Another foreign youth flying a single-engine Cessna swooped out of the Western sky into Soviet airspace Friday, but this one was met with children and flowers instead of police and a jail cell.

Tony Aliengena, an 11-year-old from California, arrived in Leningrad Friday morning. His flight was approved well in advance, unlike that of Mathias Rust, who spent 14 months in a Moscow prison after evading Soviet air defenses and landing in Red Square two years ago.Tony, from San Juan Capistrano, sent a letter to President Mikhail S. Gorbachev in May 1988 asking permission to fly across the Soviet Union, otherwise closed to foreigners, on a "friendship flight."

Thirteen months and $150,000 in expenses later, he landed in Leningrad with an entourage including his parents and sister, a documentary film crew, a 10-year-old Russian pen pal, a publicity agent, Soviet and American journalists and a world-record observer to verify his attempt to become the youngest person to pilot a plane around the globe.

After touching down in Leningrad under clear skies, the fourth-grader looked nervous but sounded nonchalant.

"What do you have to say to Soviet children?" he was asked by a crowd of reporters upon exiting his red, white and blue Cessna 210 Centurion turboprop.

"I say want to say `hi,' " the scrubbed, crew-cut youth said with a shrug.

"Do you want to be an astronaut?" asked another reporter.

"No, I want to be a brain surgeon," he replied.

Tony's Soviet host, Gennady Alferenko, director of the Foundation for Social Invention in Moscow, was not so relaxed.

Alferenko said Soviet Defense Ministry officials were at first horrified by Tony's flight plan because of the memory of Rust, a 19-year-old West German whose aerial escapade in a single-engine Cessna led to the retirement of Defense Minister Sergei L. Sokolov and the firing of the chief of Soviet air defenses. "You mean it's the same kind of plane?" Alferenko quoted the ministry officials as saying.

"Tony will show all the Soviet Union what it means - the freedom to fly," Alferenko said. "Now a Siberian boy will think, `Why do I have to stay in this village in Siberia? I live in the whole world, and I want to see it, to fly by balloon or cross the Bering Strait by kayak.' "

Alferenko's foundation is financing the Soviet portion of Tony's trip.

It is also untangling red tape so Tony can make 14 stops in the Soviet Union, mostly in isolated Siberian towns. The Soviet route will enable him to cross the Pacific Ocean on a short hop across the Bering Strait to Alaska.

The price for the Soviet portion of Tony's flight is $70,000, including $10,000 for landing rights alone.

Tony will fly to Moscow on Monday where he hopes to present a friendship scroll signed by 250,000 American children to Gorbachev.