Paul Thayer, deputy secretary of Defense and former Navy test pilot, took a ride in the second seat of an F18 fighter a few weeks back on a mission to help decide if the plane was worth buying. About $40 billion was riding on his judgment--the potential value of the F18 program to two contractors, McDonnell Douglas Corp. and Northrop Corp.

Thayer put a training version of the plane through its paces, or as the Pentagon report described the Jan. 26 flight, he exercised "all controls and displays for both air-to-ground and air-to-air combat."

But the trouble began when he prepared to land. First the nose wheel wouldn't go down and a backup system had to be used. Then, after he put the plane down on the runway at Patuxent Naval Air Station, he discovered the landing brakes didn't work. But apparently the runway was long enough that the plane was able to coast to a stop.

A Pentagon spokesman said yesterday that Secretary Caspar W. Weinberger is still pondering the plane's future. Thayer reportedly liked it as a fighter, but will need some more convincing on whether it has value as a light bomber. Some insiders are predicting that the secretary will cut back on the planned purchase of the attack version.

Hearing of Thayer's problems, Weinberger joked: "You'd think they would have had a good one for you."