ASN Wikibase Occurrence # 131507
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| Date: | Saturday 7 July 2007 |
| Time: | 16:51 |
| Type: |  Eurocopter EC 130B4 |
| Owner/operator: | Liberty Helicopters Inc |
| Registration: | N453AE |
| MSN: | 3487 |
| Year of manufacture: | 2001 |
| Total airframe hrs: | 8077 hours |
| Engine model: | TURBOMECA ARRIEL 2B1 |
| Fatalities: | Fatalities: 0 / Occupants: 8 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Hudson River, New York, NY -
 United States of America
|
| Phase: | En route |
| Nature: | Passenger - Non-Scheduled/charter/Air Taxi |
| Departure airport: | New York, NY (JRA) |
| New York, NY (JRA) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Information verified through data from accident investigation authorities |
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.
Probable Cause: 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.
Accident investigation:
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|
Sources:
NTSB: https://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20070717X00949&key=1
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 18-Jun-2011 09:08 | gerard57 | Added |
| 07-Mar-2013 03:18 | TB | Updated [Time, Aircraft type, Cn, Operator, Location, Nature, Departure airport, Narrative] |
| 30-Oct-2014 17:32 | TB | Updated [Aircraft type] |
| 21-Dec-2016 19:25 | ASN Update Bot | Updated [Time, Damage, Category, Investigating agency] |
| 04-Dec-2017 18:48 | ASN Update Bot | Updated [Operator, Other fatalities, Nature, Departure airport, Destination airport, Source, Narrative] |
| 12-Mar-2018 20:55 | Aerossurance | Updated [Nature, Narrative] |
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