The helicopter was being used to give 5-minute for-hire sightseeing rides to airshow patrons. The pilot transmitted over the local control frequency that he had a power failure and was going down. Another airborne helicopter pilot saw the helicopter about 300 feet above ground in a nose-high attitude and yawing to the right as it descended vertically. The witness said the yaw continued during the descent until it was slightly nose low at ground impact. Responding fire department units did not find fuel leaks in the tanks or lines. FAA inspectors found the fuel system intact with no evidence of tank or line rupture. About 1 pint of fuel was drained from each of the fuel tanks during recovery of the helicopter. The operator said a computed tabulation system was used to keep track of the fuel onboard the helicopter. On paper estimated fuel usage was subtracted from the amount believed onboard and fuel added during the day was added to the total. No calibrated dipstick was used to determine the amount of fuel in the tanks. All three main rotor blades were found coned upwards. The tail rotor blades were not damaged. The engine was put in a production test cell and ran normally. Tests of the fuel gage showed it read accurately at zero. As the amount of fuel was increased, a progressively higher error was noted. At 1/4 system capacity, the gage read twice the amount it should.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

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