ERA09LA317

The inadequate design of the fourth-stage turbine wheel, resulting in the fatigue failure of one airfoil and a subsequent loss of engine power.

Following completion of aerial tree cutting near power lines, the pilot returned to the designated landing zone and, while maneuvering for landing after placing the saw on the ground, the engine experienced a loss of power. The pilot attempted to perform an autorotative landing but landed hard, causing substantial damage to the helicopter. Postaccident inspection of the engine revealed one airfoil of the fourth-stage power turbine wheel fractured near the hub due to high-cycle fatigue that initiated on the pressure side of the airfoil at the trailing edge. No surface defects were noted along the root filet radius adjacent to the fracture initiation region. The enhanced fourth-stage power turbine wheel was manufactured in October 2006, installed into the turbine assembly, operated for 1,769 hours, then removed for a 1,750-hour inspection. The fourth-stage power turbine wheel remained installed in the turbine assembly at the completion of the inspection, and the turbine assembly was reinstalled onto the engine. At the time of failure, the wheel had accumulated approximately 2,690 hours since new and approximately 921 hours since the last 1,750-hour inspection was performed. Since 2005, there have been four other fatigue failures of an airfoil of an enhanced fourth-stage turbine wheel. All other failures initiated at the trailing edge near the hub. In December 2006, Rolls-Royce developed an engine alert bulletin (CEB-A-1400) alerting owners and operators of the accident make and model engine to avoid steady state N2 speed between 75 to 88 percent above 85 shaft horsepower (shp). Additionally, Rolls-Royce reported that their research of failures of airfoils of the fourth stage power turbine revealed the possibility of a higher stress state in the airfoil at the trailing edge root than originally modeled. The stress, which occurs during engine start, results from thermal differentials in the airfoil geometry that produce a residual stress at the trailing edge root that can lead to a fatigue crack. Once a crack occurs, it can propagate in high-cycle fatigue followed by overload failure if the engine is operated in a steady state in the N2 speed avoidance range (75 to 88 percent N2) above 85 shp, as specified in CEB-A-1400.

