Brownwood, TX, USA
N805SB
Bell BH 206L-3
The helicopter pilot experienced a sudden "explosion" followed by an uncommanded left yaw while initiating an approach. During the autorotation the pilot was forced to increase collective pitch in order to clear a berm resulting in a loss of main rotor RPM. The helicopter landed hard and remained in the upright position. A review of the turbine engine's maintenance records and work orders revealed the engine had been repaired and inspected in December 2005 for "a sudden stoppage." At that time, the compressor, gearbox, and turbine were replaced. A teardown examination of the engine revealed that the aft end of the turbine-to-compressor coupling had fractured in the region where it mated with the turbine splined adapter. The coupling fracture resulted in a loss of continuity in the N1 drive train and an overspeed failure of the first stage turbine wheel. Metallurgical examination of the fracture surfaces of the remnants of the first stage turbine wheel revealed there was no fatigue fractures. Examination of the splines in the aft end of the coupling and on the pieces from the fractured section revealed fretting and a fractured spline tip in the area of contact with the turbine spline adapter. There was non-uniform wear on 16 consecutive external splines on the turbine splined adapter and there was also non-uniform wear of the splines in the area where the adapter mated with the aft end of the turbine-to-compressor coupling. When the engine was disassembled, there was a discrepancy noted between the thickness of the shims (used for alignment of the gas producer and power turbine drive trains) on the mounting pad between the exhaust collector support and the accessory gearbox than what had been entered in the engine logbook or what was etched on the exhaust collector support. In addition, the balance marks on the spur adapter gear shaft and the turbine-to-compressor coupling were not aligned. According to the Rolls Royce 250-C30 series Overhaul Manual, the balance marks were required to be aligned during re-assembly of the engine. The Overhaul Manual also stated that the cracks found on the vertical fire shield could have been as a result of "abnormally high engine vibration."
On November 26, 2006, approximately 1730 central standard time, a single-engine Bell 206L-3 helicopter, N805SB, was substantially damaged during a forced landing following a total loss of engine power approximately three miles south of the Brownwood Regional Airport (BWD), near Brownwood, Texas. The helicopter was registered to Air Evac EMS Incorporated, of West Plains, Missouri, and was being operated by Air Evac Lifeteam. The airline transport pilot, the flight nurse, and the paramedic were not injured. A company flight plan was filed for the positioning flight that departed Abilene, Texas, about 1650, and was destined for Brownwood. Visual meteorological conditions prevailed for the flight conducted under 14 Code of Federal Regulations Part 91. In a written statement, the pilot reported that the flight was uneventful until he initiated his descent into the Brownwood Airport. The pilot reported that the helicopter experienced a sudden "explosion" accompanied by an uncommanded yaw to the left. He added that he observed debris in his peripheral vision, and noticed that the "over-temperature" light was illuminated. The pilot further stated that he immediately entered an autorotation from an altitude of 600 to 700 feet above ground level, continued straight ahead over a small lake, a tree, and a berm toward a clearing. The pilot added that in order to clear the berm along his route of flight, he was forced to increase collective pitch, which resulted in a loss of main rotor RPM while the helicopter was approximately 10 feet above the ground. Subsequently, the helicopter landed hard on its skids and remained in the upright position. After the pilot and the two passengers exited the aircraft, the pilot observed a small fire in the engine compartment and used the onboard fire extinguisher to put the fire out. A Federal Aviation Administration (FAA) inspector performed an on-scene examination of the helicopter. According to the inspector, the landing gear skids were partially spread, the tops of both the left and right vertical fins on the horizontal stabilizer had been sheared-off from contact with the main rotor blades, and the aft fuselage bulkhead was wrinkled. The tail rotor drive shaft cover was also damaged as result of contact with the main rotor blades. The helicopter was recovered to the operator's facilities in West Plains, Missouri, for further examination. An FAA inspector and a representative of the engine manufacturer performed a detailed external examination of the Rolls-Royce 250-C30P turbine engine. According to the Rolls Royce representative, the engine was securely mounted to the airframe and