Abstract: Two employees were installing an air line for an acoustic air horn system for a boiler on the fifth-floor boiler level of a fossil-fuel electric power generation plant. They were installing piping for the air line near the east side doors to the boiler, which were located just below the fifth floor. They reached the doors by lifting a section of the floor grating and descending to the top of the boiler ventilation duct 1.5 meters below. The workers lifted a 560- by 890-millimeter section of the floor grating so that one of them could lower himself onto the top of the ventilation duct to take measurements. When that worker returned to the fifth-floor level, he assisted his coworker in tightening the overhead portion of a pipe. The coworker was standing on a platform ladder at the south end of the boiler, approximately 1.5 to 1.8 meters away from the floor opening, and was looking upward as he tightened the south end of the pipe. The first employee was on a separate ladder, tightening the north end of the same pipe. His ladder was positioned less than 305 millimeters from the floor opening. After both workers tightened and leveled the pipe, the coworker looked down to get his wrenches. Hearing a noise, he saw the first employee falling through the opening in the floor grating. The falling worker hit the ventilation ductwork, bounced off it, and fell approximately 4.6 meters further to the fourth-floor grating. The injured worker lost consciousness and sustained two lacerations to the back of his head, a concussion, and a broken scapula. He was hospitalized for his injuries. Neither worker could recall what work surface the injured worker was standing on immediately before falling (for example, on the ladder, on the floor grating, or on a pipe). The injured worker's concussion caused some memory loss.