On April 3, 1996, about 1225 eastern standard time, a Hiller UH12-E, N14MQ, landed hard during an emergency descent near Huger, South Carolina. The helicopter was operated under the provisions of Title 14 CFR Part 137, and visual flight rules. Visual meteorological conditions prevailed. A flight plan was not filed for the local aerial application flight. The commercial pilot was seriously injured, his observer received minor injuries, and the helicopter was substantially damaged. The flight departed from a work site near Huger, about 1100. The pilot was performing controlled burning, using a long line, in the Francis Marion Forest. He reported that the engine lost power and an autorotative landing was attempted to a clear area. The pilot reported that the engine quit consistent with fuel exhaustion. He also reported that he had flown a total of about 40 minutes, and that the fuel tank contained 30 gallons of 100LL fuel prior to the initial departure. Additionally, he said the helicopter normally used 20 gallons per hour yielding about 1.5 hours of endurance. When the helicopter was moved from the accident site, it was noted that the fuel tank was ruptured, allowing fuel to leak from it. During further examination of the helicopter, the fuel tank vent was found clogged by fine, brownish dirt and insect parts.