As the two small jets approached Alamogordo, N.M., on the morning of April 12, they looked like Mig 21s.

Painted a Soviet camouflage design, with red numerals on the noses, the jets even had small red stars on each fuselage.

While the planes were chased by U.S. Air Force F15s, it became clear that the pilots were well schooled in Soviet flying tactics and battle psychology.

So when the two jets vanished from radar screens somewhere over the White Sands Missile Range, their disappearance touched off a ground and air search which even involved a U2 spy plane.

No trace of the two planes or their pilots has been found.

This disturbs the Air Force a great deal because the planes were flown by American pilots attached to a 3-year-old "aggressor" combat-training squadron.

Only two days earlier, according to a spokesman at Nellis Air Force Base near Las Vegas, another "aggressor" pilot safely ejected from his mock-Mig before it crashed.

Two squadrons of 53 "agressor" pilots are based at Nellis to give F15 pilots experience in spotting and fighting Mig-sized jets flown by pilots schooled in Soviet flying tactics and philosophy.

Largely a product of the Vietnam war, four "aggressor" squadrons have been formed by the Air Force. In Vietnam, it was found that once combat pilots safely finished 10 missions they were most likely to complete their tours of duty alive.

The Northrop F5Es the "agressors" use are close in size to the Mig21, a common Soviet fighter plane. The planes are painted in one of five Soviet designs.

The pilots wear jumpsuits with a patch bearing a red star-with cross hairs over it-and "agressors" across the top.

According to the Air Force neither the missing planes nor pilots had any particularly sensitive equipment or information.

The New Mexico incident came after a routine training mission involving four "aggressors" and two F15s. The missing pilots were returning to Holloman Air Force Base near Alamogordo, and their disappearance occurred when they were changing radio frequences to contact air traffic controllers for their approach.

Last radio contact came at about 10:45 a.m. while Capt. Michael G. Humphries, 29, of Dallas, and Capt. Thomas S. Pollock, 378 of Caldwell Canyon, Idaho, were flying at 13,000 feet in heavy winds and snow. Not far from their last location are mountains that rise above 12,003 feet.

Humphries acknowledged an air traffic controller's instructions to switch radio frequencies and contact approach controllers on a new one.

He never did.

Humphries and Pollock vanished from radar screens over the White Sands Missile Range west of Oscura. Since navigation is the responsibility of the lead pilot in a formation, a Holloman spokesman said that "anything that happens to the lead plane, there's a good chance it will happen to the wing" pilot.

Their parachutes contained emergency locator beacons, which were never detected, and both men are listed as "missing."

The search has been complicated by the vast area and its terrain. Based on their last fuel reports, the pilots could have flown anywhere within a 32,000 square-mile area stretching across the Mexico border. The terrain is a hodge-podge of white sand and black lava beds, flat farmland and mountains, foresta and deserts.

The main search area, however, is an 1,800-square-mile area around Carizozo.

Air search operations were suspended after 12 days, but a 20-man Air Force ground team has been camping in the mountains since Sunday. They are due to break this weekend and return Monday to make ever-larger circles in their search for the planes.

Air Force spokesmen declined to speculate on what may have happened to the pilots but expressed certainty that they did not disappear voluntarily. CAPTION: Picture, The missing planes are similar to these Northrop F5Es outfitted like Soviet Mig21s in an "aggressor" squadron at Nellis base in Nevada for pilot combat training. AP