Errant German flier no longer a Soviet jailbird. Mathias Rust's release could enhance warming ties between Moscow and Bonn

Mathias Rust will go down as one of West Germany's most unusual heros. The young hobby flier - who sparked an international crisis in May 1987 by piloting his single-engine Cessna to a near-perfect landing in Moscow's Red Square - was pardoned yesterday by Soviet officials. He had been serving a four-year sentence.

The announcement underscores the rapidly warming relations between Bonn and Moscow.

West German Foreign Minister Hans-Dietrich Genscher greeted the news, saying it shows that the Soviets are willing to deal with humanitarian questions.

Bonn has long pressed for lenient treatment for Mr. Rust, who was 19 years old when he made the 420-mile flight from Finland to the heart of the Soviet capital.

The flight took Rust across one of the most heavily fortified borders in the world - stunning Soviet military planners and throwing a wrinkle into West German relations with the East.

Sources in Bonn say that Mr. Genscher discussed the Rust case - among other things - during meetings with Soviet leaders over the weekend in Moscow.

During these meetings, the Soviets indicated that Rust would be released, but they didn't say when.

``It looks as though we've found a very favorable outcome for a difficult situation,'' says one West German government source.

The Rust case was one of several nettlesome issues which Bonn wanted to clear away in preparation for this fall's visit of West German Chancellor Helmut Kohl to the Soviet Union.

The freeing of Rust is certain to be held up as an example of effective West German diplomacy. Indeed, one West German analyst likens the release of Rust to the Soviets tossing the West Germans a ``little bouquet - something small, but still sweet-smelling.''

Rust, who says he made his remarkable flight in the interest of world peace, has become a bittersweet figure for many West Germans.

The West German press often refers to him as a ``dumb boy'' who got carried away by his enthusiasm for a cause.

Many say he embodies youthful German idealism - the notion that if only East and West could have more contact with one another, there would be greater understanding.

Rust's stunt created a sensation throughout the West.

In the United States, his flight was given more favorable coverage than in West Germany. As a result, Rust's trial was monitored closely, and the Soviets used the opportunity to show that their legal system functions fairly. The Soviets, meanwhile, were never quite sure what to make of the affair.

Initial charges that Rust was aided in his flight by Western intelligence forces were eventually dropped. In the end, the young flier was charged with violating Soviet airspace, entering the Soviet Union illegally, and malicious hooliganism.

He was sentenced to four years in prison last September. It could have been much worse.

The maximum penalty for violating Soviet air space alone is 10 years.

However, the biggest impact from Rust's flight may have been felt inside Soviet military circles.

The undetected intrusion of a light plane through the much-touted Soviet air-defense network raised troubling security questions.

Just a day before Rust's flight, Soviet Major General N.V. Britvin said the Soviet people ``could rest easy with regard to the sacred borders of their country.''

Two of the Soviet Union's top military leaders were booted out in the aftermath of the incident, and other military restructuring is thought to have taken place.

Meanwhile, the odyssy for Rust is not quite over.

West German officials are now discussing what punishment, if any, might await him when he gets back to his homeland.

At the least, it's possible that he will have his pilot's license permanently revoked.