Orders for durable goods, a major barometer of future economic activity, soared 4.2 percent in December, primarily because of new commercial aircraft orders, the Commerce Department reported yesterday.

Orders for all of 1985 rose 3.8 percent above the 1984 level, the third consecutive annual increase. Durable orders climbed 14.8 percent in 1984 and 16.8 percent in 1983. In 1982, they dropped 10.2 percent.

The strong rise in December was the first increase since August and the best improvement since November 1984, when orders for the "big ticket" items gained 8.2 percent. December orders overall were $108.164 billion, compared with $103.796 billion in November.

Without the orders for transportation, up 24.8 from November and totaling $30.2 billion, the month would have shown a 2 percent decline overall. The surprising boost in transportation orders followed an 8.7 percent decline in November and a 5.6 percent drop in October.

Government military orders, often a key factor in the orders for the most expensive manufactured products, were down 4.8 percent in December. If the defense capital goods were not included, the month would have shown a 4.8 percent rise, the Commerce Department said.

New orders for capital goods, which indicate industries are adding to their plants and equipment, rose 16.1 percent in December after falling 2 percent the month before. Nondefense capital-goods orders gained 21.6 percent, compared with a 4.8 percent decline in November.

New orders for primary metals were down 3.8 percent, slipping for the third month in a row, and machinery orders dropped 2.4 percent after a 5.2 percent rise in November.

Orders for household durable goods fell 3 percent in December, following a 0.7 percent decline in November.

U.S. manufacturing has been hurt by a flood of foreign imports that have forced numerous plant shutdowns and layoffs as domestic manufacturers struggled with stiff foreign competition.

The durable-goods category is watched for possible signals about business plans to expand and modernize production facilities. However, because the December jump was concentrated in orders for airplanes, which don't translate into increased factory production, analysts were likely to dismiss the gain.