Accident Overview
On April 26, 2019, at approximately 0700 Pacific daylight time, a Bell 206B helicopter (registration N206JH) was substantially damaged in an accident near Oakridge, Oregon. The pilot, who was the sole occupant, was not injured. The helicopter was operating under Title 14 Code of Federal Regulations Part 137 as an aerial application flight.
Sequence of Events
The pilot had just sprayed a load of herbicide on trees and was maneuvering for a final dispersal pass when he heard a faint buzzing sound lasting 1 to 2 seconds. No annunciator lights or alert horns accompanied the sound. However, the engine power turbine (N2) gauge oscillated in the 50-65% range, while the rotor tachometer remained within operating limits. The pilot elected to perform an autorotation. During the landing on sloping, tree-covered terrain, the helicopter rolled over.
Engine and Turbine Blade Examination
The helicopter was equipped with a Rolls Royce M250-C20B turbo-shaft engine. Initial external inspection revealed that the N2 stage had locked, and the exhaust duct and stacks showed multiple inward dents with outward expansion. Disassembly and examination found that one blade of the 3rd stage turbine wheel had fractured near the inner platform at the blade root. Visual examination of the fracture surface showed progressive growth ring signatures consistent with fatigue from the trailing edge to approximately two-thirds along the chord, transitioning to a rough texture indicative of overload failure.
Metallurgical Analysis
The 3rd stage turbine wheel was sent to the Rolls Royce materials laboratory. Evaluation determined that the fracture initiated in low-cycle fatigue near the trailing edge radius, progressed to high-cycle fatigue along the chord, and resulted in overload failure. Fluorescent penetrant inspection revealed cracks at similar locations on the blade root radius of the failed blade's packet and the preceding blade. Additionally, the pressure side wax weld adjacent to every airfoil appeared to intersect the airfoil root radius.
Component History and Service Bulletins
The 3rd stage turbine wheel was part of a post-service bulletin (SB) commercial engine bulletin (CEB)-1365, an enhanced turbine assembly design released by Rolls Royce in November 1999. The wheel had a life limit of 4,550 hours and 6,000 cycles, with a recommended time between overhaul of 3,500 hours for the turbine module. Maintenance records indicated the accident wheel had accumulated 1,661.9 hours in 1,030 cycles.
Service Bulletin CEB-A-1400, originally released in December 2006 and revised six times through January 2019, aimed to reduce high-cycle fatigue risk in the 3rd stage turbine wheel by providing speed avoidance restrictions. Rolls Royce records showed that the SB CEB-1365 wheel had been associated with five previous accidents and two incidents.
Mitigation and Redesign
As part of a mitigation plan, Rolls Royce redesigned the 3rd stage turbine wheel, incorporating a modified fillet radius at the platform and improved manufacturing processes to reduce fillet variations. Production of the new design began in December 2020. An in-service replacement program started on July 30, 2021, via SB CEB A-1428, requiring removal of the older wheel and installation of the new one no later than the next gas producer turbine wheel replacement. An FAA Airworthiness Directive (AD) was planned to mandate this replacement but had not been published as of the report's issuance.