On July 02, 2002, at 1445 Pacific Daylight Time (PDT), a Robinson R22 BETA helicopter, N4072T, was substantially damaged following a loss of control during a precautionary landing near Chatsworth, California. The private pilot and passenger sustained minor injuries. The helicopter was owned by the Dragonfly Corporation of Dover, Delaware, and was being operated by Group 3 Aviation of Van Nuys, California. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 flight for which no flight plan was filed. The local flight originated from the Van Nuys Airport at 1420 PDT. The pilot reported that the helicopter developed an abnormal vibration while en-route to a local practice area. The pilot added that he became concerned about the unusual vibration and elected to land the helicopter as soon as possible to investigate the source of the vibration. The pilot executed a power-on approach to a terraced paved area located to the right of his intended course. While on the approach on a northerly heading, as the pilot was flaring the helicopter for a landing, control was lost after the main rotor blades came in contact with the terraced terrain to his right-front. The pilot stated that the engine was performing "normally" and he did not experience a loss of engine power. He added that at the time of the accident, the winds at the accident site were from the south at an estimated 10 to 13 knots. The helicopter came to rest on its left side. The pilot, who was occupying the right seat, exited the helicopter unassisted, and then assisted the injured passenger in exiting the helicopter. There was no fire. There were no reported eyewitnesses to the accident. The pilot summoned for help with his cellular phone. The pilot was issued a private pilot certificate on June 19, 1999. At the time of the accident, the pilot held a single-engine land, single-engine sea, multi-engine land, instrument airplane and helicopter rating. Per the logbook, the pilot had accumulated a total of 2,155.7 flight hours, 452.9 hours of which were in a helicopter. In the last 90 days the pilot had accumulated a total of 24.9 flight hours, 6.1 hours of which were in a helicopter. The pilot's second-class medical certificate issued on January 3, 2002 listed the restriction that he must wear corrective lenses. The Robinson model R22 BETA helicopter (S/N 1747) has a two-bladed main rotor, a two-bladed tail rotor and is powered by a single Lycoming O-320-B2C reciprocating engine (S/N L-14470-39A) rated at 160 horsepower. A review of the maintenance records revealed that the helicopter underwent its most recent 100-hour inspection on May 13, 2002, at a total airframe time of 5,743.2 hours. At the time of the accident, the engine had accumulated 8,699.5 hours since new, 689.1 hours since the last overhaul and 87.2 hours since the last inspection. No evidence of uncorrected maintenance discrepancies was noted in the records. On June 25, 2002, the Robinson Helicopter Company issued an R22 Safety Alert Letter dealing with unusual vibrations and the possibility of a main rotor blade crack. The letter states that "IF MAIN ROTOR VIBRATION INCREASES RAPIDLY OR BECOMES SEVERE DURING FLIGHT, LAND IMMEDIATELY." The pilot recalled receiving this letter three days prior to the accident and indicated this as the reason for his landing attempt. At 1451 PDT, the Van Nuys automated surface observing system (ASOS) at the Van Nuys airport, reported the wind 130 degrees at 7 knots, few clouds at 2,300 feet AGL, 10 miles visibility, temperature 90 degrees Fahrenheit, dew point 41 degrees Fahrenheit and altimeter 29.96 inches of Mercury. The density altitude was calculated by the IIC at 5,685 feet. The helicopter wreckage was located near the ridge top parking lot where the pilot was attempting to land at about 3,000 feet MSL. The aircraft came to rest on its left side on a ledge about 15 feet wide about 10 feet below the parking lot. The wreckage was recovered and moved to the Robinson Helicopter Company facility in Torrance, California, for a detailed examination. The engine had minor impact damage and was rotated using the fanwheel assembly. A thumb compression check revealed compression in all four cylinders and ignition spark was verified at all four cylinders from both magnetos. The main rotor flight controls were separated in four places but no evidence of pre impact failure was found. The tail rotor control system was intact. The driveline to the main and tail rotors was intact. Both the main rotor and tail rotor gearboxes were intact and rotated freely. Both tail rotor blades were broken off about 3 ½ inches from their outboard mount points with severe leading edge damage to both blades. One main rotor blade was bent aft and up from its normal position leaving the trailing edge buckled. The other main rotor blade was bent aft and down from its normal position leaving the trailing edge buckled. Small cracks were observed on both main rotor blades near the root fitting and spar cover area. Due to the observed cracks in the main rotor blade root area, the sealant was removed from both blades and they were subjected to a dye penetrate inspection per Robinson Helicopter Company Service Letter 21A. No indications of cracks in the root fitting material of both blades were found. The wreckage was released to the owner's representative upon completion of the investigation.