Orders for U.S. durable goods fell unexpectedly in December, with a key category that signals business investment plans dropping by the largest amount in five months, the government reported Tuesday. The US Commerce Department said orders for durable goods—expensive manufactured items expected to last at least three years—fell 4.3 percent last month. The decline was the biggest since July and reversed November's 2.6 percent gain. Economists expected December orders to rise. The drop in orders was led by a 17.5 percent decline in the volatile category of commercial aircraft. Excluding transportation, orders fell 1.6 percent, the biggest drop in nine months, after rising 0.1 percent in November. Non-defense capital goods excluding aircraft—a closely watched indicator of business spending plans—fell 1.3 percent in December after rising 2.6 percent the previous month. Economists expected orders for core capital goods to rise last month. While durable-goods data is volatile from month to month, details of Tuesday's report could support views that factory activity will cool early this year after output grew at its fastest pace in almost two years in the fourth quarter of 2013.