Chicago ? Prosperous times for defense contracts helped Boeing Co. wow Wall Street with much better results than anticipated despite another troublesome quarter for its shrinking commercial airplane business.

The aerospace manufacturer reported a 31 percent decline in third-quarter earnings Wednesday, largely because of the heavy costs of shutting down production of the 757 jetliner. But the latest surge of business for its growing defense operations helped it turn an unexpectedly solid $256 million profit, sending its stock up sharply.

Profits from the defense unit, which include the manufacture of military aircraft, weapons and launch systems, jumped an impressive 38 percent on a 12 percent gain in revenues.

That helped offset the latest decline in the commercial airplanes division, where earnings fell 90 percent because of pension contributions and a $184 million pretax charge to scrap the 757.

While its longtime mainstay unit continues to suffer from the post-9-11 slump in aviation, Boeing is still riding relatively high because of the booming business in U.S. defense contracts.

Boeing’s net earnings for the July-through-September period amounted to 32 cents per share, down from $372 million, or 46 cents per share, a year earlier.

That was significantly higher than the 25-cents-per-share consensus estimate of analysts surveyed by Thomson First Call. As a result, Boeing shares rose $2.46, or 6.83 percent, to $38.50 in trading Wednesday.