Orders to U.S. factories fell in May by the biggest amount in three months due to weak demand for transportation equipment, while a key category that signals business investment plans declined for a second consecutive month, the government reported Thursday, indicating that manufacturing remained in a weak period. The Commerce Department said new orders to factories fell 1 percent in May following a 0.7 percent decline the previous month. Overall factory orders have dropped in nine of the last 10 months. Much of the May weakness reflected a big 35.3 percent decline in demand for commercial aircraft. But even excluding the volatile transportation category, which declined 6.5 percent, factory orders rose only 0.1 percent.