A well-dressed, self-possessed man in his early 20s hijacked an Air Florida jet out of Miami to Cuba yesterday morning, in the sixth such incident since May 1. The hijacking came just a day after U.S. officials announced that Cuba is sentencing hijackers to jail terms of up to 40 years.

"Maybe this man was in a hole all night and never saw the television or read the newspaper," said Jack Barker, a spokesman for the Federal Aviation Administration in Atlanta. Barker was in Miami Wednesday to release information just provided by the Cuban government that hijackers have been receiving lengthy prison terms.

"There is just amazement that you see an aircraft hijacked this morning, in light of the publicity that was so widespread in Florida," Barker said. "It was bannered in all the newspapers, was the lead story on the television news and was on radio all day long." Yesterday's was the second hijacking in six days.

Four of the six most recent hijackings have been by apparently homesick Cuban refugees trying to return to Cuba. The man who commandeered Air Florida Flight 8, however, was identified by FBI spokesman Dennis Erich as Robert Patrick Richter and was described as a "Caucasian, not a Hispanic, who requested to be met by English-speaking policemen" in Cuba.

The Cuban government on June 15 sent the United States a diplomatic note that detailed the fates of those who have hijacked planes to Cuba since September, 1980.

The news conference in Miami Wednesday was designed to use that information to make it clear that hijacking is not the way to get to Cuba.

"A person that wants to go home, that wants to be with their family again, well, the only way they'll ever see their family is on visiting day," Barker said.

Yesterday's hijacking began about 10 minutes after takeoff on an Air Florida Boeing 737 bound for Tampa with 47 passengers aboard. The hijacker handed to a flight attendant a note that read: "This is a hijacking. I have an explosive device. Take the plane to Habana sic . Cuba now! Liberty or death. Power to the revolution."

According to Erich of the FBI, the hijacker carried a gym bag containing a cylinder with wires running to a push-button device. "He was very silent the whole time," Erich said. "He just sat there. He asked that the two rows in front of him and behind him be cleared, and they were."

After the plane landed in Havana at 8:41 a.m. yesterday, Richter sat quietly in a seat until soldiers bearing machine guns came aboard, handcuffed him and led him away. He kissed the asphalt as he stepped off the plane.

The FBI in Miami released only a vague description of Richter, saying they were still investigating possible motives. Erich did say that Richter was not from the south Florida area.

One passenger on the flight was experiencing his second hijacking. Mayer Finkle, 67, of North Miami, told United Press International that he was on a hijacked plane 13 years ago, and he said the first time was more frightening.

Finkle said yesterday's hijacking went "very smoothly and the crew handled the situation so well we didn't even have time to get nervous." He described Richter as "extremely good-looking."

"He was a typical, all-American guy," passenger Joanne Curry of Boca Raton, Fla., told the Associated Press. "I was surprised that somebody who looked so neat and successful would be the hijacker."

Michelle Goehring, of Hollywood, Fla., told UPI: "It was a tense when the soldiers came onto the plane with machine guns. But the crew did a great job, and we had Bloody Marys and went shopping in Havana."

No one was injured in the incident. The airplane was on the ground in Cuba about two hours, returning to Miami at 10:46 a.m.

In all six recent hijackings, the hijackers have claimed to have explosive devices. That seems to have been true only on last Saturday's hijacking, when two Cubans waved a flickering cigarette lighter near a bottle of gasoline. It could not be determined immediately whether yesterday's hijacker had a real bomb.

In late May, the FAA resumed placing air marshals on selected flights out of Miami in an effort to curb hijackings. Air marshals rode planes during the spurt of hijackings in the fall of 1980, when there were 10 hijackings to Cuba in a month, including three in one day.

Marshals have been on only one plane that was hijacked, according to Barker, and in that instance elected not to identify themselves so as not to endanger the passengers.

The hijackings in 1980 stopped abruptly after Cuba returned two hijackers to the United States for prosecution. Each was sentenced to 40 years in prison. There were no hijackings between September, 1980, and July, 1981.

Barker said that "there is no recommended procedure for pilots" in a hijacking. "As a pilot, he is in command of the aircraft, and it's the pilot's discretion as to what to do," he said. "The only thing that's recommended is that the safety of the aircraft is paramount."