Orders to U.S. factories for big-ticket manufactured goods edged up slightly in January, but the strength came from a big increase in the volatile aircraft category.

In encouraging news for future growth, a key category that tracks business investment plans posted its biggest gain in six months.

The Commerce Department said Wednesday that orders for durable goods rose 0.4 percent in January, led by a 15.9 percent rise in orders for commercial aircraft. The category that serves as a proxy for business investment rose 0.8 percent after two months of declines. It was the biggest gain since a 1.5 percent July bump.

The weakness in business investment has puzzled economists who expected to see strength in this area as companies boosted investment spending to take advantage of new tax breaks.