On April 15, 1999, about 1445 eastern daylight time, a Robinson R-22 helicopter, N151RM, was substantially damaged while landing on a beach in Port Jefferson, New York. The certificated flight instructor and private pilot were not injured. Visual meteorological conditions prevailed for the dual instruction flight that had originated from Islip Airport, Ronkonkoma, New York, about 1430. No flight plan had been filed for the flight that was operated under 14 CFR Part 91. In the NTSB Pilot/Operator Aircraft Accident Report, the flight instructor stated: "We were performing a normal approach to a hover. At 20 feet we initiate a flare and brought back the airspeed to approx. 20 kts. Level out and started to bring the power back, the descent rate at this point 200-300 feet/min. The power/engine did not fully recover and descent continued down to ground. A slightly left law occurred then a hard right way. We hit the ground slightly on the left skid it...[dug] down in the sand and the aircraft rolled over on it's left side...." The private pilot receiving instruction stated: "...At the time of adding collective to slow the ship, the ship failed to slow down. It appeared that the engine failed to respond, although no unusual sounds were heard. Prior to touchdown, the nose yawed to the right. An inspector from the Federal Aviation Administration (FAA), reported that he interviewed the flight instructor and private pilot who reported that the initial approach was flown by the private pilot, and as the helicopter neared about 15 feet above the ground, the flight instructor was also on the flight controls. Passing through about 10 feet, the nose of the helicopter rotated to the right, and the flight instructor heard the LOW ROTOR warning horn sound. The flight instructor increased the collective to cushion the landing. However, the helicopter struck the ground hard and rolled over on its left side. The main rotor head sheared from the main rotor shaft, and the main rotor blades were damaged. The flight instructor was asked if the engine was running throughout the descent and ground contact, and he replied that it had been running. The investigation revealed that the helicopter was equipped with both a throttle compensator, and an electronic governor. The mechanical compensator worked with strictly mechanical linkage and as the collective was raised or lowered it would change the throttle to increase or decrease engine RPM accordingly. The electronic governor was designed to keep the engine/main rotor RPM operating near the top of the green arc. The Chief Accident Investigator for Robinson Helicopters reported the low rotor horn would sound at about 97 percent main rotor RPM. He also reported that once the helicopter was in a low rotor condition, an increase in throttle alone would not correct the condition, because as long as the low rotor conditions existed, the engine would not be able to develop full power. In addition, a nose right yaw is consistent with low main rotor RPM. He said that the tail rotor looses thrust about 3 times faster than the main rotor blades. According to the Robinson R-22 Flight Manual, Safety Notice SN-22: "Many R22 accidents have been caused by the pilot reducing his airspeed to near zero during an approach before reducing his rate-of-descent. As the pilot then raises the collective and flares to stop the rate-of-descent, he flares into his own downwash, greatly increasing the power and collective pitch required. The aircraft begins to enter the vortex ring state (settling-with-power) and a hard landing occurs frequently followed by a roll-over. This can occur during a steep approach either power-on or power-off." "It can be avoided by always reducing your rate-of-descent before reducing your airspeed. A good rule to follow is never allow your airspeed to be less than 30 knots until your rate-of-descent is less than 300 feet per minute."