WASHINGTON — US companies boosted their orders for manufactured goods by the largest amount in 18 months in September, but firms stayed cautious in ordering goods that signal plans to expand and modernize. Factory orders rose 4.8 percent in September compared to August, when orders fell 5.1 percent, the Commerce Department said Friday.

The September gain was the biggest since March 2011 and was driven by a surge in demand for commercial aircraft, a volatile category which had seen orders plunge in August.