History of Flight
On September 16, 2008, at 1843 eastern daylight time, a Hiller UH-12E, registration N3324F, operated by Summit Helicopters, Inc., sustained 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 14 CFR Part 137.
The pilot reported that while in straight and level flight preparing for the next pass and about to activate the boom switch, the helicopter began a violent horizontal circular shake or wobble, and the engine sounded abnormal. Main rotor rpm decreased, and the helicopter began to descend. The pilot entered an autorotation, initiated a left turn to avoid trees, and touched down. During the landing, the tail rotor contacted the ground. After engine shutdown, a main rotor blade struck the tailboom when the rotor head tilted and the blade moved forward.
Personnel Information
According to Federal Aviation Administration 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
The helicopter was manufactured in 1964. Its most recent 100-hour inspection was completed on August 28, 2008, at which time it had accumulated 9,529 total hours of operation.
Meteorological Information
A weather observation taken approximately 11 minutes after the accident at Lynchburg Regional Airport, about 18 nautical miles north-northwest of the accident site, recorded calm winds, visibility 10 miles, overcast ceiling at 7,000 feet, temperature 19°C, dew point 15°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 damage to the transmission mounts, the fifth driveshaft Cardan joint, the one-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 main rotor blade drag strut.
Tests and Research
Examination of the failed main rotor blade drag strut by NTSB investigators showed failure at the inboard threaded portion. A vibro-peen marked serial number (S/N 10058) was visible. The threaded end of the rod remained attached to the rod clevis with a nut still on the thread remnant. Binocular microscope examination of the fracture face revealed crack arrest marks typical of fatigue cracking emanating from the root portion of the thread. Based on orientation, the fatigue crack origin was on the top or bottom side of the drag strut depending on mounting orientation. The origin showed no gouge or corrosion. Fatigue crack propagation extended through approximately 80 percent of the thread cross section, with the remaining portion showing a matte feature consistent with overstress separation.
Maintenance Records
A review of the operator's Maintenance Tracking Report indicated the failed drag strut was listed as part number (P/N) 52120-5, but this could not be verified because no part number was present on the failed strut. Helicopter manufacturer drawings specified that all part identification marks should be co-located with the serial number. According to maintenance records, the main rotor head assembly had been reassembled from spares, and the failed drag strut had accumulated 2,236.9 total hours of operation. The helicopter manufacturer stated that if the drag strut was P/N 52120-5, its life limit was 2,500 hours.
Previously Issued Service Bulletin
On January 21, 1974, Hiller Aviation issued Service Bulletin UH12D and UH12E No. 51-2, titled "Main Rotor Blade, Assembly Inspection of the Main Rotor Drag Strut," to identify drag struts that may not meet hardness requirements. The bulletin required that before the next 50 hours of flight, P/N 52120 drag struts should be removed and the hardness of the hexagonal rod portion measured. If hardness requirements were not met, the strut would need replacement by a P/N 52120-5 strut. If hardness was met, an "H" was to be etched in front of the serial number. Examination of the failed drag strut revealed no part number and no "H" in front of the serial number; the NTSB could not determine if it was a P/N 52120-5 or an earlier P/N 52120 subject to the bulletin, nor could it verify the documented hours in service. Rockwell hardness testing was unsuccessful in determining the strut's lineage.
Additional Information
The FAA's Suspected Unapproved Parts Office notes that many methods for designing and producing major aircraft products require specific FAA approval after review of design, facilities, processes, and quality control. Some parts do not require specific FAA sanction, such as those produced by owners/operators for maintenance or standard parts meeting industry criteria. Parts that may not meet requirements are termed suspected unapproved parts (SUPs). Advisory Circular 21-29C defines SUPs and counterfeit parts. The FAA states that none of these types of parts should be installed on an aircraft.