DESTIN, FL, USA
N88MP
Hughes 369D
The pilot said he had been flying the helicopter at an altitude of about 500 feet, 100 knots indicated airspeed, and 65 to 70 pounds per square inch torque, when he felt a high frequency vibration through the antitorque pedals. He said the vibration dissipated but soon began again, and became excessive with 'strong' forces in the pedals. The forces were soon followed by a loud 'bang' and the aircraft yawed violently to the right, and the nose pitched down. He said the antitorque pedals had no effect, so he lowered the collective control and applied aft cyclic control input to the stop the yaw, and level the aircraft. He said the yaw rate slowed, but did not stop, and at about 70 to 80 degrees of right yaw, he reduced the throttle in an attempt to further reduce/control the yaw condition, and the aircraft yaw stopped. He was then able to execute an autorotative landing to an open field. The on scene examination of the aircraft showed that the stabilizer, as well as an 18-inch section of the hollow portion of the tail had separated from the aircraft in flight. The 90-degree gearbox also partially separated from the aircraft, and was only held by one remaining bolt, and there were contact marks and paint transfer signatures, consistent with those from multiple tail rotor blade strikes at the fracture point. The aft 6 inches of the tailboom with attach points, tail rotor gearbox, and tail rotor assembly were retained for metallurgical examination. The examination revealed that the fractured tail rotor gearbox studs were fatigue failures. In addition, both elastomer bearings in the tail rotor drive fork were examined, and were found to have failed due to fatigue.
HISTORY OF FLIGHT On July 2, 2000, about 1150 central daylight time, a Hughes 369D, N88MP, registered to the Florida Department of Natural Resources, and operated by the Florida Fish and Wildlife Commission, as a Title 14 CFR Part 91 public-use flight, experienced a tail boom separation while in flight, in Walton County, Florida. Visual meteorological conditions prevailed, and no flight plan was filed. The commercial-rated pilot, the sole occupant, was not injured, and the aircraft incurred substantial damage. The flight originated in Panama City, Florida, the same day, about 0930. The pilot stated that he had performed a preflight inspection on the aircraft, and had completed a compressor wash, washed the engine, and applied WD40 to the tail rotor blades as a routine anti-corrosion procedure, prior to the flight. He further stated that he then departed Panama City International Airport at approximately 9:30 a.m. on a routine law enforcement flight, and was en route back to Panama City after approximately 2.2 hours of flight. While en route, he was conducting boating activity checks along the way, in the vicinity of the Destin Pass. The pilot said that at about 1150, while orbiting a jet ski in the water, in the vicinity of Walton County substation, on Highway 331, and about 0.5 mile north of Highway 98, at an altitude of about 500 feet above ground level (AGL), at 100 knots indicated air speed (KIAS), and with 65 to 70 pounds per square inch (PSI) torque applied, he felt a high frequency vibration through the antitorque pedals. He said he attributed this vibration to having flown through his own rotor wash during the tight orbit. He said the vibration dissipated but soon began again, and soon became excessive, with "strong" feedback forces in the pedals. He said the strong feed back forces, were soon followed by a loud "bang" and the aircraft violently yawed to the right, and the nose pitched down. He said the antitorque pedals had no effect, so he lowered the collective control and applied aft cyclic control input to the stop the yaw, and level the aircraft. He said the yaw rate slowed, but did not stop, and at about 70 to 80 degrees of right yaw, he "rolled off" the throttle in an attempt to further reduce/control the yaw condition, and the aircraft yaw excursion stopped. He said it took almost full aft cyclic control input to maintain a level flight attitude. He said he then turned the aircraft to the right in a southerly direction, toward the only available landing site, which was within autorotational gliding distance, and into the wind. He said it took almost full aft cyclic control input to maintain a level flight attitude, and he kept the airspeed up to about 80 to 90 knots during the descent to control the right yaw, keep the aircraft streamlined, and prevent it from spinning. He said he had to extend his glide distance to reach the landing area, and he performed an autorotative landing in a level flight attitude. He said that prior to touchdown, he increased collective control input to decrease the rate of closure and the aircraft yawed to the right approximately 180 degrees. The aircraft touched down without forward movement, and yawed a further 20 degrees on the heels of the skids after touching down. He then turned off the avionics, generator, battery switches, engaged the rotor brake until the rotor stopped, exited the aircraft, and looked it over for any sign of fuel leaks or fire. He then opened the engine cowling for cooling and rotated the main rotor blades backward to prevent coking of the bearings. After looking over the aircraft, he climbed back in, and informed the dispatch office of the accident. PERSONNEL INFORMATION Records obtained from FAA and the Florida Fish and Wildlife Conservation Commission revealed that the pilot held a commercial pilot certificate, with helicopter and airplane single engine, multi engine land ratings, as well as instrument helicopter and airplane ratings. The pilot's commercial certificate was last issued on November 29, 1999. The pilot held an FAA second-class medical certificate, last issued on August 31, 1999. The medical certificate did not specify any limitations or waivers. According to Florida Fish and Wildlife Conservation Commission records, the pilot had accumulated 4,200 total flight hours, with 88 flight hours flown in the last 90 days. The pilot had accumulated 4,000 total flight hours in helicopters, and 150 flight hours in the accident helicopter make and model. AIRCRAFT INFORMATION N88MP is a Hughes 369D helicopter, serial number 911027D. At the time of the accident, the aircraft had accumulated 4555.6 hours total flight hours on the airframe. The last scheduled maintenance was an annual inspection/100 Hr, which was performed at 4474.4 TT on April 7, 2000. The aircraft is equipped with an Allison 250-C20B turboshaft engine, serial number CAE-833994, rated at 420 horsepower. The engine had accumulated 4,555.6 total flight hours, and 79.6 hours since