Orders placed with U.S. factories fell for the first time in four months, the Commerce Department said today, with demand dropping sharply for commercial airplanes and parts.

Factory-goods orders decreased by 0.1%, following an upwardly revised 1.7% advance in July. However, most of the weakness in the headline number was attributable to a stark drop in demand for aircraft, a segment of the data that economists say is susceptible to wide month-to-month swings. Excluding the volatile transportation sector, factory orders rose 1.3%.

Demand for transportation-related goods decreased by 7.4% after rising 6.5% in July. Non-defense aircraft and parts orders plunged 42.9%, while defense aircraft and parts orders fell 3.0%. Orders for motor vehicle bodies and parts advanced by 1.4%.

Consumer-goods orders increased by 1.4%, after a 0.1% dip in July, with demand for consumer durables, or products meant to last three years or more, up 6.2%. Orders for computers and electronic components advanced by 4.6%. Demand for metals and machinery also moved higher, as did orders for electrical equipment and appliances.

A report Friday by the Institute for Supply Management showed economic activity in the manufacturing sector grew in September for a 16th straight month. The ISM’s index of manufacturing activity moved to 58.5, versus 59.0 in August and 62.0 in July. Readings above 50 point to expansion in activity.

Elsewhere, non-defense capital goods orders excluding aircraft declined by 0.8%. Capital-goods orders tumbled 6.4%. Defense capital-goods orders rose by 4.6% while orders for business equipment meant to last more than 10 years slid 7.7%. Shipments rose by 1.1% and unfilled orders gained 0.3%.