On August 29, 2004, about 1802 eastern daylight time, a Robinson R-22, N871CL, was substantially damaged when it impacted water near Northport, New York. The certificated flight instructor, and private pilot were fatally injured. Day visual meteorological conditions prevailed for the local instructional flight that departed Long Island Mac Arthur Airport (ISP), Islip, New York, about 1715. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. HISTORY OF FLIGHT According to radar data, a target matching the performance characteristics of the accident helicopter, and "squawking 1200," was flying east along the north shore of Long Island, New York, about 600 feet agl. The target then initiated a descent, and radar contact was lost approximately 300 feet agl. Several minutes later, radar contact was reestablished, and the target continued to proceed eastbound about 600 feet agl. In the vicinity of the accident site, about 2 minutes before the accident, radar contact was lost, and was not reestablished. According to a witness that was lying on the beach watching a kite, the helicopter approached from the west along the shoreline 100 to 160 feet agl. The witness then saw the helicopter collide with the kite, the main rotor system separate, and about a second later, heard a loud "pop," which he described as sounding like a "balloon popping but much louder." The helicopter then started to rotate rapidly in a counter clockwise direction, then impacted the water. The witness also recovered the kite string, minus the kite, and gave it to local authorities. According to another witness that was sea kayaking about 300 feet from the shoreline and about 200 feet from the accident site, the helicopter approach from the west about 200 feet agl. He then heard a "loud crack noise," and saw the main rotor system "snap off." The helicopter then entered an uncontrolled descent, and the witness heard engine rpm increase. The witness added that the wind was approximately 7 knots out of the south, and the sky was clear, except for "perhaps a few clouds." In his written statement, the witness made no mention of a kite. Other witnesses interviewed reported a kite flying at the time of the accident, but could not recall if the helicopter had contacted it or not. The accident occurred during the hours of daylight. The wreckage was located about 80 feet from the shoreline in approximately 3 feet of water at 40 degrees, 55.700 minutes north latitude, 73 degrees, 20.500 minutes west longitude. PERSONNEL INFORMATION The instructor held a commercial pilot certificate with a rating for rotorcraft helicopter, and a flight instructor certificate with a rating for rotorcraft helicopter. His most recent Federal Aviation Administration (FAA) second-class medical certificate was issued on April 19, 2004. On that date he reported approximately 332 total hours of flight experience, all of which were in rotorcraft. Additionally, the instructor had worked for the helicopter operator for approximately 3 weeks. During that time he had flown approximately 92 hours, 10 hours of which were within 24 hours of the accident. The pilot receiving instruction held a private pilot certificate with ratings for airplane single-engine-land and rotorcraft helicopter. On his most recent FAA third-class medical certificate, which was issued on July 24, 1997, he reported 135 total hours of flight experience. According to the operator, the pilot had accumulated about 10 hours of flight experience in the 30 days preceding the accident. METEOROLOGICAL INFORMATION A weather observation was taken about 4 minutes after the accident, at Republic Airport (FRG), Farmingdale, New York, located approximately 14 miles to the southwest of the accident site. The weather was recorded as winds 200 degrees at 11 knots, visibility 10 miles, a broken ceiling at 1,200 feet, temperature 75 degrees Fahrenheit, dewpoint 73 degrees Fahrenheit, and an altimeter setting of 29.97 inches of mercury. WRECKAGE AND IMPACT INFORMATION The wreckage, less the main rotor hub and both main rotor blades, was recovered to a police impound yard the night of the accident. Examination of the main wreckage revealed that impact damage was greater towards the front of the helicopter. The cockpit displayed damage consistent with impact and main rotor blade contact. The engine and transmission cowlings displayed impact damage. The tailboom displayed impact damage, but no damage consistent with main rotor blade contact was observed. The tail rotor gearbox was attached, and both tail rotor blades had separated approximately 1-inch outboard of the blade grips. Flight control continuity was confirmed from the cockpit to the main rotor pitch change links, and to the tail rotor blade grips. The fracture surfaces for both main rotor pitch change links were deformed, and exhibited 45-degree shear lips. Examination of the main transmission revealed that the case and mounts were intact, and no noticeable deformation to the supporting structure was observed. A rotational force was applied to the input drive of the transmission, and the mast rotated. Drive train continuity was confirmed from the engine to the main transmission, and to the tail rotor gearbox. The main rotor mast had separated approximately 1-inch below the main rotor hub. The fracture surface was deformed, and exhibited a 45-degree shear lip. The main rotor hub and both associated rotor blades were recovered on September 27, 2004. The upper portion of the main rotor mast fracture surface also exhibited a 45-degree shear lip, and marks consistent with hub assembly contact. Fuel system continuity could not be confirmed because of impact damage. The fuel selector was in the "On" position. The carburetor was removed from the engine to facilitate the examination. The throttle plate, along with all the carburetor subsystems were intact, and other than saltwater, no contaminants were identified in the system. Examination of the ignition system revealed that the ignition switch was in the "Both" position. The left magneto was attached to the engine case, and the right magneto was separated. A rotational force was applied to both magnetos, and spark was observed on all four left magneto towers, but not on any of the right magneto towers. Continuity of the ignition leads from both magnetos to their respective sparkplugs could not be confirmed because of impact damage. The sparkplugs were removed, and contaminants ranging from oil, to rust, and to sludge, were identified. Examination of the engine revealed impact damage to several valve covers, along with the air induction and exhaust systems. Engine control continuity was confirmed from the cockpit to the carburetor, and carburetor heat box. A rotational force was applied to the engine crankshaft, and compression was obtained on all four cylinders. Additionally, continuity to the accessory gears, and the valve train was confirmed. The oil induction screen was absent of debris. Saltwater and sludge were present in the oil system, but nothing metallic was identified. ADDITIONAL INFORMATION According to the pilot's operating handbook for the helicopter, "Pushing the cyclic forward following a pull-up or rapid climb, or even from level flight, produces a low-G (weightless) flight condition. If the helicopter is still pitching forward when the pilot applies aft cyclic to reload the rotor, the rotor disk may tilt aft relative to the fuselage before it is reloaded. The main rotor torque reaction will then combine with tail rotor thrust to produce a powerful right rolling moment on the fuselage. With no lift from the rotor, there is no lateral control to stop the rapid right role and mast bumping can occur. Severe in-flight mast bumping usually results in main rotor shaft separation and/or rotor blade contact with the fuselage." The main wreckage was released to the owner on September 1, 2004. The main rotor system was released on October 6, 2006.