N4250N
Hughes 369HS
The pilot was spotting tuna off a ship and was approximately 10 minutes from the ship when the cyclic went full left travel. The pilot, who had not attended any factory training courses on this helicopter, said he tried to get the trim motor to reengage but to no avail. He was unable to get the cyclic to return to a neutral position. He stated that he came in to land on the boat and lost control of the helicopter because he could not hold the cyclic with one hand. The helicopter struck an antenna, rolled to the left, and landed upside down in the water. The pilot indicated to others he had problems with the trim switch on previous flights. The owner's manual states that the cyclic stick forces with a runaway cyclic would be approximately 30 pounds. It states that the helicopter will respond normally to all cyclic inputs by the pilot. The switch was returned and disassembled for detailed examination. The manufacturer could not find any identifiable markings on the switch during the examination and tool marks and other internal evidence disclosed that the switch had been disassembled and reassembled in the field. Non-standard parts were found on the inside of the switch. Boeing Helicopters sent out a mandatory service information notice dated March 10, 1994, which required all operators of the 369/500 helicopters to replace all four-way trim switches with the revision 'D' four-way trim switch. The switch is not a repairable item. No repair manual or spare parts programs exist for this switch.
On June 20, 1997, about 1030 hours local ship time, a Hughes 369HS helicopter, N425ON, collided with a ship antenna, rolled over, then collided with the ocean during a landing approach to a ship while fish spotting in international waters. The helicopter was destroyed. The certificated commercial pilot sustained serious injuries and the observer onboard sustained fatal injuries. The helicopter was operated by Hansen Helicopters and leased to the Korean owned tuna vessel M/V GRANADA for aerial observation missions in support of fishing activities. The location of the accident site is about 2,000 miles southeast of Guam at 0 degrees 58 minutes north, 174 degrees 38 minutes east. The operator told the insurance adjuster the main rotor blade struck a ship antenna during an attempted landing. The helicopter rolled over and went into the water on its right side, and remained floating inverted in the water. The ship recovered the helicopter. The pilot reported in his pilot/operator report that the trim switch in the cyclic malfunctioned, forcing the cyclic stick into a full left position. He said that attempts to return the cyclic to a neutral position failed, and that he "flew around trying to get the trim motor to re-engage," without success. As he approached the boat and came to the landing deck, he lost control of the helicopter "because he could not hold the cyclic with one hand." The helicopter then rolled to the left and came to rest upside down in the water. A mechanic for the operator witnessed the accident sequence. He reported that the approach to the ship appeared normal until the helicopter was 4 feet off the landing deck. An abrupt power increase was heard followed by the helicopter rearing backwards, turning 270 degrees to the left, and then going into the water. The pilot stated to the witness that he had been having trouble with the trim switch on prior flights. The operator sent the cyclic trim switch to the Safety Board for examination and disassembly. The switch was examined under the supervision of the Safety Board on August 18, 1997, at Boeing Helicopters, Mesa, Arizona. The examination and subsequent disassembly revealed that the switch had been disassembled and then reassembled in the field at an undetermined point in time. A series of tool marks, a cracked insulator, and loose terminals were found during the examination. According to the manufacturer of the switch, Guardian Electric, there were no markings of any kind on the switch to indicate the manufacturer or date code. The appended drawing A218-100646-03, originally released 5/29/90, indicates that the revision of the part "should be stamped on the bottom of the bracket." This stamp is a metal stamp and occurred in revision "D" dated 11/16/93. According to the manufacturer, the lack of the revision "D" stamp indicates that the inspected switch was manufactured prior to this change. Additionally, during the inspection, it was noted that the stainless steel ball had worn groves into the soft silver of the disc contact. A review of the design history revealed that the contact disk with the copper-beryllium liner was released on 12/2/53 and is associated with drawing A012-900006-00. A scanning electron microscope analysis was completed on the contact disk. The results of the analysis revealed that the switch that was examined was made up of silver. A previous version of this switch, drawing X-818 originally released in 11/2/53, had no copper-beryllium liner and a non-metallic plunger to actuate the contact disk. This version was revised on 12/3/53 with the copper-beryllium lined version and the ball assembly added as identified by drawing XA-6. The cyclic trim switch is located on top of the cyclic stick grip. The switch has five positions: normally off; in the center; forward; aft; and right. When the trim switch is moved off center to any of the four trim directions, the electrical system energizes one of the trim motors to apply trim spring forces in the desired direction. A review of the owner's manual discussed cyclic trim failures. According to the pilot, the cyclic trim malfunctioned, causing the cyclic stick into the full left position. The note in the owner's manual stated that "runaway cyclic trim failures can produce cyclic stick forces of approximately 30 pounds in the direction. . .although the forces required normally to all cyclic inputs by the pilot." Hansen helicopters stated that the helicopter was originally 12 miles from the ship when the malfunction occurred. They estimated it would take the pilot about 10 minutes of flight time to reach the ship for landing. The manual instructs the pilot to utilize left hand and legs, as necessary, to apply pressure against the cyclic stick to relieve the right hand loads and conserve strength for landing. Use collective friction to prevent unwanted collective movement and associated power changes. Be prepared to respond to any emergency requiring the use of collective pitch. The Safety Board did a test flight in an attempt to replicate as closely as possible the conditions and force pounds necessary to control the helicopter with a cyclic trim failure. The test flight aircraft was a MD520N, equipped with a five bladed rotor system, which produces a heavier load and control force than the 369. Cyclic trim failures were replaced in all four axes in various flight configurations and were successfully flown to touchdown in a confined area-landing site. The chief pilot, flight test at Boeing helicopters stated that cyclic trim failure procedures are routinely reviewed with students who attend factory training courses. The pilot received a biennial flight review on May 8, 1996, in a Bell 47 helicopter. He stated that he had a total of 247.8 hours of flight time in the Hughes 369 helicopter. The operator stated that the pilot had not completed any factory training in the specific helicopter. Boeing Helicopters checked their database and could not find any record that the pilot had attended any of their factory training courses. There is a warning associated with the cyclic trim failure. It states that "control of the helicopter is the primary consideration of a pilot confronted with any type of trim motor or switch malfunction. The pilot-in-command should land the helicopter immediately if the pilot's physical condition, strength, or threshold of fatigue, would compromise their ability to safely control the helicopter in continued flight." The operator stated in a facsimile to the Safety Board that the pilot was 6-foot-tall and weighed 175 pounds. Additionally, the helicopter was equipped with fixed floats. Boeing Helicopters send operators a mandatory Service Information Notice dated March 10, 1994. The affected aircraft included all 369/500 series helicopters with Guardian Electric A218-100646-03 four-way trim switches installed in the cyclic grip. The assembly/components affected by this notice were A218-100646-03 prior to revision "D" four-way trim switches and A218-9667114-02 and 369D27133-501 grip assemblies. The procedure outlined in these notices required operators to replace affected four-way trim switches with switches that have been upgraded by Guardian to a revision "D" configuration.
The unapproved field modification of the cyclic trim switch, including the use of non-standard parts, which resulted in a hard-over lateral trim failure, and the pilot's subsequent failure to maintain control of the helicopter during a landing approach. A factor in the accident was the operator's failure to comply with a factory service bulletin, which required replacement of the switch with a new version, and the pilot's continued operation with a known discrepancy.
Source: NTSB Aviation Accident Database
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