continuity was established for the mechanical linkage between the cockpit and both the gas producer fuel control and the power turbine governor. Oil was present in the delivery and return lines between the engine and oil cooler, and oil was present in the oil pressure and torque-meter oil lines on the forward side of the accessory box. There was no evidence of foreign object ingestion on the leading edge of the compressor housing or on the first stage compressor blades. The gas producer turbine support was separated from the power turbine support at the mating flange between the two components around approximately two-thirds of their circumferences. In addition, the gas producer turbine support was split open in several places exposing the elongated absorbing ring and remnants of the gas producer turbine rotor. A review of the engine's maintenance records and work orders revealed that during December 2005, at a total engine time of 4,440.4 hours, the engine was inspected and repaired by an overhaul facility in Dallas, Texas, due to "a sudden stoppage." The maintenance records revealed that the compressor, gearbox, and turbine were replaced at that time. The maintenance records confirmed that the engine had accrued a total of 5,003.5 hours at the time of the accident, with 563.1 hours since the December 2005 inspection. A teardown examination of the engine was conducted on January 4, 2007, at the Rolls Royce's engine teardown facilities in Indianapolis, Indiana, under the supervision of the FAA. Representatives from the operator were also present for the examination. The engine examination revealed that the "aft end of the turbine-to-compressor coupling had fractured in the region where it mated with the turbine spline adapter. The coupling fracture resulted in a loss of continuity in the N1 drive train and an overspeed failure of the first stage turbine wheel. Metallurgical examination of the fracture surfaces of the remnants of the first stage turbine wheel revealed there was no fatigue fractures. Examination of the splines at the aft end of the coupling and on the pieces from the fractured section revealed fretting and a fractured spline tip in the area of contact with the turbine spline adapter." The examination further revealed non-uniform wear on 16 consecutive external splines on the turbine splined adapter and there was non-uniform wear of the splines in the area where the adapter mated with the aft end of the turbine-to-compressor coupling. This is also the area where the turbine-to-compressor coupling fractured. During the disassembly of the engine, a discrepancy was noted between the thickness of the shims (used for alignment of the gas producer and power turbine drive trains) on the mounting pad between the exhaust collector support and the accessory gearbox than what had been entered in the engine logbook or what was etched on the exhaust collector support (the etched values and the logbook values were identical). The shims on the mounting bolts at the 2 and 4 o'clock positions were reversed with the bolts at the 10 and 8 o'clock positions respectively. Additionally, the balance marks on the spur adapter gear shaft and the turbine-to-compressor coupling were found not-aligned. According to the Rolls Royce 250-C30 series Overhaul Manual, the balance marks are required to be aligned during re-assembly of the engine. A crack was also noted on the vertical fire shield emanating from a mounting bolt hole. According to Rolls Royce "a binocular examination of the crack identified the crack as a fatigue progression." According to the Rolls Royce 250-C30 series Overhaul Manual, these cracks could have been as a result of "abnormally high engine vibration." The pilot held an airline transport pilot certificate with a rating for rotorcraft-helicopter, a commercial certificate with ratings for airplane single-engine land, and instrument airplane. In addition, the pilot was a certificated flight instructor for rotorcraft-helicopter. His last FAA second-class medical certificate was issued on May 26, 2006. According to the operator, the pilot had accumulated a total of 6,845.3 hours; 6,734.1 hours in rotorcraft, of which 2,181.5 hours were in the Bell 206L-3. Weather reported at the Brownwood Regional Airport at 1725 was wind from 160 degrees at 8 knots, visibility 10 statute miles, few clouds at 4,300 feet, broken clouds at 6,000 feet, temperature 70 degrees Fahrenheit, dewpoint 55 degrees Fahrenheit, and a barometric pressure setting of 30.02 inches of Mercury.
The fracture of the turbine-to-compressor coupling and subsequent overspeed failure of the first stage turbine wheel as result of the improper re-assembly of the engine by maintenance personnel. A contributing factor was the lack of suitable terrain for the forced landing.
Source: NTSB Aviation Accident Database
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