The Navy will seek a second source to produce engines for its F18 fighter as part of a larger effort to increase competition in military procurement, Navy Secretary John F. Lehman Jr. said yesterday.

Lehman's decision turns the tables on General Electric Co., which holds a monopoly on F18 engine production and would now have to share its know-how with Pratt & Whitney, a division of United Technologies Corp. Earlier this year Pratt & Whitney lost its long-held monopoly on Air Force fighter engines when General Electric won a competition for future Air Force business.

Lehman confirmed his decision to seek competition in F18 engines, which was reported in Defense Week yesterday, in the course of announcing a new contract for Navy training jets. The Navy signed a contract with McDonnell Douglas Corp. to produce the T45 trainers in what could become a $3.2 billion program between now and 1997.

Lehman said seeking competition for the trainer program and turning it from a "cost-plus" contract into a fixed-price contract reduced development costs from $810 million to about $438 million. Lehman said fixed-price contracts hold the company's feet to the fire but also prevent the Navy from continually changing its demands as the plane is built.

"This requires the Navy to discipline itself, to knock off gold-plating," he said. "It requires us to knock off the 'nice-to-haves.' "

The Navy was criticized earlier for not joining with the Air Force to develop a new trainer, used to teach pilots to fly advanced fighters. But the Democratic-controlled House Appropriations Committee said in its report on the defense budget last week that the Navy, in particular, "should be commended for its effort to increase competition."