Bell 206 N13AT accident near St Thomas, CB

Probable cause

A total loss of engine power due to fatigue failure of two of the stage 3 compressor blades. Contributing to the failure of the compressor blades was the failure of maintenance personnel to inspect the compressor at the recommended interval for operation in corrosive environments.

NTSB analysis

During a short sightseeing flight around an island, the helicopter lost total engine power and descended into heavily wooded, steep terrain. The majority of the helicopter was consumed by a postcrash fire. The helicopter’s proximity to the rising terrain at the time of the power loss likely precluded the pilot from performing a successful autorotation.

Examination of the engine found evidence of fatigue fractures on two of the stage 3 compressor blades and fatigue fractures on the majority of the stage 6 compressor blades. Analysis of the remainder of the stages 3 and 6 compressor wheels showed that they met material requirements. Thermal damage to the compressor case halves precluded determining whether the stages 3 and 6 blades were rubbing against the case and its plastic coating, which could have initiated the fatigue fractures. While pitting corrosion could also result in fatigue fractures, no evidence of pitting corrosion was found and only surface corrosion was present. The liberation of the two stage 3 compressor blades led to their ingestion within the remaining compressor stages, resulting in the total loss of engine power in flight.

The engine maintenance manual contained provisions for more frequent inspections when the engine is operating in an erosive and/or corrosive environment, such as where the accident operator was located. A caution within the 300-hour inspection table stated that the inspection must not exceed 300 hours or 12 months for coated compressor wheels, including the accident engine’s stage 2-3 compressor wheel. The engine logbook showed that a 300-hour engine inspection was completed about 3 years and 800 flight hours before the accident. Paperwork associated with the two most recent inspections, conducted by the accident operator about 11 months and 1 month before the accident, respectively, did not indicate that the 300-hour inspection had been performed. Had maintenance personnel completed the 300-hour inspection of the compressor case halves, blades, and vanes within the recommended 12-month interval, the presence of corrosion or damage to the plastic coating could have been detected and addressed.

Text from the NTSB. Open the NTSB docket for ERA21FA130 for the full record.