Naruna, VA, USA
N3324F
Hiller UH-12E
While in straight and level flight, the pilot indicated that the helicopter began a "violent horizontal circular shake or wobble," and the main rotor rpm and began to decrease. The pilot entered an autorotation, turned the helicopter to the left to avoid trees, and touched down. During the forced landing, the tail rotor contacted the ground, and after the engine had been shut down, one of the main rotor blades contacted the tailboom. Post accident examination revealed that a main rotor blade drag strut had failed at the inboard threaded portion of the strut. Further examination revealed that a vibro-peen marked serial number was visible on the failed drag strut and that the drag strut had failed due to fatigue cracking that emanated from the root portion of the threads. The helicopter's maintenance records listed the failed drag strut as being a part number that should have had a life limit of 2,500 hours, but failed at 2,236 hours. Contrary to the helicopter manufacturer’s drawings, there was no part number on the failed drag strut, which put into doubt the actual hours of operation on the part.
HISTORY OF FLIGHT On September 16, 2008 at 1843 eastern daylight time, a Hiller UH-12E, N3324F, operated by Summit Helicopters, Inc., incurred substantial damage during a forced landing after encountering a severe vibration and reduction of main rotor speed near Naruna, Virginia. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the aerial application flight, conducted under Title 14 Code of Federal Regulations Part 137. According to the pilot, while in straight and level flight for his "next pass," and getting ready to turn "the boom switch on," the helicopter began a "violent horizontal circular shake or wobble," and the engine did not sound right to the pilot. It then experienced a reduction in main rotor rpm and began to descend. The pilot then entered an autorotation, started a left turn to avoid trees, and touched down. During the forced landing, the tail rotor contacted the ground, and after the engine had been shutdown, one of the main rotor blades contacted the tailboom when the rotor head tilted, and the blade moved forward. PERSONNEL INFORMATION According to Federal Aviation Administration (FAA) and pilot records, the pilot held a commercial pilot certificate with multiple ratings, including rotorcraft helicopter. He had accumulated 756 total hours of flight experience in rotorcraft, of which 249 hours were in the accident helicopter make and model. AIRCRAFT INFORMATION According to FAA and operator records, the helicopter was manufactured in 1964. The helicopter’s most recent 100-hour inspection was completed on August 28, 2008. At the time of the inspection, it had accumulated 9,529 total hours of operation. METEOROLOGICAL INFORMATION A weather observation taken about 11 minutes after the accident at Lynchburg Regional Airport (LYH), Lynchburg, Virginia, located approximately 18 nautical miles north-northwest of the accident site, recorded calm winds, visibility 10 miles, overcast at 7,000 feet, temperature 19 degrees C, dew point 15 degrees C, and an altimeter setting of 30.15 inches of mercury. WRECKAGE AND IMPACT INFORMATION Postaccident examination by company maintenance personnel and an FAA inspector revealed that the helicopter had received damage to the transmission mounts, the 5th driveshaft Cardan joint, the 1 inch driveshaft slip joints, the tail rotor gearbox, the tail rotor assembly, the landing gear, the tailboom, the engine to transmission drive shaft, the main rotor blades, and one of the main rotor blade drag struts. TESTS AND RESEARCH Examination of the main rotor blade drag strut by NTSB investigators revealed that it had failed at the inboard threaded portion of the strut. A vibro-peen marked serial number (S/N), of 10058 was visible. Further examination revealed that the threaded end of the rod had remained attached to the rod clevis and a nut was still attached to the thread remnant. Binocular microscope examination of the fracture face on the thread remnant revealed crack arrest marks typical of fatigue cracking that emanated at the root portion of the thread. Based on the orientation of the fatigue crack origin relative to the clevis, the origin of the fatigue crack would be located either on the top or bottom side of the drag strut depending on mounting orientation. The origin of the fatigue crack contained no evidence of gouge or corrosion damage. Fatigue crack propagation was through approximately 80 percent of the thread cross section. The remaining portion of the fracture showed a matte feature consistent with overstress separation. Maintenance Records A review of the operator's Maintenance Tracking Report form revealed that the failed drag strut was listed as being part number (P/N) 52120-5. However, this could not be verified by NTSB investigators due to the fact that no part number was present on the failed drag strut. The helicopter manufacturer's drawings specified that, all part identification marks should have been co-located with the S/N. According to the helicopter's maintenance records, the helicopter's main rotor head assembly had been reassembled from "spares" and the failed drag strut had accumulated 2,236.9 total hours of operation when it failed. According to the helicopter manufacturer, if the drag strut was a P/N 52120-5 drag strut, it should have had a life limit of 2,500 hours. Previously Issued Service Bulletin On January 21, 1974, Hiller Aviation had issued Service Bulletin (SB) UH12D and UH12E No. 51-2, titled "Main Rotor Blade, Assembly Inspection of the Main Rotor Drag Strut," to identify drag struts which may not meet hardness requirements. The SB indicated that before the next 50 hours of flight, that P/N 52120 drag struts should be removed from the helicopter and the hardness of the hexagonal rod portion should be measured. If the hardness of the hexagonal rod portion did not meet the hardness requirements the drag strut would have needed to be replaced by a P/N 52120-5 drag strut. If the drag strut met the hardness requirements that were specified, an "H" was required to be etched into the drag strut in front of the S/N. Examination of the failed drag strut did not reveal a P/N, and no "H" was present in front of the S/N, the NTSB was unable to determine if the drag strut was P/N 52120-5 drag strut or an earlier P/N 52120 drag strut, that would have been subject to the SB. It was also unable to be determined if the documented hours in service were accurate. Although no deficiency in the hardness of the bar stock itself was discovered, Rockwell hardness testing was unsuccessful in helping to determine the lineage of the failed drag strut. ADDITIONAL INFORMATION According to FAA's Suspected Unapproved Parts Office, many of the methods for designing and producing major aircraft products, such as airframes, engines and propellers, need specific FAA approval. The FAA grants approvals only after a review of design criteria, facilities, processes, and quality control systems. The FAA monitors firms that have these production approvals to ensure they comply with regulations and the terms of their approvals. Some approved parts do not require specific FAA sanction. For example, the owner or operator of an aircraft can produce parts to maintain or alter their own product. Manufacturers often allow the use of "standard parts," such as nuts and bolts, that meet specified industry-accepted criteria. Maintenance personnel also can make parts in the course of their aircraft repair work as long as such parts meet applicable design criteria. All these parts must comply with regulations and meet industry standards. Parts that may not meet applicable requirements sometimes do enter the aviation system. They can range from parts that lack proper documentation to parts that are actually counterfeit. Until the FAA determines whether items actually meet requirements, the agency calls them suspected unapproved parts, or "SUPs." Advisory Circular 21-29C "Detecting and Reporting Suspected Unapproved Parts", defines a SUP as "A part, component, or material that is suspected of not meeting the requirements of an 'approved part.' A part that, for any reason, a person believes is not approved. Reasons may include finding such as different finish, size, color, improper (or lack of) identification, incomplete or altered paperwork, or any other questionable indication." The Advisory Circular (AC) defines a counterfeit part as "A part made or altered to imitate or resemble an "approved part" without authority or right, and with the intent to mislead or defraud by passing as original or genuine." SUPs can be parts rejected during production because of defects, missing required documentation , improperly maintained, and that have not been shown to comply with FAA requirements. SUPs may also include items from a supplier who produces parts for an approved manufacturer and then ships them directly to end users without the approved manufacturer’s authorization. Counterfeit parts may be new parts that are deliberately misrepresented as designed and produced under an approved system or other acceptable method. Counterfeit parts also may be used approved parts that have reached a design life limit or have been damaged beyond possible repair, but are altered and deliberately misrepresented as acceptable. According to the FAA, none of these types of parts should be installed on an aircraft.
The fatigue failure of a main rotor blade drag strut.
Source: NTSB Aviation Accident Database
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