WASHINGTON (MarketWatch) — Orders for long-lasting goods sank 13.2% in August after a big drop in bookings for airplanes and autos, the Commerce Department said Thursday.

Even after subtracting the volatile transportation sector, however, orders were weak. Bookings also fell for machinery, computers and primary metals in another sign the U.S. manufacturing sector has softened considerably after a more than two-year hot streak.

Orders for durable goods, or items expected to last at least three years, provide a good idea of how fast the economy is growing. Orders surge when growth accelerates and droop when the economy falters.

Economists surveyed by MarketWatch had expected durable-goods orders to sink a seasonally adjusted 5.3% in August, mainly because of fewer aircraft bookings.

Sales of aircraft in particular are hugely expensive and can swing dramatically from month to month, skewing the headline figures for orders. Orders for military weapons and equipment, which plummeted 28% in August, can have the same effect.

In July, Boeing signed a flurry of new deals at the big Paris Air Show and orders spiked 51%. As a result, orders dried up in August, plunged 102%. And orders for autos tumbled 11%.

Subtracting transportation, orders fell a much smaller 1.6% in August.

On the brighter side, an even more critical number, known as core capital goods, rose 1.1% in August after falling 5.2% in July and 2.7% in June.

Bookings for these goods, which strip out defense and transportation, tend to provide a clearer window into how well the broad manufacturing sector is doing. The increase might suggest that demand is stabilizing.

The weaker level of orders since the end of spring reflects a global economic slowdown — U.S. exports to Europe and China have softened — and fresh worries about the U.S. government going off a so-called “fiscal cliff” in 2013.

Deep spending cuts and big tax increases are slated to take effect Jan. 1 unless a divided Washington acts to change current law. Defense contractors could be hard hit if planned reductions in military spending occur. Read more on what executives say about fiscal cliff.

Shipments of core capital goods, perhaps reflecting the anxiety of business, fell 0.9% in August to mark the second straight decline. That number is used to help calculate quarterly gross domestic product.

“Hiring and investment are suffering greatly from paralyzing uncertainty, as voters face a choice between two drastically different visions for economic policy in less than six weeks, and businesses are willing to wait for the outcome before settling on and executing plans for the future,” said Stephen Stanley, chief economist of Pierpont Securities.

Shipments of all durable goods fell 3.0% in August. Inventories of durable goods rose 0.6% last month.

Orders for July were revised down to a 3.3% increase from an initial report of a 4.1% gain.

Separately, Commerce revised down its estimate of second-quarter U.S. growth.Read story on U.S. GDP.

The Labor Department meanwhile reported a sharp drop in initial jobless claims.Read story on jobless claims.