Two Americans released by the Angolan government this week spent part of their captivity in a bleak new prison designed and built by Cubans, according to one of the freed prisoners.

"You'd have to see the prison to believe it," said civilian pilot Geoffrey Tyler yesterday on an airliner enroute to Paris and New York after being released along with two American mercenaries in a prisoner exchange in Lusaka, Zambia.

Tyler, 33, formerly of Seabrook, Md., spent more than three months in the recently constructed Central Prison in Luanda. He was incarcerated in Angola for 21 months after he crash-landed his single-engine plane on a road near the Namibian border in 1981 while ferrying it to a buyer in South Africa.

His one-man cell was about the size of a small bathroom with a cement slab for a bed and a hole in the concrete for a toilet, Tyler said. Water was brought from a cistern in which dead rats often floated.

The windows in the small cells are slanted so that prisoners cannot look outside, he said.

Tyler said there are no recreational facilities or reading materials in the prison.

Tyler said he was told that there are only two other prisons like the new installation -- in Cuba and the Soviet Union.

Tyler and one of the mercenaries, Gustavo Grillo, were in the prison from last Dec. 21, when their scheduled release broke down while they waited at the airport, until April 2, a day after they were visited by Michael Ranneberger, the State Department's desk officer for Angola.

They were then returned to Luanda's main prison, Sao Paulo. graphics /photo: UPI Geoffrey Tyler greeted by niece in Washington yesterday after release from Angola.