N7173S

On February 1, 2020, about 1400 mountain standard time, a Hiller UH-12E helicopter, N7173S, was substantially damaged when it was involved in an accident near Dinosaur, Colorado. The pilot and two passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 air taxi flight.

According to the pilot, the purpose of the flight was to search for lost cattle. While in level flight about 700 ft above ground level, the pilot heard a grinding noise for about three seconds, followed immediately by a loss of power. The pilot conducted an autorotation to a forced landing, during which the tail boom was substantially damaged.

Examination of the engine at the recovery location revealed that the gas producer rotor was seized. Inspection at the compressor inlet found 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 (P/N 6890550, S/N CAC 36300) was removed from the engine and shipped to the manufacturer for further examination.

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 a S4 blade failure during operation with liberated debris resulting in the 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 of these areas found that although fine fracture features of the thumbnail-shaped regions were partially smeared, there were intact areas on the blade stubs. The intact areas displayed fracture features near the suction side crown location that were 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 of the corrosion pits 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. The 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 requirement.

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.