TAHOLAH, WA, USA
N1096L
Hughes 369D
While conducting external load operations the helicopter sustained substantial damaged after a loss of engine power, autorotation, and forced landing. A witness reported hearing a loud bang, followed by the engine spooling down. The witness also reported seeing smoke coming from the engine as the helicopter descended through the trees, landing hard on a tree stump under the pilot's seat area. Postaccident inspection of the turbine engine revealed excessive blending in the web of the first stage turbine wheel. Within the blending, a notch exhibited a radius sharper than allowed by blend limits, which the manufacturer' metallurgists believed to have been created by a hand-held grinder. It could not be determined whether the manufacturer of the turbine wheel or an overhaul facility made the subject grinding marks.
HISTORY OF FLIGHT On December 8, 1998, about 1145 Pacific standard time, a Hughes 369D, N1096L, registered to and operated by Eagle Air Helicopters as a 14 CFR Part 133 external load operation, sustained substantial damaged after a loss of engine power, autorotation, and forced landing while conducting external load operations near Queets, Washington. The commercial pilot, who was the sole occupant, received minor injuries. During the landing, the helicopter landed hard and rolled over. Visual meteorological conditions prevailed and no flight plan was filed. There was no report of an ELT actuating. The flight originated from the Forks Airport (S18) approximately three hours and 45 minutes prior to the accident, although landings had been performed near the work site for refueling during that period of time. A witness reported hearing a loud bang, followed by the engine spooling down. The witness also reported seeing smoke coming from the engine as the helicopter descended through the trees, landing hard on a tree stump under the pilot's seat area. The pilot reported in a written statement that "while flying to the landing with a load of wood there was a loud bang. The a/c [aircraft] rocked backwards almost throwing me out of the seat, smoke started coming into the a/c and I could hear the engine winding down...entered autorotation." AIRCRAFT INFORMATION A review of the engine log book indicated that the Allison turbine engine was last overhauled on July 30, 1997 by Standard Aero Limited, Winnipeg, Canada, on work order L I 55292. The overhaul included the replacement of all turbine wheels. The 1st stage turbine wheel, part number 688407, s/n X140670, was shipped to Standard Aero Limited as a new spare part on April 28, 1997. At the time of the accident, the turbine had accumulated 1223 hours and 195 cycles since the last overhaul. Additionally, the aircraft had sustained major damage to the airframe from a hard landing due to the loss of the tail rotor gear box in August 1998, at 5498.8 hours total time in service. This damage had been repaired and the aircraft returned to service. Repairs included removal of the engine drive shaft. The helicopter had 5582 hours time in service at the time of this accident. WRECKAGE AND IMPACT INFORMATION An on-site investigation was not performed at the crash site. However, evidence and witness information indicated that the tail boom was damaged as a result of the hard landing. The left side skid gear and struts were broken. The right side engine compartment door exhibited multiple exit holes consistent with an uncontained failure of a turbine wheel or turbine blades. After recovery from the accident site, the helicopter and its engine were inspected by NTSB investigators and parties to the investigation at the operator's hangar facility in Forks, Washington. The engine mounts were found to be intact. The first stage turbine wheel was observed to be fragmented, with damage to the gas producer support, right-side compressor air discharge tube, and right side of the horizontal fire shield. The TOT harness had the appearance of having been cut by exiting fragments. Removal of the outer combustion case revealed that the gas producer tiebolt had penetrated the number 8 bearing sump cover. The turbine was removed from the accessory gearbox. The turbine-to-compressor coupling was noted to be intact with no damage to the splines. N2 rotated free and smooth; N1 was locked up. The turbine was shipped to Rolls Royce Allison for further disassembly and metallurgical examination. TESTS AND RESEARCH During disassembly and inspection at Rolls Royce Allison, it was noted that there was blending in the web of the first stage wheel that was deeper and larger than allowed. Within the blending, a notch exhibited a radius sharper than allowed by blend limits. (Rolls Royce Allison noted that the balance grind on the balance ring of the first stage wheel is normally accomplished after the gas producer rotor is assembled as a single unit). Allison's metallurgical laboratories confirmed that the wheel material was EMS 73646-1, as specified by the applicable engineering drawing. They noted that the first stage wheel had a .024 inch deep undercut below the inner balance ring to web radius. Within the undercut, there was a .0065 inch deep notch, which the Allison lab believed to have been created by a hand-held grinder. After disassembly and inspection, the first stage turbine wheel was sent to the NTSB materials laboratory for further inspection. Safety Board metallurgists were unable to ascertain whether the manufacturer of the turbine wheel or overhaul facility made the subject grinding marks. A Rolls Royce Allison representative advised them that as the wheel was shipped as a single spare part, the wheel would not have been ground (balanced) at the factory; such balancing would have been performed by an overhaul facility. ADDITIONAL INFORMATION The Safety Board did not retain possession of the airframe, which remained at the operator's hangar after inspection. The engine and the first stage turbine wheel were returned to the operator, who acquired the salvage to the aircraft, after disassembly and testing.
Turbine wheel failure as the result of a low-cycle fatigue crack that initiated in a beyond-limits grinding gouge. Factors include improper maintenance by unknown parties and tree stumps that prohibited a successful emergency landing.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
