New orders for durable goods dropped 2.5 percent in March, the Commerce Department reported yesterday, but the new figures did not erase concerns that American industry is in the midst of an inflationary boom.

Three-fourths of the fall came in the highly volatile aircraft and parts sector, which declined 19.8 percent from February. Even the second consecutive monthly dip in orders for primary metals, welcome though it was to Carter administration economists, may have been due in large part to steel companies closing their order books for deliveries through June.

Nevertheless, one administration economist termed the report "comforting." He said the government has been "worried that business was beginning to build inventories too rapidly. This reduces the likelihood that that is taking place."

The 2.5 percent decline, to $80 billion, was the largest drop since a near-record 5.3 percent decrease last July. That previous decline seemed to set the stage for a major advance. New orders for primary metals, for instance, rose at a 90 percent annual rate between August and January.

And even with the March drop, new orders for the first quarter as a whole were up at a 19 percent annual rate compared to the fourt-quarter level, according to administration calculations.

The closely watched non-defense capital goods sector dipped $1 billion, or 4.3 percent, to $22.3 billion. Except for communication and railroad equipment, all industries reported declines. Again, however, aircraft and parts was responsible for much of the decline.

Shipments of durable goods increased $2.5 bilion, or 3.3 percent, to $76.7 billion in March. But because $80 billion of new orders were booked, the unfilled-order backlog rose by 1.3 percent to $249.1 billion. That the smallest such increase since last July.

All in all, the report on orders was taken by the administration as a "welcome indication that the overheating is not as serious as we feared," as that administration economist put it. On the other hand, there was nothing in the report to suggest that the economy is about to slip into a recession, he said.

The Commerce report noted, "The March primary-metals order decline, and a February drop of $1.1 billion for that industry, followed heavy ordering in January, when a $2.4 billion, or 20.2 percent, increase was recorded." CAPTION: Chart, no caption, The Washington Post