NTSB Identification: LAX99LA253.
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Accident occurred Monday, June 07, 1999 in PACIFIC OCEAN
Probable Cause Approval Date: 12/07/1999
Aircraft: Hughes 369HS, registration: N4278M
Injuries: 2 Minor.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
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.'
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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. Full narrative available
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