Abstract: Electric power generation plant employees from an idled area of the plant were assigned temporary duty to assist regular maintenance personnel in performing equipment checks and light repairs of various machines. The employees were to inspect some air poppet damper control units inside an induced-air duct. Three of the workers had unbolted a 457- by 610-millimeter access hatch on the north side of a large air handler assembly and entered the structure to inspect the damper-actuating equipment located to the south. Two of the three workers, who were experienced, entered the ductwork first, followed by the third worker, who was about 6 meters behind them. Walking in darkness, the third worker walked toward the light from a flashlight carried by one of the first two workers. The third worker stepped into a near-vertical air shaft, 11 meters in depth, to the east and south of his initial location. He was hospitalized with cuts and lacerations from the fall.