What happened
On September 01, 2022, at approximately 0735 Pacific daylight time, a Sikorsky S-61N helicopter, registration N617CK, was involved in an accident near White City, Oregon. The aircraft was operating as a rotorcraft external load flight for timber logging operations.
Two days prior to the accident, during a flight on August 30, 2022, the crew experienced an aborted engine start on the No. 2 engine. During the sequence, the T5 temperature increased to 720°C, and the digital gauge eventually displayed 001°C. The pilot in command did not report this event to the director of maintenance, as he believed a hot start required a temperature of 950°C sustained for two seconds. No entries were made in the maintenance log regarding this event.
On the day of the accident, the crew heard a loud "bang" during climb out from a log landing site. After assessing the instruments and finding them within normal limits, the crew continued the operation. Two additional sorties were completed, but the "bang" sound occurred again, prompting a precautionary landing. Maintenance personnel could not identify a definitive source for the noise, and the crew elected to continue operations.
During a subsequent departure from the log landing site, the crew experienced several successive "bangs" followed by a grinding noise. The No. 2 engine torque value dropped to zero, and the Master Caution light illuminated. The helicopter descended to the ground, striking the terrain hard on the main landing gear before rolling onto its right side. The accident resulted in 2 minor injuries to the crew and ground support member, and 0 fatalities.
The investigation
Post-accident examination of the No. 2 engine revealed overtemperature distress throughout the gas generator turbine (GGT) section. Specifically, all stage 1 turbine blades showed severe overtemperature distress, with four blades separating at their mid-span and others exhibiting cracks and profile "cupping." Metallurgical examination of the blades showed evidence of bulk deformation, coating melting, and creep voiding consistent with overtemperature exposure.
Additionally, the turbine coupling shaft was difficult to remove, and the turbine forward shaft showed a dark blue discoloration indicative of thermal deformation. While the engine temperature gauges were found to be operational, a fuel flow divider removed for testing was found to not meet operational standards.