Aircraft Accident/Incident Report

New York, New York 10001
Saturday, July 7, 2007 16:51 EDT

The helicopter was being operated on a revenue sightseeing flight when the accident occurred. Approximately 8 minutes into the flight, about 350-400 feet above the Hudson River while on an approach to land, a 20-inch section of the composite main rotor blade trailing portion, aft of the spar, fractured from the rest of the blade. The pilot reported an immediate decay in main rotor speed with a prominent and abnormal vibration. She also saw a piece of debris, most likely the liberated piece from the main rotor blade, fly from the left rear of the helicopter forward, past the cabin. She made an emergency autorotation onto the water after activating the emergency float system. The helicopter landed upright on its floats; however, the main rotor blades struck the water and the tail boom, resulting in substantial damage to the tail boom. The occupants were rescued by boaters and were not injured. Detailed examination of the helicopter revealed no evidence of flight control system component failures or malfunctions other than the main rotor blade fracture. The main rotor blades (part number 355A-11-0030) were manufactured from glass fiber reinforced composite material with a foam core. From the leading edge to trailing edge of the blade, the blade is constructed with a spar, wedge-shaped foam core, trailing edge roving, and trailing edge tab sandwiched between skin layers and skin reinforcement layers. The skin, skin reinforcement, and trailing edge tab layers are made with glass fabric reinforcement. The trailing edge roving is made of unidirectional glass fibers aligned approximately parallel with the spanwise direction of the blade. The main rotor blade that fractured was examined at the manufacturer’s facility with oversight by France's Bureau of Investigation and Analysis (BEA), and results of the examination were reviewed by the NTSB Materials Laboratory. The National Transportation Safety Board Materials Laboratory also examined sectioned pieces of the blue and yellow main rotor blades submitted by Eurocopter. Physical and microscopic examination of the main rotor blade showed that the fracture was due to fatigue cracks that initiated near the trailing edge of the blade near blade station 1300. It was discovered that the fatigue cracking most likely occurred due to out-of-specification deviations in the alignment of the trailing edge roving fibers within a transition region where the trailing edge roving shifts toward the trailing edge and where skin reinforcement layers end. In the areas of the deviations, the unidirectional fibers of the roving were not properly aligned with the spanwise direction of the blade, likely resulting in localized changes in stiffness at the trailing edge. With this type of fiber misalignment, some of the longitudinal stresses normally carried by the roving layers would be shed to adjacent skin and trailing edge tab areas, which can result in fatigue cracking in these adjacent layers. The undamaged fracture features in the trailing edge roving of the blue main rotor blade section revealed fiber fractures with mirror fracture surfaces across the fiber diameters indicating they were substantially weakened when they fractured. The mirror fractures in the fibers could be evidence of progressive fracture through the trailing edge roving due to fatigue or environmental attack. The fracture surface overall was relatively rough and did not form a flat plane typical of fatigue fracture in tension, such as observed in some areas of the skin. However, some of the fibers had a step at one side or both sides of the fracture that could suggest a mixed mode of loading including tension and transverse shear, which could theoretically explain the overall roughness of the fractures. Also, as cracking progresses the matrix material surrounding the fibers will crack, allowing environmental exposure that could potentially weaken the fibers. The fractographic evidence indicates the trailing edge roving was likely significantly weaker than expected. The weakening of the trailing edge roving fibers could occur due to mechanical damage to the surfaces of the fiber during the fatigue cracking process or could be due to an environmental attack of the fibers as cracking progressed. In either case, the strength of individual fibers was less than expected in a large area of the trailing edge roving. The total extent to which the trailing edge roving was weakened is unknown due to post-fracture damage and the unknown extent to which the trailing edge roving might have cracked before final fracture. In previous cases of skin cracking, an intact trailing edge roving was required to maintain crack stability. In this case however, the evidence shows the trailing edge roving was likely significantly weaker than expected. The previous cases cannot be used as evidence for crack stability in this case due to the weakened condition of the trailing edge roving. Given the extent of cracking in the skin and evidence of weakening in the trailing edge roving, it is likely that the cracking at the trailing edge of the blue blade had proceeded to an extent to cause fracture of the blade. The deviation in the trailing edge roving fibers occurred during the manufacture of the accident blade and it is likely that the trailing edge fibers shifted during the curing process. The skin layers of the blades are somewhat translucent after curing, and manufacturing records showed that the inspector who performed the visual examination after curing flagged the transition area of this blade for a radiographic inspection. It is possible that the inspector observed an anomaly in the trailing edge roving; however, the out-of-specification deviations in the trailing edge roving were not detected by the radiographic examination. A record search by the blade manufacturer of 9,761 similar blades revealed that one other blade had been flagged for radiographic inspection near blade station 1300 during visual examination and subsequently passed radiographic inspection. That blade was returned to the manufacturer’s facility and examined. The blade was sectioned, and the trailing edge roving showed no significant deviations from the as-designed position. The two other blades from the accident helicopter were also examined at the manufacturer. The trailing edge roving in these blades did not show any significant deviations from the as-designed position. The main rotor blade was rated for a service life of 20,000 hours and the fracture/separation occurred after about 8,077 hours. The manufacturer stated that prior to this event there had been no reported similar main rotor blade failures.

The fatigue fracture and in-flight separation of a 20-inch section of the blue composite main rotor blade trailing edge, aft of the spar, due to inadequate manufacture, and the manufacturer’s subsequent failure to detect an out-of-specification deviation in the rotor blade’s trailing edge roving.