The night before the accident, the work packs were prepared for the midnight shift to replace the left and right horizontal stabilizer leading edges. Toward the end of the evening shift, a supervisor assigned two of his mechanics to begin removing the leading edge, reasonably believing that an early start would help lessen the workload of the next shift. The two mechanics began removing screws from the top and bottom of the right leading edge. They were soon joined by the quality assurance inspector, who climbed on top of the horizontal stabilizer and removed the top right-hand leading edge screws and the left-hand leading edge screws. The supervisor was unaware that work had started on the left side.