BELL HELICOPTER TEXTRON 206L-1

Full Narrative

On December 28, 2023, about 1340 eastern standard time, a Bell 206L-1 helicopter, N571CJ, was substantially damaged when it was involved in an accident near Walland, Tennessee. The pilot and five passengers were not injured. The helicopter was operated by Whirl'd Helicopters, Inc. (doing business as Scenic Helicopter Tours) as a Title 14 Code of Federal Regulations Part 91 revenue sightseeing flight. 
The flight departed from Eagles Landing Heliport (03TN), Sevierville, Tennessee, about 1300 for a local tour flight. About 35 minutes into the flight, the pilot observed an engine chip light announcement on the instrument panel. The pilot immediately began maneuvering toward a landing area and, within a few seconds, he heard an abnormal noise from the engine and saw the engine oil temperature and pressure indications decrease to zero. The pilot immediately entered an autorotation while continuing to maneuver to the landing area. About 3 to 5 seconds later, the engine stopped producing power. During the autorotation landing to a field, the helicopter touched down hard, skidded forward several feet, and came to a stop. During this time, the main rotor blades contacted one of the vertical stabilizers, nearly severing the tailboom.
The engine was a Rolls-Royce 250-C28B and had accumulated 13,378.7 hours time since new (TSN). The engine was last repaired on July 27, 2021. The gearbox assembly was installed on July 14, 2021, and the compressor assembly was installed on July 27, 2021, at an engine TSN of 12,823.3 hours. The turbine assembly was installed on July 24, 2019, at an engine TSN of 11,980.9 hours.
A postaccident field examination revealed that all the 4th stage turbine blades were intact, but most of the blade shrouds were worn away. Manual rotation of the N1 rotor spool resulted in rotation of the N2 rotor spool, consistent with a mechanical lock between shafts. The upper chip detector showed a coating of black paste and metallic debris, and the lower chip detector was clean of ferrous debris.
The engine to airframe oil tank lines were found secure with no evidence of leakage. Residual oil was observed in the supply line from the oil tank to the engine, the return line was void of oil, and the engine oil tank contained less than half a tank of oil. About 16 ounces of oil was drained from the accessory gear box.
Engine disassembly revealed that the compressor module was intact and undamaged, and the compressor impeller turned freely and smoothly. The spur adapter gearshaft (SAG) was intact and undamaged, and the turbine-to-compressor coupling spline was intact. The aft SAG O-ring was disintegrated into coarse black paste, and there was no evidence of an O-ring in the forward SAG O-ring groove. The accessory gearbox internal components and oil scavenge and pressure pumps were undamaged and well lubricated.
The 4th stage turbine wheel was intact, but the curvic coupling mating it with the 3rd stage turbine wheel showed rotational smearing. Further, most of the blade shrouds were rotationally abraded away, consistent with contact against the 4th stage wheel tracks. The 4th stage turbine nozzle was intact but showed rotational scoring on the 3rd and 4th stage turbine wheel tracks.
The power turbine bearing support was intact, but the 3rd and 1st stage turbine wheels could not be withdrawn because the turbine-to-compressor coupling and the power turbine inner shaft were fractured and deformed outward, which locked the 3rd stage turbine wheel. The 3rd stage turbine wheel was intact, but the curvic coupling mating with the 4th stage turbine wheel showed rotational smearing. All 3rd stage turbine wheel blades were intact.
The combustor outer case and the combustor liner were intact and undamaged, and the fuel nozzle was intact, undamaged, and clean.
The No. 8 bearing sump cavity was found dry and coated with a black residue, and the No. 8 bearing spanner nut was found untorqued and backed out until about the last thread. The No. 8 bearing spanner nut exhibited one partial crimp and two blended locations.
Generally, when a No. 8 bearing spanner nut is removed during overhaul, the previous locking crimp must first pried open before the nut can be unscrewed. The overhaul manual allows each crimp to be used only once, so the previous deformed crimped material on the nut must be blended away and a new crimp location must be chosen on the nut after it is re-torqued onto the shaft during re-assembly of the engine.
Examination of the partial crimp that was found on the No. 8 bearing spanner nut, revealed a pry mark on the inner surface of the crimp ring, which was consistent with the removal of the crimp during the last disassembly. Additionally, the inner crimp surface was oxidized, due to exposure to the hot gases during engine operation, consistent with a non-contact condition the turbine wheel shaft. No evidence was found that locking crimp was made on the No. 8 bearing spanner nut during the last assembly.
The No. 8 bearing sump cover showed rotational scoring consistent with contact with the No. 8 bearing spanner nut during engine operation. The No. 8 bearing exhibited significant wear and thermal damage to the bearing balls and inner ring, as well as a loss of the forward shoulder of the inner raceway ring.
Rub damage on the power turbine support and 2nd stage turbine wheel was consistent with loss of axial and radial retention of the gas producer rotor during operation.