N793SFBell 407 · 1999

On July 26, 2024, at 10:40 Central Daylight Time, a Viking Aviation LLC, doing business as Survival Flight, Bell 407 helicopter, powered by a single Rolls-Royce Corporation model M250-C47B turboshaft engine, experienced an uncontained engine failure while traveling to refuel after dropping off a patient. The engine failure necessitated a forced landing in a field. The pilot reported the incident occurred during the third flight of the day while using night vision goggles (NVG) from Oklahoma City, Oklahoma (OK) to Okmulgee, OK with the autopilot engaged and the helicopter in level flight at 2,000 feet mean sea level (MSL), 1,400 feet above ground level (AGL). The pilot reported that the cockpit gauges indicated the engine torque to be 78%, the gas producer temperature (NGT) 707°C-710°C, and gas producer rotor speed (Ng also commonly referred to as N1) at 98.6%.

Shortly after completing a fuel check, the pilot heard a loud boom and saw a bright flash of light out the left side of the helicopter, and the helicopter lost engine power. The bright light and explosion were also reported by the paramedic on board. The helicopter then yawed and rolled left. The pilot corrected for the yaw and roll and entered an autorotation, making a turn to a field to his right.

During the descent, the pilot noticed wires running left to right across the flight path and pitched the nose down to avoid the wires. The pilot and the paramedics stated that prior to the engine failure, there were no abnormal cockpit indications or warning lights. The pilot stated that an engine power assurance check was completed at the beginning of the shift and that the check was good. The pilot also stated that the oil levels were normal and there were no bypass indications.

The paramedics reported that this particular helicopter had a high pitch “whine” since it was received at their operations base; other helicopters at their operations base did not have the same “whine” sound. None of the three occupants on board were injured. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 135 on-demand helicopter air ambulance operation.

Persons from Survival Flight and the Federal Aviation Administration traveled to the accident site to document the wreckage. The wreckage was then transported to the Viking Aviation facility in Batesville, Arkansas where Rolls-Royce joined the investigating team to assess the aircraft and engine damage, download the flight data from the engine control unit, and witness the engine removal from the helicopter for additional examination. The NTSB did not travel for the initial on-scene examination of the airframe and engine nor for the follow-on examination at the Viking Aviation facility.

Initial examination of the helicopter revealed that it was intact but the following damage was identified: 1) the main landing gear skids were slightly splayed out with the helicopter still resting on the skids and not on its belly, 2) the tops of the tail rotor right- and left-hand finlets were damaged, and 3) engine exhaust duct was pock-marked outwards and exhibited impact damage and two exit penetration through-holes.

Examination of the engine through the engine exhaust duct revealed that the exhaust collector support exhibited impact marks and multiple penetrations. Additionally, debris impact marks/tears were also observed on the horizontal firewall shield. Bolts used to secure the gas producer support case to the power turbine support case were fractured and the two cases flanges were separated creating a gap where the energy absorbing ring was visible.

The containment ring located around the outside of the gas producer nozzle assembly appeared distorted but intact. The containment ring is designed to absorb energy in the event of gas producer turbine stage 1 rotor failure. Multiple pieces of turbine engine debris were found in the engine bay on top of the horizontal firewall shield. One piece was consistent with part of the gas producer stage 1 turbine wheel.

The engine was shipped to the Rolls-Royce facility in Indianapolis, Indiana where the engine was examined, disassembled, and documented with persons from the Federal Aviation Administration, Rolls-Royce, Arrow Aviation LLC., Viking Aviation, and National Transportation Safety Board in attendance. The gas producer stage 1 turbine wheel experienced a disk burst and fragmented into multiple pieces. Approximately 80% (by weight) of the wheel fragments were recovered within the engine and helicopter airframe.

The stage 1 and stage 2 disk curvic coupling teeth were rotationally smeared and very little tooth height remained. The stage 2 turbine disk was intact, and all the blades were present. The tie bolt that secures the stage 1 and stage 2 turbine wheels together was found fractured in-line with the stage 1 turbine wheel. All the fragments of the gas producer stage 1 turbine wheel and the tie bolt were sent to the Rolls-Royce materials laboratory for metallurgical evaluation. The stage 2 turbine nozzle inner stationary air seal exhibited rotational damage, distortion radially outwards, and the knife edges were flattened. All eight anti-rotation lugs that engage with the gas producer turbine support were fractured.

