N4278M
Hughes 369HS
About 10 minutes into a fish spotting mission from a fishing vessel, the pilot felt an intense vibration through the tail rotor pedals and helicopter control became increasingly difficult. About 250 feet above the water, the 90-degree gear box and tail rotor assembly separated. The helicopter spiraled down toward the water, spinning to the right with an estimated 55- to 60-degree nose-down attitude. The pilot reported the last moments before contacting the water were 'uncontrollable.' The helicopter was recovered, except for the tail rotor system rotating components and gearbox. The operator reported that he had removed the tail rotor abrasion strips from the blades during an annual inspection about 73 hours before the accident. He reported that he applied 'direct and intense heat' to the blades to remove the strips. According to the manufacturer, the abrasion strips can be removed; however, it must be done by an approved blade overhaul facility and cannot be accomplished in the field. The manufacturer stated that the use of heat is unauthorized and likely induced a bond separation within the tip cap area. A review of the McDonnell Douglas maintenance manual revealed that tail rotor blade repairs are not specifically addressed. A McDonnell Douglas representative reported that if a repair is not addressed in the manual, it is not authorized. He stated that certified blade repair stations are authorized expanded repairs. The pilot/mechanic reported that he was aware that the operator had removed the tail rotor abrasion strips and had observed a small gap between one of the tail rotor blade tip caps and the blade during a routine inspection before the accident. He contacted the operator and requested a new blade. He then applied clear epoxy resin at the joining surfaces then test-flew the helicopter and noticed 'no change in the flight characteristics of the aircraft.' He stated that he felt the 'blade [was] airworthy and was as originally constructed.' McDonnell Douglas Service Bulletin HN-195 directs mechanics to visually inspect the tip cap-to-blade bonding for failure and states that 'if any evidence of debonding is noted, replace blade.'
HISTORY OF FLIGHT On June 7, 1999, at 0900 hours local time, a Hughes 369HS, N4278M, crashed into the Pacific Ocean following separation of the tail rotor gear box approximately 300 miles southwest of Majuro in the Marshall Islands. The helicopter, owned and operated by Hoffman Helicopters, Yona, Guam, was destroyed. Both the commercial pilot/mechanic and the observer suffered minor injuries. The local flight was conducted under the provisions of 14 CFR Part 91 as a tuna spotting operation, and had originated from a fishing vessel about 0845. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he was flying at 600 feet above the water. The helicopter had all doors removed for flight and had permanent floats installed. The pilot and observer were both wearing life jackets and had an inflatable life raft onboard. About 10 minutes into the flight, the pilot felt a vibration through the tail rotor pedals. He stated that the vibration started out "light" then became a "violent vibration" which he felt through the entire airframe. The vibration was such that he couldn't read the instrument panel. The pilot reported that he decreased the power and airspeed, hoping to stabilize the ship and further assess the situation. He stated that control of the aircraft became increasingly difficult. He suddenly heard a loud metallic grinding noise from the rear of the aircraft, which stopped after about 10 seconds. About 250 feet above the water, the pilot said that there was an "extreme forward CG [center of gravity] shift" and he heard a "whirring or spinning noise." The helicopter spiraled down toward the water, spinning to the right with an estimated 55- to 60-degree nose-down attitude. The pilot remembered pulling back hard on the cyclic but said that the last moments before contacting the water were "uncontrollable." He reported that he found himself upside-down in the water; he released his seatbelt and swam to the ocean surface, which he estimated to be about 6 to 7 feet above him. He stated that the observer came to the surface just after he did. They waited in a life raft until they were rescued by their ship, which was approximately 40 minutes away at the time of the accident. The helicopter was recovered, except for the tail rotor system rotating components and gearbox, and taken aboard the ship. After rescue, it took about 26 hours for the ship to get to the closest hospital; there was no doctor or paramedic aboard the ship. PERSONNEL INFORMATION The pilot held a Federal Aviation Administration (FAA) commercial pilot's certificate with a rotorcraft-helicopter rating. The pilot reported that at the time of the accident he had 3,046 hours of total flight time, including 2,856 total rotorcraft hours and 1,825 hours in the accident helicopter make/model. He reported that he did not have a current biennial flight review at the time of the accident. The pilot also held an FAA mechanic's certificate, with both airframe and powerplant ratings. He held a second-class medical, dated April 26, 1999, with no waivers or limitations. The pilot explained that he worked on the ship in the capacity of both pilot and mechanic. There were no other pilots or mechanics on board. AIRCRAFT INFORMATION The operator reported that he had purchased the