Juneau, AK, USA
N57958
Aerospatiale AS-350BA
The commercial certificated pilot was departing on a local area sightseeing flight under Title 14, CFR Part 135, in a skid-equipped helicopter. During the initial climb, the engine lost power about 75 feet above the ground. The pilot entered an autorotation and attempted to cushion the landing, but the helicopter landed hard, collapsing the landing gear, and wrinkling the tail boom. A post-touchdown fire was burning in the engine compartment as the occupants exited the helicopter. Postaccident examination of the engine revealed that a retaining leaf spring, normally installed within engine Module 1 to hold several oil tubes in place, became unclipped. The legs of the spring made contact with the adjacent rotating engine power shaft. The shaft was severed, which resulted in an overspeed and failure of the free turbine. The engine had accrued 737.9 hours since Modules 2 and 3 were repaired in 2003, and Module 1 underwent a visual inspection. No maintenance had been done on Module 1 since the 2003 inspection.
On July 25, 2005, about 1206 Alaska daylight time, a high skid-equipped Aerospatiale AS-350BA helicopter, N57958, sustained substantial damage following a loss of engine power and subsequent emergency landing during takeoff initial climb at the Juneau Airport, Juneau, Alaska. The helicopter was being operated as a visual flight rules (VFR) local area sightseeing flight under Title 14, CFR Part 135, when the accident occurred. The helicopter was operated by Temsco Helicopters Inc., Juneau. The commercial certificated pilot and the six passengers were not injured. Visual meteorological conditions prevailed, and VFR company flight following procedures were in effect for the flight to the Mendenhall Glacier. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on July 25, the pilot reported that the helicopter was third in a flight of six helicopters that departed the operator's hangar facility on a sightseeing tour. The accident helicopter was about 75 feet agl when another helicopter pilot reported seeing a puff of black smoke from the engine exhaust, which continued to trail from the helicopter as it made an emergency landing. The accident helicopter pilot said that the aircraft yawed to the right, and the low rotor annunciator horn sounded. He entered an autorotation and attempted to cushion the landing, but the helicopter landed hard and slid about 30 feet. The landing gear collapsed, and the tail boom was wrinkled. A post-touchdown fire was burning in the engine compartment as the occupants exited the helicopter. Airport firefighting personnel extinguished the flames. A Federal Aviation Administration (FAA) aviation safety inspector, Juneau Flight Standards District Office (FSDO), responded to the scene and documented the site. He reported that the engine compartment fire was primarily on the right side of the engine. Postaccident examination of the engine revealed damage to the power turbine module, shrapnel-like exit holes in the exhaust tube, and twisting of the number four engine module. Engine Information The engine is comprised of several modules/components. Module 1 includes the accessory section, the fuel control unit, which is a hydro-mechanical design, and the freewheeling unit shaft and power shaft housed in a liaison tube that runs longitudinally under the engine between the accessory case and the reduction gearbox. Module 2 is comprised of the axial compressor wheel. Module 3 includes the centrifugal compressor, combustion chamber, and two gas generator turbines. Module 4 is the power (free) turbine wheel and shaft. Module 5 is the reduction gearbox. Engine power, generated by rotation of the power turbine shaft, is transmitted to the power shaft via the reduction gearbox. The power shaft drives the free wheeling shaft via the free wheeling unit, which drives the main rotor gearbox, and the tailrotor shaft. The liaison tube between the reduction gearbox and the freewheeling unit houses the freewheeling shaft and the power shaft, and three oil supply tubes, all running parallel to each other, within Module 1. Within the liaison tube, the oil tubes are retained in-place by teflon rings. The rings are held in place around the inner circumference of the liaison tube by two leaf springs that are metal clips with semicircular indentations for the teflon rings. The leaf springs are held in-place under tension, against the liaison tube, by a prong on one leg of the spring that fits into a slot on the other leg of the spring to form an interlocking joint. Photographs of the leaf springs are contained in the public docket of this report. When installed within the liaison tube, the distance between the power shaft and the lower side of the leaf spring interlocking joint is about 0.08 inches. The