Republic Airlines Inc., a major carrier based in Minneapolis, said yesterday that it had operating profit of $5 million in the first quarter, but large interest expenses left it with a net loss of $15.2 million.

Meanwhile, New York Air, which is one of the nation's newest airlines, reported an operating loss of $1.9 million and a net loss of $1.6 million. The net loss was smaller because the company gained about $300,000 in net interest income.

Republic was one of the few airlines to show a profit from operations in the first quarter, a three-month period in which passenger traffic was generally low and fuel costs high. Its net loss resulted mainly from interest expenses of $20.3 million.

In the first quarter of last year, Republic had a net loss of $13.8 million. Republic's revenues in the latest quarter were $351.1 million, compared with $169.8 million a year ago.

Boeing Co. has reported a rather flat first quarter as its earnings rose only 3 percent from the comparable period last year, reflecting airlines' reticence to order certain aircraft in the face of rising fuel costs and competition.

The Seattle-based company said Monday that it earned $144.1 million ($1.49 a share) compared with $139.8 million ($1.45) in the first quarter of 1980.

Sales rose to $2.4 billion from $2.1 billion.

Kennecott Corp.'s first-quarter earnings fell 44 percent from the comparable quarter last year, and its profit was mainly due to the transfer of property.

Kennecott, which is being sought by Standard Oil Co. of Ohio, said Monday that it earned $34.6 million ($1.17 a share) in the most recent period compared with $61.5 million ($1.42) in the 1980 first quarter.

Sales fell to $547.9 billion from $645.2 million.