On August 4, 2001, at 1500 eastern daylight time, a Blanik L-13 glider, N9297Z, collided with a tree during an initial takeoff run at the Marlette Township Airport, Marlette, Michigan. The certified flight instructor (CFI) received a serious injury and the student pilot was not injured. The glider received minor damage. The 14 CFR Part 91 instructional flight was operating in visual meteorological conditions without a flight plan. The local flight was originating when the accident occurred. The CFI reported, "The slack out - go - signal was given from the operations trailer and take-off power was applied to the winch, whereupon the left hook of the winch harness released, uncommanded, from the center of gravity attachment. The uneven force caused the glider to yaw to the left and head for the ditch on the west side of the runway. I applied right aileron and full right rudder, but was unable to keep the glider from floating into the ditch." The CFI reported the glider ended up bridging the ditch on a heading of east. He reported that the ditch was full of broken and cut down brush and saplings. During the impact, a broken branch penetrated the left side of the cockpit and impaled itself in the CFI's right calf. The CFI remained hospitalized for five days following the accident. Members of the glider club reported that they had hooked both the left and right side cables prior to the takeoff. Inspection of the tow winch by an inspector from the Federal Aviation Administration Detroit Flight Standards District Office failed to reveal any mechanical failure/malfunction.