WASHINGTON (AP) — Orders to U.S. factories fell in August by the largest amount in eight months, led by a drop in demand for commercial airplanes and weakness in a key category that tracks business investment spending.

The Commerce Department says factory orders declined 1.7 percent in August after a slight gain of 0.2 percent in July. It was the biggest setback since orders dropped 3.7 percent in December.

Demand in a key category that serves as a proxy for business investment slipped 0.8 percent in August, following solid gains of 1.9 percent in July and 1.5 percent in June.

Manufacturing has been under stress this year as a strong dollar has hurt export sales. The big fall in energy prices has also resulted in cutbacks in investment by energy companies.