Abstract: On December 4, 1997, a crew was stringing guylines on the Thunderbird TMY50 yarder. One guyline was anchored and tightened. The second guyline was being strung with a dozer, with the guyline drum in free spool. Evidence indicates that the drum locked while the line was being pulled off, causing the yarder to tip backward. The backward tipped yarder slackened the anchored guyline. Employee #1 was watching the drum for backlash and ran when the yarder tipped. A coworker, who was a cat driver, saw the yarder tipping and released the winch line, allowing the tower to rock back to vertical. This caused the first guyline to tighten. Employee #1 was caught under the left arm by the guyline and was flung approximately 25 ft vertically. He landed on his back in the road and sustained a fractured pelvis. Employee #1 was hospitalized.