Orders for U.S. durable goods fell sharply in December, pushed lower by a significant drop in demand for commercial aircraft, while a closely-watched measure of business investment plans fell unexpectedly, the government reported Tuesday. The Commerce Department said orders for durable goods - expensive manufactured items expected to last at least three years - fell 3.4 percent last month to $230.5 billion. Orders fell 2.1 percent in November. The December drop was led by a 55.5 percent plunge in the volatile category of commercial aircraft. There also was weakness in other areas, with demand for machinery, computers, and primary metals falling. Non-defense capital goods orders excluding aircraft - a measure of business spending plans - fell 0.6 percent in December following a similar decline in November and a 1.8 percent drop in October. Economists expected core capital goods orders to increase 0.5 percent, and the consecutive declines may signal that slowing global growth and falling crude-oil prices were beginning to have an impact on the U.S. economy. The recent weakness in manufacturing contrasts with earlier months when U.S. factories were enjoying large gains in orders and production. Analysts believe that the recent retreat in orders will be reversed later this year, but they are closely monitoring the impact of the stronger U.S. dollar on exports.