MIA07FA116

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.

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.

HISTORY OF FLIGHT On July 7, 2007, about 1651 eastern daylight time, a Eurocopter EC 130 B4 helicopter, N453AE, registered to Meridian Consulting Company, Inc., and operated by Liberty Helicopters, Inc., sustained substantial damage following an in-flight separation of a section of one of the main rotor blades and subsequent autorotation onto the Hudson River, New York, New York. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan was filed for the flight, which was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 and 136 as a revenue sightseeing flight. The flight departed from the West 30th Street Heliport (JRA), New York, New York about 1643. There were no injuries to the certificated commercial pilot or seven passengers. The pilot stated that she had flown the accident helicopter on three previous flights that day and reported no discrepancies on those flights. The accident flight departed for a 10-minute sightseeing flight. All passengers were wearing inflatable life vests, which were contained in a pouch that was strapped around their waists. Approximately 8 minutes into the flight, while flying southbound on a left base leg, or approximately 1/2 mile from the point where she would have turned onto final approach for the heliport, she heard a loud bang, and felt an abnormal vibration (medium to low). At that time, she said she was flying about 350 to 400 feet above the water and between 100 and 110 knots indicated airspeed. The bang was the first thing that got her attention, and noted there was no yawing motion associated with the noise. She saw gray colored debris that was rectangular in shape, and approximately 8 inches in length, fly from the aft left to front left, before it went out of sight. She then heard a "winding down" of the main rotor rpm, but did not hear the low rotor warning horn. The main rotor rpm decay was immediate. She looked at the dual tachometer, but did not recall the main rotor rpm reading. She entered autorotation by applying down collective and aft cyclic, and also deployed the pop-out floats. The vibration was prominent and abnormal. She made a slight flare at 25 feet, and the helicopter settled from that altitude. She applied forward cyclic and the helicopter landed "soft" on the choppy water. The helicopter was level at the point of touchdown and at that time, she had one-half “up” collective applied. She reported there was no binding of the flight controls from the time of hearing the sound to the point of touchdown on the water. After touchdown on the Hudson River, she noted that the main rotor blades were tilted to the right, the tail rotor was still spinning, and the engine was still running. She did not report hearing any horn or seeing any lights, and she did not make any radio calls. She helped the front seat passengers remove their restraints and exit the helicopter. One passenger in the rear seat helped the other rear seat passengers exit the helicopter. All occupants were rescued from the water by private boaters. A witness on a boat reported suddenly hearing a very loud banging noise. The banging noise continued until the main rotor blades contacted the water and became damaged along with what appeared to be pieces of the engine cowling. The banging sound decreased, but the engine remained running “very smoothly,” though it appeared to him to be out of its normally installed position. He said the banging sound as being a thumping sound, metallic in nature, that in his opinion was consistent with the main rotor blades contacting something metallic. The witness, who is an airplane mechanic, reported that the sound was consistent with the sound of tapping of a hard plastic screwdriver handle on aluminum skin. PERSONNEL INFORMATION The pilot, age 37, held a commercial pilot certificate with rotorcraft helicopter and instrument helicopter ratings, issued September 27, 2006. She also holds a private pilot certificate with an airplane single-engine land rating. She was issued a second-class medical certificate with no limitations on February 13, 2007. There was no record of any previous accidents or incidents or enforcement actions by the Federal Aviation Administration (FAA). The pilot was hired by Liberty Helicopters, Inc., on February 8, 2007. Just prior to employment, she reported having a total time of 2,286 hours, of which 1,103 hours were in rotorcraft and 1,183 hours were in airplanes. Three of the 1,103 hours in rotorcraft were in Aerospatiale helicopters. Her last airman competency/proficiency check in accordance with 14 CFR Part 135.293 titled, “Initial and recurrent pilot testing requirements” and also 14 CFR 135.299 titled, “Pilot in command: Line checks: Routes and airports” was performed on February 8, 2007. The flight duration was recorded to be 1.0 hour and the results were listed as “Approved.” The flight was in the accident helicopter. She was qualified to act as pilot-in-command (PIC) in the following make and model helicopters: Eurocopter EC 130 B4, Aerospatiale models AS350B1, AS350B2, and AS350BA. Since being hired by Liberty Helicopters, Inc., the pilot recorded approximately 239 hours in various make and model helicopters including time spent in flight training. She reported on the NTSB Pilot/Operator Aircraft Accident/Incident Report having a total time of 2,752 hours, of which 1,569 were in rotorcraft. In the previous 90 days she reported accruing 357 hours in rotorcraft, of which 214 hours were in the accident make and model helicopter. AIRCRAFT INFORMATION The helicopter was manufactured in December 2001, by Eurocopter as model EC 130 B4, with serial number 3487. It was equipped with an Arriel 2B1 engine, rated for 5 minutes at 747 shaft horsepower. Main rotor blades part number (P/N) 355A-11-0030.00, serial numbers (S/N’s) 22312, 22716, and 22741 were installed at the time of manufacture. Following manufacture, the helicopter was disassembled, shipped to Liberty Helicopters, Inc., and reassembled on January 3, 2002. At that time the main rotor blades were installed in accordance with (IAW) the maintenance manual. A Standard Airworthiness Certificate was issued on April 9, 2002. The type certificate data sheet indicates that for the accident make and model helicopter, the maximum number of passengers is 6. On September 27, 2002, the helicopter was modified IAW Service Bulletin (SB) 25.028 which allowed the installation of 8 seats. The helicopter was placed on Liberty Helicopters, Inc., Operations Specifications on April 22, 2002, and was maintained in accordance with a Federal Aviation Administration (FAA) Approved Aircraft Inspection Program (AAIP). With respect to the airframe, the following inspections are required: 3-day check, 100, 500, 1,000, 2,000, and 2,500-hour inspections. The 3-day check stipulates that the main rotor blades are to be checked for security and general condition of the skin, trim tabs, and polyurethane protective strips. A visual inspection of the main rotor blade for scratches, cracks, impacts and distortions is also indicated. Following the 3-day check, a caution indicates, “Ensure all cowlings and fairings are closed and latched.” Review of the 100-hour inspection checklist revealed that the skin, and leading edge of the main rotor blades, are to be checked for delamination and cracks. The inspection of the blades is accomplished IAW the manufacturer’s aircraft maintenance manual (AMM) 62-11-00, section 6-1. Although the manufacturer Master Servicing Recommendation (MSR) manual specifies to inspect the main rotor blades at intervals of 110 hours, the AAIP work card specifies that the blades are to be inspected for cracks at intervals of 100 hours. Review of the maintenance records revealed the helicopter had a 100-hour inspection on June 23, 2007. The helicopter total time at the time of the in