Employees were preparing to unload a wind turbine gearbox from a flatbed trailer. An employee went to reposition himself on the flatbed trailer and grabbed onto a wooden piece of the shipping cribbing for support. The piece of wood began to come loose, causing the employee to lose his balance. The employee jumped to the ground about 4 feet below and landed on his feet. His left ankle rolled and the employee was hospitalized with a fractured ankle that required surgery.