Sikorsky S-61N N617CK

Two days before the accident flight, the helicopter flight crew conducted a timber logging flight. The No.1 engine started without issue, but the No. 2 engine start was aborted after it sustained a hot start, as confirmed by the No. 2 engine T5 temperature digital gauge indication of 999° C, a subsequent indication of --- ° C, and finally 001° C. The engine was allowed to cool, and about five minutes later the PIC started the No. 2 engine without issue, followed by the No. 1 engine; the flight crew then completed an eight-hour flight day. The helicopter remained in service until the day of the accident. On the day of the accident, while performing their normal logging operation flight, the flight crew heard a loud “bang” twice and decided to make a precautionary landing. With the engines turning, no anomalies were confirmed by maintenance personnel, and they continued flight operations. The flight crew did not perform an out-of-ground-effect hover power check to determine whether they had single-engine capability. After picking up about a 4,000 pound load during climb out of the confined area, another loud “bang” occurred, followed by a series of successive “bangs” that resulted in a vertical descent and hard impact with terrain, which resulted in a subsequent rollover onto the helicopter’s right fuselage. Postaccident examination of the helicopter’s No. 2 engine revealed overtemperature distress of the gas generator turbine (GGT) section. All stage 1 turbine blades revealed overtemperature distress. Most of the stage 2 turbine blades exhibited deformation about the leading edges and blade tips. During examination, the turbine coupling shaft was unusually difficult to remove, indicative of overtemperature effects, and the turbine forward shaft exhibited a dark blue discoloration indicative of thermal deformation. The metallurgical examination of 11 stage 1 turbine blades revealed evidence of bulk deformation, coating cracking/melting, and creep voiding/surface cracking consistent with overtemperature exposure. The PIC stated that he did not report the hot start event to the operator’s director of maintenance because he understood a hot start to entail a T5 temperature of 950° C sustained over 2 seconds. However, guidance from the engine manufacturer indicated that hot start criteria are based on a combination of temperature over time, and that hot starts may occur at temperatures of approximately 750° C within about 15 seconds. Given the circumstances of the event and physical evidence from the postaccident engine examination, it is likely that the No. 2 engine hot start that occurred 2 days before the accident contributed to the failure of the stage 1 turbine blades due to overtemperature, which resulted in the No. 2 engine loss of power and subsequent impact with terrain. According to the FAA, serious engine damage occurs if a hot start is allowed to continue.

Quoted verbatim from the NTSB record.