The power turbine support includes the stage 3 turbine nozzle support and the Nos. 6 and 7 roller bearing housing. The Nos. 6 and 7 roller bearing housing is attached to the stage 3 turbine nozzle support by four struts. All four bearing radial support struts were fractured, and the bearing support was loose within the power turbine section; the Nos. 6 and 7 bearings were intact but dry.

The power turbine stage 3 turbine wheel was completely corn-cobbed with almost all the airfoils fractured transversely across the platform while all the power turbine stage 4 turbine wheel blades were present and exhibited hard body impact damage and were fractured transversely across the airfoil at various lengths along the outer 25% of the blade span. Both the power turbine stage 3 and stage 4 nozzle assemblies exhibited vane impact damage and circumferential scoring and material transfer. The power turbine bearing support, the power turbine inner shaft, the power turbine rotating labyrinth seal, and the power turbine stage 3 wheel were sent to the Rolls-Royce materials laboratory for metallurgical evaluation.

Metallurgical examination of the gas producer stage 1 turbine wheel fracture surfaces using binocular and scanning electron microscopes (SEM) revealed dendritic and ductile dimple features consistent with overload. A cross section through the rim of the wheel and through one of the partial blades revealed a coarsened microstructure and partially solutioned gamma prime region in the airfoil section consistent with thermal distress while the rim section did not exhibit those signs of thermal distress. Semi-quantitative energy dispersive x-ray spectroscopy analysis confirmed that the gas producer stage 1 turbine wheel material composition was consistent with the manufacturing print.

The tie bolt fracture surfaces showed evidence of ductile dimples consistent with tensile and torsional overload; the material composition was consistent with the manufacturing print. During the last engine overhaul, the gas producer stage 1 turbine wheel and the tie bolt were installed as new, zero-time components.

Metallurgical examination of the remnants of the power turbine stage 3 turbine wheel blade airfoils revealed that one airfoil exhibited features consistent with a high cycle fatigue fracture with initiation on the trailing edge and progressing forward approximately a half inch before the blade ultimately failed in overload. SEM images and semi-quantitative energy dispersive x-ray spectroscopy (EDS) of the fracture surface revealed no chemical or material anomalies near the fatigue crack origin; oxidation was also present which is consistent with the crack being exposed to engine gas path operating temperatures. The material composition of the power turbine stage 3 wheel was consistent with the manufacturing print. Grit blasting and micro-etching confirmed that the material grain size was as specified, and 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 Federal Aviation Administration issued Airworthiness Directive 2006-20-07, to prevent loss of power, possible engine shutdown, or uncontained failure by requiring operators to track power turbine speed excursions in those speed ranges that induce a vibratory response that can cause the stage 3 turbine wheel blades to crack and fail. The stage 3 and stage 4 turbine wheels are allowed a maximum of six power turbine speed excursion events before they must be removed from service. These tracking requirements are also included in the Rolls-Royce 250-C47B operation and maintenance manual along with the speed ranges to avoid.

Review of the Viking Aviation engine inspection and components card for the accident helicopter revealed that as of May 2023, the last overhaul of the turbine, the stage 3 and stage 4 wheels had all 6 allowable power turbine speed excursions remaining. Furthermore, the power turbine stage 3 turbine wheel was visually inspected and fluorescent penetrant inspected at the last engine overhaul, 45.5 hours and 99 cycles, prior to the accident.

The engine electronic control unit stores engine operating data and fault data. The data was downloaded and examination of the engine operating and fault data showed that the first indications of an engine problem were a loss of torque with a corresponding decrease in power turbine rotational speed and helicopter rotor speed and an increase in gas turbine temperature; all this occurred at essentially the same time.

At this time, faults for power turbine speed and engine torque limit exceedances were also recorded coinciding with the engine parameter shifts noted above. The faults and initial engine operating parameter movements were all consistent with a failure with the power turbine section first. A few seconds later, gas producer speed and fuel flow started to drop, indicating an issue in the gas producer section. The engine control unit summary report is programmed to list the power turbine exceedances above the do not exceed speeds for the engine. The report contained no recorded exceedances for the gas producer or power turbine.