed and fuel flow dropped, indicating an issue in the gas producer section. The ECU summary report listed no exceedances for gas producer or power turbine speeds.