HISTORY OF FLIGHT: On July 12, 1993, at 1359 Pacific daylight time, a Robinson R22B, N531JD, cruised into energized transmission lines located about 3 miles east of Weimar, California. One witness reported observing and hearing the helicopter seconds prior to the mishap, and no unusual engine sounds were noted. Another witness reported hearing the helicopter collide with the power line(s). The witness reported that the helicopter then rolled upside down and fell. The helicopter was destroyed and the commercial pilot, who possessed a flight instructor certificate, was fatally injured. The second pilot, who possessed a private pilot certificate (which had been issued on July 9, 1993) was also fatally injured. According to a relative of the private pilot, during the accident flight the private pilot had planned to receive dual flight instruction. Visual meteorological conditions prevailed at the time of the mishap. The flight originated from Auburn, California, at an estimated time of 1300. Following the collision, a fire started on a hillside adjacent to the crash site. The fire burned an estimated 50 acres of native vegetation before it was extinguished. WRECKAGE, IMPACT AND POWERLINE INFORMATION: According to the Placer County Sheriff, the helicopter crashed into the north fork of the American River. The helicopter came to rest in shallow water a few yards from the river's east shoreline. The tail boom assembly was found about 15 feet from the main wreckage. About 1/2 dozen electrical burn arc marks were noted on the main rotor blades. The Federal Aviation Administrator's coordinator examined the accident site and helicopter wreckage on-scene. The coordinator verbally reported observing a power line on the helicopter's rotor mast. No evidence was found of pre-impact failures of the engine (Serial Number L-16229-39A), transmission, or rotors. A spokesperson from the Pacific Gas and Electric Company (PG & E) reported that, as a result of the crash, a power outage occurred on the electric company's Middle Fork-Weimar 60kV lines at 1359. Electric power was restored to PG & E's 2,273 customers by 1515. The PG & E spokesperson further reported that the poles from which its 3 transmission lines (conductors) had been suspended were about 2,274 feet apart from each other. The conductors measured 3/8-inch in diameter, and were composed of aluminum wrapped around a solid steel core. The west and east bank poles #19/1 and #19/0 were each about 45 feet tall. The vertical distance between the river and the impacted lines was estimated at 550 (plus or minus 50) feet. The collision occurred over the river (gorge) about midway between the poles. Neither the transmission lines, nor their supporting wood poles had been marked with devices designed to increase their conspicuousness. ADDITIONAL INFORMATION: The Safety Board examined the San Francisco Sectional Aeronautical Chart, 50th edition, dated April 1, 1993. The examination revealed that the accident lines were depicted. The chart was also observed to include the geographic area of the pilot's departure airport. The accident site was located on the chart at approximately 39 degrees 1 minute north latitude, by 120 degrees 55 minutes west longitude. A California Highway Patrol officer, who responded to the crash site in a helicopter, verbally reported to the Safety Board that the accident site wires and their supporting poles were not particularly conspicuous. The poles were located on the sides of the mountains which formed the gorge, and they were partially hidden by native vegetation. The same Middle Fork-Weimar power lines had previously been involved in an accident. In 1987, a military helicopter collided with and severed the lines, and the helicopter crashed into the American River. Thereafter, PG & E reviewed the accident facts and concluded that the mishap had been "...an isolated incident involving a...pilot who was not familiar with the area." PG & E also concluded that "...aircraft normally did not fly at or below the canyon rim." At the time of the 1987 accident, the lines were depicted on the aeronautical chart. PG & E took no action to mark the lines or change their configuration until after the current accident.