Incident Overview
On February 1, 2020, about 1400 mountain standard time, a Hiller UH-12E helicopter, registration N7173S, was substantially damaged during an accident near Dinosaur, Colorado. The pilot and two passengers were not injured. The helicopter was operating under Title 14 Code of Federal Regulations Part 135 as an air taxi flight.
Flight Details
According to the pilot, the flight was conducted to search for lost cattle. While in level flight at approximately 700 ft above ground level, the pilot heard a grinding noise lasting about three seconds, followed immediately by a loss of engine power. The pilot performed an autorotation to a forced landing, during which the tail boom was substantially damaged.
Engine Examination
Examination of the engine at the recovery location revealed that the gas producer rotor was seized. Inspection of the compressor inlet showed no guide vane or airfoil impact (foreign object) damage, but damage was observed further downstream. The compressor bleed valve was removed, and metal debris was found in the bleed manifold passage. The compressor module (Part Number 6890550, Serial Number CAC 36300) was removed from the engine and shipped to the manufacturer for further examination.
Compressor Teardown and Metallurgical Findings
Compressor teardown evaluation revealed that all S4 and S5 compressor blade airfoils were separated near the root. The first and second compressor stages were undamaged. The trailing edges of the S3 blades and all axial compressor components downstream of S3 were battered. The damage was consistent with an S4 blade failure during operation, with liberated debris resulting in downstream damage.
Metallurgical examination of the S4 compressor wheel found considerable corrosion pitting on the forward rim face and at both suction and pressure sides of the blade stubs. Although most of the blade fracture surfaces were entirely obscured by secondary damage (smearing), characteristic thumbnail-shaped regions of fatigue were observed on three stub fracture surfaces. Scanning electron microscopy (SEM) examination found that intact areas displayed fracture features consistent with fatigue progression aft from the leading-edge area in one blade and fatigue cracks emanating from the suction side of the blade at the crown in two blades. Two of the fatigue regions were approximately centered around areas of intergranular damage identified as corrosion pits.
SEM examination of one corrosion pit revealed smooth intergranular surface morphology, consistent with corrosion, and semi-quantitative energy dispersive x-ray spectroscopy (EDS) analysis found elevated oxygen content, also consistent with corrosion. These features indicated that multiple cracks progressed in fatigue across the blade thickness away from corrosion-damaged areas. The material microstructure, hardness, and composition were consistent with the drawing requirements.
Maintenance Background
The Rolls-Royce 250-C20B Operation and Maintenance Manual Table 602 (Scheduled Inspections) includes a periodic detailed inspection requirement for compressors installed on engines operating in corrosive environments. The operator reported that the compressor had not been operated in corrosive conditions since overhaul. Review of the helicopter service records found that the compressor was installed on the accident helicopter in May 2012 with 0 operating time since overhaul (TSO) and had accumulated 505 flight hours since installation. The logbook contained an 8130-3 (serviceable) tag indicating that the compressor was overhauled in 2006. A compressor impeller life limited part record card was also found. The records provided no time since new (TSN) or operating history for the compressor module prior to 2006 and no information about the storage during the period of unuse between 2006 and 2012.