WPR19LA122

During the agricultural aerial application flight, the helicopter lost engine power, and the pilot performed an autorotation into sloping, tree-covered terrain. Examination of the engine revealed that a blade from the 3rd stage turbine wheel had been liberated, resulting in the loss of engine power. The blade’s fracture surface indicated that it was initiated in a low-cycle fatigue mode near the trailing edge radius of the blade and progressed to a high-cycle fatigue process along the blade chord, which finally resulted in overload failure. The initiation event for the failure could not be determined; however, it was likely caused by operation within an engine speed avoidance range, an engine speed/airframe interaction of unknown origin, inadequate engine temperature management on startup, inadequate wheel design or undetermined manufacturing defect, or combinations thereof. The blade had not been operated beyond its overhaul or inspection time limit. Similar blade failure was associated with five previous accidents and two previous incidents. An in-service replacement program was started by the engine manufacturer on July 30, 2021, requiring removal and replacement of the 3rd stage turbine wheel with a new design at the next scheduled replacement time.

On April 26, 2019, about 0700 Pacific daylight time, a Bell 206B, N206JH, was substantially damaged when it was involved in an accident near Oakridge, Oregon. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot had just sprayed a load of herbicide on trees and was maneuvering the helicopter for a final pass to complete the dispersal when he heard a faint buzzing sound that lasted about 1 to 2 seconds. There were no accompanying annunciator light warnings, and he did not hear any alert horns, but noticed that the engine power turbine (N2) gauge was oscillating about the 11 to 12 o'clock position (50-65% range). The rotor tachometer speed remained within the operating range, and he decided to perform an autorotation. The helicopter landed on sloping, tree-covered terrain and rolled over. The helicopter was equipped with a Rolls Royce M250-C20B turbo-shaft engine. Initial external inspection indicated that the N2 stage had locked, and that the exhaust duct and stacks exhibited multiple dents to their inner surfaces expanding in an outward direction. Disassembly and examination revealed that one blade of the 3rd stage turbine wheel, immediately following a shroud slot, was fractured and had liberated near the inner platform at the blade root. Visual examination of the fracture surface revealed progressive growth ring signatures consistent with fatigue from the trailing edge to approximately 2/3 along the chord, at which point the fracture surface changed texture, becoming rough, consistent with overload failure. The 3rd stage turbine wheel was sent to the Rolls Royce materials laboratory for further assessment. Evaluation revealed that the fracture initiated in a low-cycle fatigue near the trailing edge radius of the blade and progressed to a high-cycle fatigue process along the blade chord, which resulted in overload failure. Fluorescent penetrant inspection further revealed that the trailing edges of all airfoils within the packet that contained the failed blade, along with the preceding blade, exhibited cracks at similar locations on the blade root radius. Furthermore, the pressure side wax weld adjacent to every airfoil appeared to intersect the airfoil root radius. The wheel was identified as part of a post-service bulletin (SB) commercial engine bulletin (CEB)-1365 or “enhanced” turbine assembly design released by Rolls Royce in November 1999. The life limit for the wheel was 4,550 hours, and 6,000 cycles. The recommended time between overhaul for the turbine module was 3,500 hours. Maintenance records indicated that the accident 3rd stage wheel had accumulated 1,661.9 hours in 1,030 cycles. SB CEB-A-1400, Steady State Operation Avoidance Range Limit, was originally released in December 2006 and revised six times through January 2019. The SB was intended to reduce the possibility of high cycle fatigue in the 3rd stage turbine wheel by providing a seriesof speed avoidance restrictions. According to Rolls Royce records, the wheel released as part of SB CEB-1365 had been associated with five previous accidents and two previous incidents. As part of a mitigation plan, Rolls Royce undertook a redesign of the 3rd stage turbine wheel, featuring a modified fillet radius at the platform and improved manufacturing processes to reduce fillet variations, with production that began in December 2020. An in-service replacement program was begun on July 30, 2021, via SB CEB A-1428, requiring removal of the SB CEB-1365 3rd stage turbine wheel and installation of the new SB CEB A-1428 3rd stage turbine wheel no later than the next gas producer turbine wheel replacement time. An FAA Airworthiness Directive (AD) was planned to mandate the incorporation of SB CEB A-1428; however, as of the time of publication of this report, the release of the AD has not yet been established.