power shaft operates about 6,000 rpm. The power shaft and the freewheeling shaft rotate within the liaison tube adjacent to the oil tubes, but receive no oil lubrication within the tube. Maintenance Information The engine was overhauled by Turbomeca Engine Corp, on March 1, 1997. At that time, the engine had accrued 4081.9 hours. On November 28, 2003, Modules 2 and 3 were repaired by ACRO Aerospace, Richmond, British Columbia, Canada, during which Module 1 had a visual inspection. At that time, the engine had accrued 6121.6 hours. On June 4, 2004, with 6214.7 engine hours, and on August 4, 2004, with 6442.5 engine hours, the operator changed the exhaust duct, which requires removal of Module 5. On July 4, 2005, at 6794.8 engine hours, the operator complied with a service bulletin, which was an inspection of the gas generator 2nd stage turbine blades in Module 3. The most recent engine inspection by the operator was during a 200 hour inspection on July 7, 2005. At that time, the engine had accrued 6811.4 hours. At the time of the accident, the engine had accrued 6859.5 hours, and 737.9 hours since the visual inspection in 2003. The operator reported they had not performed any maintenance on the engine that required service on Module 1 since it was overhauled. A review of the operator's maintenance logs revealed that between May 31, and July 7, 2005, the airframe oil filter by-pass indicator popped five times. Maintenance personnel changed the filter element and the oil each time, and also changed the by-pass indicator. Engine oil samples were submitted by the operator for analysis, which indicated normal samples. Engine Examination The engine was shipped to the manufacturer, and was examined on August 5, 2005. An FAA inspector from the Rotorcraft Directorate, Ft. Worth, Texas, supervised the inspection. He reported that the power shaft, within the Module 1 housing, was fractured about 11 inches from the output end of the reduction gearbox. The shaft did not have any signs of torsional twist. It had scoring marks on the outer surface of the shaft, adjacent to the fracture. A steel retainer (leaf spring), normally positioned within the Module 1 liaison tube housing, was out of position, and had contact marks on it as well. The freewheeling shaft had rotational scoring marks. The axial and centrifugal compressor wheels rotated freely, and were relatively free from damage. Module 3, was not damaged. The power turbine shaft, within Module 4, was sheared. About 2/3 of the power turbine wheel outer circumference was fractured along the base of the blade slots. All of the turbine blades were fractured and missing from the wheel. The engine case, adjacent to the turbine wheel, had extensive damage and the case was twisted. The output section of the reduction gearbox in Module 5, was difficult to turn. The case contained fractured segments of metal, and the muff coupling was broken. The fuel control unit was examined and found to have the delta P diaphragm installed backwards. It was reassembled with the diaphragm in place as found, and placed on a test bench. The bench testing revealed a fuel flow that was within normal specifications. Metallurgical Examination A metallurgical examination of removed components from the accident engine was conducted by the NTSB's Materials Laboratory on September 2, 2005. The examination revealed that the power turbine disk and blades, the power turbine shaft, the muff coupling, the outer race of the aft drive pinion bearing, and the power shaft were all fractured. The fractured power shaft had a continuous band of circumferential rub marks and heat tinting from 1.13 inches forward of the fracture to 0.285 inches aft of the fracture. The location of the fracture corresponded to the location of the forward leaf spring. The inner surface of the liaison tube was blackened. The forward leaf spring had blackening and wear on the inner surfaces of each leg that corresponded to contact with the power shaft. The angle of the rub marks was consistent with the upper side of the leaf spring being tilted aft in relation to the lower side of the spring. The aft tang of the leaf spring slot was bent outward. The freewheeling shaft had circumferential rub marks and heat tinting at two locations, one of which was adjacent to the power shaft fracture. Additional Information The helicopter was released to the operator on July 27, 2005, and the engine was released to the operator on November 3, 2005.
A loss of engine power during takeoff initial climb due to the separation of the engine power turbine shaft, the failure of which was due to a disconnected adjacent leaf spring, which resulted in an emergency landing after takeoff and subsequent hard landing.
Source: NTSB Aviation Accident Database
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