On October 31, 2004, at 0922 Pacific standard time, a Bell 47D1, N903B, experienced a loss of engine power and rolled over during an autorotation at South County Airport of Santa Clara County, San Martin, California. The student pilot/owner was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and the student pilot were not injured; the helicopter sustained substantial damage. The cross-country instructional flight departed Reid-Hillview Airport, San Jose, California, about 0910. The flight was destined for San Martin to refuel and ultimately terminate in Hollister, California. Visual meteorological conditions prevailed, and a flight plan had not been filed. The Federal Aviation Administration (FAA) inspector interviewed the CFI after the accident. The CFI stated that while performing a right turn onto the final approach leg of the landing pattern, about 500 feet above ground level (agl), the engine experienced a sudden loss of power. The CFI initiated an autorotation, and descended toward the soft muddy field below. Upon touchdown, the helicopter's right skid dug into the soft dirt and the helicopter tilted to the right. The main rotor blades impacted the ground, sustaining substantial damage. In a written statement, the CFI reported that prior to departure he performed a preflight inspection and verified fuel quantity via the use of a measuring stick and the cockpit fuel gauges. He confirmed about 10 gallons of fuel remained in the 28-gallon tank. In his preflight planning the CFI had estimated that the 22-nautical mile (nm) flight would take an estimated 18 minutes. With a planned ground speed of 65 knots, he calculated a corresponding fuel consumption of slightly less than 5 gallons. While en route, both the CFI and student pilot noted that an unusually high application of carburetor heat was required to keep the inlet temperature needle between the yellow and green line. The CFI further stated that with the helicopter about 1 mile from the destination airport, he noted that the cockpit fuel gauge indicated a little less than 1/4 tank. He further observed that carburetor heat indication remained between the yellow and green reference markings. With the helicopter about 900 feet agl, the student pilot initiated a descent. In addition, he maneuvered the helicopter in a right turn in an effort to enter the base leg of a left traffic pattern for a landing zone on the northwest corner of the airport. With the helicopter configured in the right turn, about 500 feet agl, the engine experienced a momentary power interruption and subsequently stopped. The student pilot executed an autorotation and the helicopter touched down in a crop field. The right skid dug into the soft muddy field, resulting in the helicopter rotating around the skid and coming to rest on its right side. During a telephone conversation with a National Transportation Safety Board investigator, the CFI stated that after egressing the helicopter he noted about 2 gallons of fuel leaked out of the tank, seeping into the terrain. He noted about 3 gallons remained in the tanks after recovery, and therefore, estimated that the tank contained about 5 gallons of fuel just prior to the accident. AIRCRAFT INFORMATION The Bell Helicopter 47D1, serial number 847, was manufactured in 1954, and had accrued a total time in service of 4,827.1 hours. The helicopter was equipped with a saddle-type fuel tank whose shape is similar to an oblong oval. The fuel tank is attached to the fuselage structure by two clamps. The clamps are affixed around the outboard sections of the tank surface and bolted onto the structure behind the mast and cockpit area. The tank is designed for the fuel to egress from the bottom left side, where the fuel flows from the tank and is routed to the gascolator, eventually reaching the carburetor and engine. Inside the tank, the fuel outlet area has two triangular concave divots located on either side. These indents serve as a funnel to the outlet and outward in the same direction as the tank. See the public docket for the pictorial representation of the fuel tank and its installation. According to the FAA inspector, due to the orientation of the fuel outlet, the helicopter is susceptible to unporting with low quantities of fuel present during maneuvers. With the outlet begin located on the left side of the tank, when a helicopter is in a right turn with minimal fuel onboard, the outlet can intake air resulting in fuel starvation to the engine. Bell Helicopter did not have knowledge of any tilt-table test information of unusable fuel for the 47D1 or any other model helicopter with the same installation of fuel tank. During a conversation with a Safety Board investigator, a Bell representative stated that there is no unusable fuel amount mentioned in the helicopter's flight or maintenance manuals. He opined that since this helicopter uses a gravity-feed fuel system, little, if any, unusable fuel exists. He further stated that Bell does not have any official fuel consumption data; however, most Franklin-powered 47 operators use around 12 gallons/hour for planning purposes; the actual consumption depends on the amount of hovering performed. METEOROLOGICAL INFORMATION The closest official weather observation station was in Watsonville, California, which was located about 13 nautical miles (nm) southwest of the accident site. A routine aviation weather report (METAR) for Watsonville was issued at 0953. It stated: skies clear; visibility 10 miles; winds from 200 degrees at 4 knots; temperature 60 degrees Fahrenheit; dew point 54 degrees Fahrenheit; and altimeter 30.20 InHg. According to a standard reference icing probability chart, the atmospheric conditions were in the range considered favorable for carburetor ice formation, with the ambient temperature and dew point conditions being within the region marked "serious icing at cruise power" and adjacent to the area marked as "serious icing at glide power." The FAA Rotorcraft Flying Handbook (FAA-H-8083-21) states that "although carburetor ice can occur during any phase of flight, it is particularly dangerous when you are using reduced power, such as during a descent."