On August 4, 2001, about 1915 central daylight time, a Bell 47G-2, N440TG, piloted by a commercial pilot, sustained substantial damage on impact with terrain following an in-flight loss of engine power while maneuvering near Blooming Prairie, Minnesota. The observation flight was operating under 14 CFR part 137. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot sustained no injuries. The local flight departed from a trailer in a field near Blooming Prairie, Minnesota, at time unknown and was maneuvering at the time of the accident. The pilot reported: Part 137 operation. Observation of sweetcorn field. Observation executed at 75' AGL [above ground level] and below 20 mph. Engine loss of power. Autorotation to neighboring soybean field. Damage to aircraft. No Injuries. Aircraft had 20 gallons of fuel and no agricultural treatment products on board. The pilot also reported a passenger was onboard during the flight. After the accident, Federal Aviation Administration (FAA) Inspectors observed the operator test run the accident engine. The engine was observed to "function properly." At 1915, the Owatonna Degner Regional Airport (OWA) weather was: Wind 160 degrees at 9 knots; sky condition clear; temperature 29 degrees C; dew point 23 degrees C; altimeter 29.96 inches of mercury. At 1915, the calculated density altitude at OWA was 3,025 feet. A copy of a Transport Canada Carburetor Icing chart was reviewed. The temperature and dew point were plotted on that chart and their intersection falls in the serious icing - descent power area of the chart. See appended icing chart. A Bell 47G-2 helicopter airspeed vs. altitude limitations chart was reviewed. The observation airspeed and altitude were plotted on that chart and their intersection falls in the avoid continuous operation area of the chart. See appended airspeed vs. altitude limitations chart. The Bell 47G-2 flight manual stated: PASSENGERS PROHIBITED DURING SPECIAL PURPOSE OPERATIONS. Persons other than minimum crew necessary for the purpose involved shall not be carried during special purpose operations in restricted category aircraft. FAA regulation section 91.313 stated: Restricted category civil aircraft: Operating limitations. ... (d) No person may be carried on a restricted category civil aircraft unless that person- (1) Is a flight crewmember; (2) Is a flight crewmember trainee; (3) Performs an essential function in connection with a special purpose operation for which the aircraft is certificated; or (4) Is necessary to accomplish the work activity directly associated with that special purpose.