Accident Description
About 10 minutes into a fish spotting mission launched from a fishing vessel, the pilot felt an intense vibration through the tail rotor pedals, and control of the helicopter became increasingly difficult. At an altitude of approximately 250 feet above the water, the 90-degree gear box and tail rotor assembly separated from the helicopter. The helicopter spiraled downward with a right spin and an estimated nose-down attitude of 55 to 60 degrees. The pilot reported that the final moments before contacting the water were 'uncontrollable.' The helicopter was recovered from the water, except for the tail rotor system rotating components and gearbox.
Maintenance History
The operator reported that about 73 flight hours before the accident, during an annual inspection, they removed the tail rotor abrasion strips from the blades. The operator stated that they applied 'direct and intense heat' to the blades to remove the strips.
Manufacturer's Guidance
According to the manufacturer, the abrasion strips can be removed, but only by an approved blade overhaul facility; this operation 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 of the tail rotor blade. 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, and that certified blade repair stations are authorized to perform expanded repairs.
Pilot's Actions
Prior to the accident, the pilot/mechanic was aware that the operator had removed the tail rotor abrasion strips. During a routine inspection, the pilot observed a small gap between one of the tail rotor blade tip caps and the blade. The pilot contacted the operator and requested a new blade. He then applied clear epoxy resin at the joining surfaces and test-flew the helicopter, noticing 'no change in the flight characteristics of the aircraft.' The pilot stated that he felt the blade was airworthy and 'as originally constructed.'
Service Bulletin
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, the blade should be replaced.