Abstract: Employee #1 was in a model #1045A Skyworker aerial lift bucket about 30 feet above the ground, trimming trees. The boom apparatus was extended; the lower boom was positioned straight up and down; and the upper boom was extended almost completely out the left side of the truck. When the boom fell its midpoint landed on telephone cables that were approximately 10 feet above the ground. During the boom's rapid descent, Employee #1. who was not tied off in the bucket, was ejected and killed. An investigation revealed that the threaded lower end of the piston for the lift's lower boom drive cylinder had broken cleanly. The boom was supported for a period of time, but as the cylinder went up and down, the piston rotated, causing it to become almost totally unthreaded (about 1/4 inch of thread remained of a total of approximately 2 inches).