Mathias Rust, the West German aviator who a year ago slipped across one of the world's most secure borders and landed a plane at Red Square, remains jailed at Lefortovo Prison with hopes fading for an early release. The dramatic May 28, 1987, flight shook the foundation of Soviet security and oiled the way for the ouster of Defense Minister Sergei L. Sokolov and Air Defense chief Alexander Koldunov, but Soviet citizens still grin at the audacity of the young pilot and his success in putting one over on the mighty Kremlin. Soviet jets scrambled when Rust crossed into restricted airspace over the republic of Estonia, but the Soviet Air Defense forces took no action to halt the flight. Then, with apparently no warning to security forces in the nation's capital, the single-engine Cessna swept toward the heart of Soviet power and buzzed Kremlin landmarks such as Lenin's tomb and Spassky Tower. Rust, now 20, was sentenced Sept. 4 to four years at a general-regime labor camp on charges of malicious mischief. By mutual consent with the Soviet government, he is serving his time at the infamous Lefortovo Prison in eastern Moscow, which in the past contained American journalist Nicholas Daniloff and U2 pilot Francis Gary Powers. A source close to the case, who spoke on condition of anonymity, said Rust asked to remain at Lefortovo rather than transfer to a less restrictive penal institution because the little-used KGB facility is more easily accessible to his parents and West German diplomats who are allowed to make regular visits. At Lefortovo, Rust has been allowed more visits than he is officially entitled to. His parents, Karl-Heinz and Monika Rust, and his 16-year-old brother Ingo, have visited Lefortovo at least five times since June. Under Soviet corrections regulations, prisoners are allowed one visit every three months. A representative of the West German Embassy is allowed to visit once each month to bring mail, books and special foods to augment the prison diet. Rust's appeal of his four-year sentence was turned down late last year. The young amateur pilot from suburban Hamburg has issued a personal request for clemency that does not require a formal reply. To date, there has been no reply. Soviet officials have commented privately that Rust will likely serve at least half of his term, to show that the Kremlin does not take such stunts lightly and to discourage any similar incidents in the future. Rust was accused during his two-day trial of endangering the lives of thousands of airline passengers when he piloted his borrowed Cessna 172-B from Helsinki, Finland through the flight paths of foreign airliners headed for Moscow's Sheremetyevo Airport. Witnesses testified that he could have injured some of the hundreds of pedestrians at Red Square on the cool, sunny evening when he landed. Rust told the court he felt ``remorse and deep repentance'' and never intended to endanger or insult anyone. Before landing in the square, Rust piloted his blue and white craft at a low altitude through more than 500 miles of closely guarded Soviet territory. The dramatic stunt was followed by a short-lived moment of glory for Rust as tourists and Muscovites crowded around the plane that came to a rest between St. Basil's Cathedral and the red-brick wall surrounding the Kremlin. They asked for the pilot's autograph and took his picture as he reclined against the fuselage before his arrest. Less than 48 hours later, the ruling Politburo met in special session and sent Sokolov into retirement and fired Koldunov for negligence.