ERA14LA361

The airline transport pilot was conducting an aerial application flight in the helicopter. About 75 ft above ground level, the pilot heard three engine compressor stalls, which were followed by a partial loss of engine power. During the ensuing forced landing to a nearby road, the helicopter rolled over into a drainage ditch and came to rest on its right side. Examination of the engine revealed that the accessory gearbox No. 2 bearing had degraded and fractured. The bearing displayed evidence of heat damage associated with its degradation. The cage of the bearing revealed fatigue fractures with multiple origins, indicative of relatively high-stress concentrations to the bearing. The cage fractures were likely secondary failures following the initial degradation of the bearing. The bearing balls were black in appearance. In addition, they were undersized when compared to engineering drawing requirements and displayed signatures consistent with material loss, likely from wear. Considering that the engine had accumulated only about 92 hours since the gearbox was installed following an overhaul, it was possible that the bearings experienced an abnormal load. The cause of the abnormal load could have been due to installation error, an alignment issue from a previous hard landing that was noted in the gearbox maintenance records, or from insufficient lubrication within the gearbox assembly. However, the investigation could not determine which of these factors, if any, caused the failure of the No. 2 bearing and resulted in the partial loss of engine power.

HISTORY OF FLIGHT On July 26, 2014, about 1545 eastern daylight time, a Hiller UH-12E, N107HA, was substantially damaged when it impacted terrain near Wadesboro, North Carolina. The airline transport pilot was not injured. Day visual meteorological conditions prevailed and no flight plan had been filed. The local aerial application flight originated at a temporary staging location about 1540. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 137. According to the pilot, while applying the agriculture product about 75 feet above ground level, he heard three engine compressor stalls, which was followed by a partial loss of engine power. During a forced landing to a nearby road the helicopter rolled over into a drainage ditch and came to rest on its right side, which resulted in substantial damage to the fuselage, tailboom, main rotor blades, and tail rotor blades PERSONNEL INFORMATION According Federal Aviation Administration (FAA) records, the pilot held an airline transport pilot certificate with ratings for airplane multiengine land and helicopter, a commercial pilot certificate with ratings for airplane single-engine land and instrument helicopter, and a flight instructor certificate for helicopter and instrument helicopter. He held an FAA second-class medical certificate, which was issued on April 9, 2014. The pilot reported 9,604 hours of total flight experience, of which 5,800 total hours were in rotorcraft and 1,210 hours were in the accident helicopter make and model. AIRCRAFT INFORMATION The helicopter was issued a standard airworthiness certificate on June 7, 1974. The most recent 100-hour inspection was conducted on June 18, 2014; at the time of the inspection the helicopter had approximately 11,884 total hours in service. The helicopter was powered by an Allison 250-C20B turboshaft engine, manufactured on April 11, 1980. The engine had been operated for 7,566.6 total hours at the time of the accident. A review of the maintenance records revealed that the engine was converted from a 250-B17C engine to a 250-C20B engine in September of 1993. At the time of the conversion the engine had 1,381.4 total hours in service. An entry located in the gearbox assembly service record dated October 30, 2007, stated "repair after hard landing." The gearbox repair was completed on November 8, 2008, and no other entries were located revealing any time in service between the repair and when the gearbox was installed on the accident engine. The compressor, gearbox, and turbine assemblies were installed on the accident engine on December 16, 2013, and the engine was subsequently installed on the accident helicopter the next day. At the time of installation on the accident helicopter, the engine had accrued 7,474.4 total hours in service and the compressor had 5,651.9 total hours in service and 0 hours since overhaul. On May 30, 2014, the gearbox was removed from the accident helicopter with a maintenance record entrythat indicated it was "making metal." The entry further stated that the "bearings on N2 tach/gov spur gear shaft going bad." The manner in which the assemblies and the engine were stored prior to installation on the accident helicopter could not be determined. METEOROLOGICAL INFORMATION The 1553 recorded weather observation at Monroe Airport (EQY), Monroe, North Carolina, located about 22 nautical miles northwest of the accident location, included wind from 210° at 6 knots, 10 statute miles of visibility, scattered clouds at 4,200 ft above ground level, temperature 31° C, dew point 21° C, and an altimeter setting of 29.99 inches of mercury. WRECKAGE AND IMPACT INFORMATION According to photograph provided by an FAA inspector that responded to the accident location, the helicopter came to rest on its right side in a ditch. The ditch was located along a roadway that was perpendicular to the field that the pilot was spraying. The tailboom exhibited damage consistent with being severed by the main rotor blades, the skids were impacted separated, and the windscreen was damaged. TEST AND RESEARCH Engine Disassembly On October 7, 2014, the engine was examined by an NTSB investigator at the engine manufacturer's facility. All fittings were found secured and in place, with the associated torque stripe showing no evidence of rotation. Compressed air was utilized and an air leak was observed at the B-nut on the Pc line; however, according to the engine manufacturer, the leak would not have precluded normal operation of the engine. The compressor was separated and the shims were counted and measured. The measurement was the same as the vibropeened number at the respective case points. Following case separation to facilitate further examination, the N1 rotated but had both tactile and audible resistance noted. N2 rotated smoothly with no resistance noted. The accessory gearbox was disassembled; the bearings and transfer tube were all labeled as PMA (Part Manufacturer Approval) parts. The No. 2 bearing was examined in place and upon removal the inner race separated and three ball bearings were located outside of the race. The No. 2 bearing and associated hardware was sent to the NTSB Materials Laboratory for further examination. [Further information pertaining to the engine disassembly and examination can be found in the "Engine Examination Report" located in the public docket for this accident.] NTSB Materials Laboratory Examination of the No. 2 bearing The No. 2 bearing was examined by NTSB Materials Laboratory personnel. The components of the bearing exhibited discoloration consistent with heat damage and the bearing cage was fractured in multiple locations consistent with fatigue fracture features. The outer diameter of the outer race exhibited fretting damage and coked oil spray patterns. The inner race half, on the compressor-side of the bearing, was uniformly tinted black and was darker in color than the other race half and the outer race revealedcircumferential sliding contact marks. The bearing balls were black in appearance. In addition, they were undersized when compared to engineering drawing requirements and displayed signatures consistent with material loss.