HISTORY OF FLIGHT On June 2, 2009, about 1110 eastern daylight time, a Hughes 369D, N8356F, registered to Aviation Advantage LLC, operated by Aerial Solutions, Inc., was landed hard during an autorotative landing to a clearing near Greenville, Virginia, following loss of engine power. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local 14 Code of Federal Regulations (CFR) Part 133 external load flight from Greenville, Virginia. The helicopter sustained substantial damage and the commercial pilot, the sole occupant was seriously injured. The flight originated about 55 minutes earlier from Greenville, Virginia. The purpose of the flight was aerial tree trimming near power lines, with approximately 1 hour duration. The pilot stated that a ground crew member gave him a 45 minute fuel check, and he then spent 5 minutes performing a “bottom pass.” He pulled up out of the right of way and while returning to a landing zone (LZ), he performed a power check. He also did an instrument scan while on final approach to the LZ; all instruments were in the green. He lowered the saw wheel to a marked spot at the LZ, and maneuvered the helicopter backwards and down to lower the saw engine to the ground. During set down stabilization, he heard a loud sound from the engine and the helicopter yawed to the left. The chief pilot of the operator estimated that the helicopter was between 90 and 100 feet above ground level (agl) when the engine failure occurred. The pilot further stated that he reduced collective, attempted to release the saw, and nosed the helicopter over to increase airspeed to achieve flare, but the helicopter impacted the ground. He released his seatbelt and shoulder harness and exited the helicopter. DAMAGE TO AIRCRAFT Damage to the helicopter consisted of a deformed tailboom, and a crease in the left side of the tailboom. The left skid was separated, and compression wrinkles were noted above and aft of the aft right skid tube. The pilot’s seat exhibited downward deformation, and extensive structural damage was noted in the area of the pilot’s seat. Bending damage to two of the main rotor blades was noted. One main rotor blade was fracture near the blade root, and one tail rotor blade was bent. PERSONNEL INFORMATION The pilot, age 41, holds a commercial pilot certificate with a rotorcraft helicopter rating, and was issued a second class medical certificate with no limitations on January 20, 2009. The NTSB Pilot/Operator Aircraft Accident/Incident Report submitted by the pilot and operator indicated his total flight time in all aircraft was 8,676 hours, of which 7,780 were in the accident make and model. During the last 90 days, 30 days, and 24 hours he reported accruing 105, 32, and 7 hours respectively, all of which were in the accident make and model helicopter. The pilot’s last flight review or equivalent was performed in the accident make and model helicopter on April 28, 2009. AIRCRAFT INFORMATION The helicopter was manufactured in 1976 by Hughes Helicopters as model 369D, and was designated serial number 1260063D. It was equipped with an engine torque exceedance instrument, and powered by a Rolls-Royce Corporation (formerly Allison Engine Company) 250-C20B engine rated at 420 shaft horsepower (SHP) with 375 SHP usable. Review of the maintenance records revealed the engine installed at the time of the accident, serial number (S/N) CAE 823103, had been installed in various airframes since October 1999. The engine was installed into the accident helicopter on July 16, 2008, at helicopter total time of approximately 9,597 hours, and remained installed until removed for postaccident examination. The helicopter total time at the time of the accident was approximately 10,518 hours. According to personnel from the engine manufacturer, a new Enhanced fourth stage power turbine wheel (P/N) 23055944 and S/N X555507 was manufactured, then sold in a kit on August 24, 2006. The maintenance records associated with the turbine assembly revealed during an overhaul by a Canadian repair station in October 2006; the new Enhanced design fourth stage turbine wheel part number (P/N) 23055944 and S/N X555507 was installed. At the time of overhaul, the turbine assembly total time and cycles since new were approximately 7,077 hours, and 8,124, respectively. The turbine assembly was removed for a 1,750 hour inspection on May 30, 2008, and sent to a U.S. FAA certified repair station for the inspection; the fourth stage turbine at that time had accumulated 1,768.9 hours since new. Records from the U.S. repair station indicate in part that the fourth stage turbine wheel remained installed at the completion of the 1,750 hour inspection. The turbine assembly was approved for return to service, installed on July 14, 2008, and the engine was installed in the helicopter on July 16, 2008. The turbine assembly remained installed until removed for the postaccident investigation. The engine had been operated approximately 2,690 hours since the new Enhanced design fourth stage turbine wheel was installed, and approximately 921 hours since the 1,750 hour inspection was last performed. The engine was last inspected in accordance with a 100-Hour inspection on May 12, 2009. The engine total time at that time was recorded to be approximately 22,889 hours. METEOROLOGICAL INFORMATION A surface observation weather report taken at Shenandoah Valley Regional Airport (SHD), Staunton/Waynesboro/Harrisonburg, Virginia, at 1120, or approximately 10 minutes before the accident indicates the wind was from 190 degrees at 7 knots, the visibility was 10 miles, and clear skies existed. The temperature and dew point were 29 and 21 degrees Celsius respectively, and the altimeter setting was 30.08 inches of Mercury. WRECKAGE AND IMPACT INFORMATION Initial inspection of the engine following recovery of the helicopter was performed by a representative of the engine manufacturer with Federal Aviation Administration (FAA) oversight. The inspection revealed neither the N1 nor N2 drive trains could be manually rotated. No visible damage was noted to the compressor module. Both exhaust stacks exhibited small areas of peening on the inside surface. The exhaust collector was in position, but exhibited breaching in the path of the No. 4 power turbine wheel. The opening extended counter-clockwise from the 2 o’clock to the 6 o’clock positions. The outer rim and knife seals of the No. 4 power turbine wheel as viewed from the opening of the exhaust collector exhibited separation of a section of the No. 4 wheel rim spanning an area of approximately 3 airfoils. The middle airfoil was fractured near the hub. The engine was removed from the helicopter and shipped to the manufacturer’s facility for further examination. Disassembly inspection of the engine at the manufacturer’s facility with FAA oversight confirmed one airfoil of the fourth stage turbine wheel was separated near the hub. Components of the engine consisting of the first, second, third, and fourth stage turbine wheels, second, third, and fourth stage turbine nozzle assemblies, power turbine outer shaft and outer shaft nut, power turbine inner race shaft and nut, No. 6 bearing, exhaust collector support, and power turbine support were submitted for detailed examination by the Safety Board’s Materials Laboratory located in Washington, DC. The power turbine governor and fuel control unit were retained for bench testing at the manufacturer’s facility. The Safety Board’s Materials Laboratory examined the fourth stage turbine wheel with a representative of the engine manufacturer (materials engineer) present. The examination revealed impact damage to several airfoils, one airfoil was fractured at the hub root filet, and portion of the outer shroud was fractured from the wheel in the area of the fractured airfoil. Neither the fractured air