accident helicopter in January 1999, and had just completed 2 months of work overhauling the helicopter. The logbooks reflected that the helicopter had undergone an annual inspection on April 16, 1999, approximately 73 flight hours before the accident. The airframe total time at the time of the accident was reported to be 8,168 hours. The operator reported that he removed the tail rotor blade abrasion strips from the blades during the annual inspection. He stated that he did this "in the interest of safety", as he believed that "the abrasion strips have caused many accidents when they come off in flight." The operator stated that he used "direct and intense heat" to remove the strips. He reported that following the accident, he informed his workshop staff that the practice of removing the tail rotor abrasion strips was "illegal and unauthorized and must not re-occur." The operator reported that he wished he had never removed the strips and stated that he was "sorry that I took it upon myself to remove the abrasion strips without trying harder to get the factory to do them." The airframe logbooks were reviewed and a copy of the relevant portions is appended to this file. The entry for April 16, 1999 did not reflect any major alteration or repair to the tail rotor blades. The pilot/mechanic reported that he arrived in Guam on April 20, 1999 to resume his contract aboard the fishing vessel. He stated that when he went to the maintenance facility on April 27, 1999 to begin the track and balance procedures, he observed that the tail rotor blades had been removed from the helicopter. He reported that the operator informed him that another of his contract helicopters needed the blades and he would have serviceable blades for N4278M in 2 days. The pilot saw maintenance personnel repainting a set of tail rotor blades on April 28, 1999, and the next day he noted that those tail rotor blades had been installed on N4278M. His inspection of the helicopter revealed that the tail rotor blade abrasion strips had been removed from the tail rotor blades and the leading edge of the blades had been painted silver/gray. He stated that the tip caps had been removed, then reinstalled after the blades were repainted. The pilot stated that he questioned the operator regarding the alteration and the operator informed him that it was his standard practice to remove the abrasion strips from tail rotor blades that were affected by salt-water corrosion. The pilot test-flew the aircraft. He stated that he believed the aircraft to be in an "airworthy condition" and indicated that he noticed "no change in the flight characteristics of the aircraft" prior to the accident. The pilot reported that during a routine postflight inspection on May 30, 1999, he observed a small gap at the mating surfaces of one of the tail rotor blade tip caps and the blade. He stated that he thought it was a void in the new paint and that the tip cap had been installed slightly askew. He performed a tap test inspection of the area and "confirmed the security of the tip cap area and rivets." The pilot reported that to determine potential movement, he applied a light bead of clear epoxy resin at the joining surfaces. After curing of the epoxy, he performed a ground test run to 100 percent power, shutdown, inspected the area again and observed no change. He stated that he felt the "blade [was] airworthy and was as originally constructed." After another test flight he observed no change. The pilot stated that he continued to monitor the area on pre- and postflight inspections. Additionally, the pilot contacted the operator and requested a new tail rotor blade. The pilot reported that during conversations with the operator after the accident, the operator informed him that the method he used to remove the abrasion strips was "the application of intense and direct heat to the tail rotor blade(s) specifically in the area of the abrasion strips and the tail rotor blade tip cap." The operator further stated that "he now recognized that this method destroyed the internal bonding, structurally weakening the tail rotor blade." The operator reported that "there is no doubt the debonding allowed the tip cap to become unsecured and dislodged, which caused the total imbalance of the tail rotor blades, resulting in the assembly failure." ADDITIONAL INFORMATION A review of the McDonnell Douglas Basic Handbook of Maintenance Instructions was conducted. Tail rotor blade repairs were not covered in the handbook. According to a representative from McDonnell Douglas, if a repair is not specifically addressed in the maintenance handbook, it is not authorized. He stated that McDonnell Douglas certified blade repair stations are authorized expanded repairs. McDonnell Douglas Service Bulletin HN-195 was reviewed and a copy is appended to this file. It directs the mechanic to visually inspect the tip cap-to-blade bonding for failure and states that "if any evidence of debonding is noted, replace blade."
The removal of the tail rotor blade abrasion strips by the use of an unapproved method by the maintenance personnel which resulted in the imbalance of the tail rotor blade assembly and subsequent separation of the 90-degree gear box; the pilot/mechanic's unapproved repair to the tail rotor blade tip caps; and, his continued operation of the helicopter with known deficiencies.
Source: NTSB Aviation Accident Database
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