WPR19FA091 · Bell UH1B

The pilot departed on his first external load flight of the day; shortly thereafter, employees heard a mayday call over the company radio. The helicopter was found inverted in the designated logging area, with the cargo line attached to the cargo hook and the grapple attached to a log. Examination of the airframe and cargo hook revealed no anomalies or mechanical failures that would have precluded normal operation. Examination of the engine revealed that the gas generator first stage sealing disk had fractured and separated into three major pieces. Multiple internal components of the engine were subsequently damaged as a result, including all four turbine rotors and nozzles. An examination of the first stage sealing disk revealed fracture features consistent with low cycle fatigue. The overhauled engine was installed into the accident helicopter about 8 months before the accident. The sealing disk was installed into the engine at the time of overhaul and had previously accrued 1,067.3 hours and 2,134.6 cycles. The operator had not kept an accurate documentation of engine cycles, and it was unknown who was responsible for documenting the engine cycles into the daily flight logs. A review of maintenance records and the helicopter's electronic cycle counter revealed that the sealing disk had exceeded the published life limit of 6,900 cycles. The last documented cycle value was 9,023.13. The cycle counter had recorded 12,023.19 cycles. Following the sealing disk separation, and subsequent internal damage, the engine lost all power, necessitating a forced landing via autorotation. Due to the low altitude and densely tree covered terrain, it is unlikely that the pilot had reaction time to release the load which became entangled in the trees as he was attempting autorotation.

Verbatim NTSB analysis and probable cause from the NTSB dataset