the last inspection. Heli-Tech Inc., of Panama City, performed all routine maintenance services and unscheduled maintenance on the aircraft, and a review of the aircraft maintenance records indicated that it was maintained in accordance with the applicable manufacturer service and maintenance manuals. Aircraft records also indicated that maintenance was in accordance with Manufacturer Service Notices and applicable FAA Airworthiness Directives. METEOROLOGICAL INFORMATION Visual meteorological conditions prevailed at the time. The Elgin Air Force Base surface weather observation at 1155 was few clouds at 4,000 feet, ceiling 30,000 broken, visibility 6 statute miles in haze, wind from 150 degrees at 13 knots, temperature 85 degrees F, dew point temperature 60 degrees F, altimeter setting 30.06 inHg. WRECKAGE AND IMPACT INFORMATION The aircraft came to rest upright in a level attitude, with no visible damage to the airframe and fuselage, except for the tail, which had been severed. The aircraft was located in a level open spot, in a recently logged forested area, at 30 degrees, 22.45 minutes north latitude, 086 degrees, 12.27 minutes west longitude. Examination of the helicopter showed that the stabilizer, as well as an 18-inch section of the hollow portion of the tail had separated from the aircraft in flight. The 90-degree gearbox also was partially separated from the aircraft, and was only held by one remaining bolt. The tailboom was severed aft, in the area of station 258.0. The fractured and separated section from station 258.0 aft (approximately 23 inches) of the tailboom, remained attached to the vertical and horizontal stabilizers and tail rotor assembly. There was evidence of contact marks and paint transference indicative of multiple tail rotor blade strikes at the fracture point. The vertical and horizontal stabilizer, tail rotor (T/R) gearbox, T/R hub and blades remained attached to the aft portion of the tailboom, and were located approximately 2100 feet north of the fuselage. The vertical stabilizer had been warped and gouged at several locations along its length. There was evidence of tail rotor blade contact to the upper section. The horizontal stabilizer also was warped and dented, but remained attached to the vertical fin, and both tip plates remained attached. The tail rotor assembly remained attached to the separated aft portion of the tailboom. There was visible damage to the T/R gearbox, and the tail rotor gearbox attaching bolts had been pulled from both upper Rosan fittings. The lower left attaching stud was severed and the lower right stud remained attached, but had become elongated and distorted. Gearbox operation appeared normal when rotated by hand. The tail rotor swashplate slid freely on the output shaft of the tail rotor gearbox. The tail rotor assembly, to include the tail rotor hub, pitch control assembly, and control links, showed little exterior damage except for some small marks, consistent with that from flying debris and possible ground contact. The pitch control mechanism function appropriately when manipulated. The drive fork appeared undamaged except for the two elastomeric bearings. Both drive fork elastomeric bearings had failed at the inner cone. One elastomeric bearing had debonded and exhibiting considerable abrasion/wear over half the exterior metal of the inner cone. The opposite bearing separated approximately 1/16th of inch in from the cone. There was elastomeric material still bonded to the entire surface around the circumference of the inner cone. The drive fork bolt and nut were not damaged. The tail rotor blades remained attached to the hub but they had been damaged. Both blades were scratched and gouged and exhibited paint transference marks, consistent with having made multiple strikes to the tail boom. The tail rotor driveshaft remained attached at the transmission output Bendix coupling. There was rotational scoring present at the front of the driveshaft tunnel. The driveshaft had one rotational/torsion fracture approximately 18 inches forward of the aft Bendix coupling. The aft portion of the Bendix coupling remained attached to the T/R gearbox. The aircraft had been configured for single pilot operation, and its cyclic, collective main rotor, N2 governor, N1 control linkages exhibited continuity throughout the full range of movement. The antitorque flight control linkage exhibited continuity up to the point where the control tube had been severed at the tailboom, i.e. station 258.0, and the severed section of the tail rotor blade pitch change mechanism moved in an appropriate manner in response to movement of the control rod. The NTSB retained the aft 6 inches of the tailboom with attaching points, tail rotor gearbox, and tail rotor assembly, including the drive fork with its elastomeric bearings for further examination. TESTS AND RESEARCH Metallurgical examinations of the aft 6 inches of the tailboom with its attach points, tail rotor gearbox, and tail rotor assembly with the drive fork, as well as an initial examination of the elastomeric bearings, were conducted on August 15, 2000, at the Boeing Company, Mesa, Arizona. The examination revealed that the three fractured tail rotor gearbox studs were fatigue failures, consistent with low load, high cycle fatigue. In addition, examination of the aft Bendix coupling indicated that the coupling failed in overload consistent with that of a sudden stoppage of the tail rotor gearbox, as it became loose from the frame fittings. After the initial examination of the damaged parts, a detailed examination of the elastomeric bearings was conducted at the manufacturer, Lord Corporation, Erie, Pennsylvania. According to the manufacturer's records, the approximate time the tail rotor elastomeric bearings had been in operation was about 2,000 hours, and the examination revealed that both elastomeric bearings had failed due to fatigue fractures. ADDITIONAL INFORMATION The NTSB released the aircraft to the Florida Fish and Wildlife Conservation Commission shortly after the accident except for the parts that were retained for testing. The remaining aircraft parts were released to Mr. John S. McDonald, Aviation Support Officer, Florida Fish and Wild life Conservation Commission, Tallahassee, Florida, on July 7, 2000.
improper maintenance inspection of the tail rotor by maintenance personnel and improper preflight inspection of the tail rotor by the pilot, which resulted in worn elastomeric bearings being continued in service, resulting in failure of the tail rotor assembly, and damage to the helicopter during a hard landing.
Source: NTSB Aviation Accident Database
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