Space is always at a premium on flights with so many V.I.P. guests.

During the flight to Asia for tsunami relief, Mr. Clinton let the elder Mr. Bush have the only bed on the government plane, while he stretched out on the floor, and the two discovered that they liked each other.

“I thought I knew him,” Mr. Bush later wrote, “but until this trip, I did not really know him.”

On the flight to the 1995 funeral of Prime Minister Yitzhak Rabin of Israel, Mr. Clinton relegated Newt Gingrich, the speaker of the House, to a seat toward the back of the plane and spent little time talking with him. Mr. Gingrich’s subsequent public complaint that he was forced to deplane from the back earned him ridicule, including a New York Daily News front-page cartoon depicting him as a wailing infant under the headline “Cry Baby.”

On Monday, the younger Mr. Bush twice strayed to the back of the modified Boeing 747, where about a dozen reporters sit, to chat — off the record — for about 90 minutes. Mrs. Clinton also visited with reporters on the plane just after it stopped for refueling in Dakar, Senegal.

Mr. Obama has said repeatedly that his earliest political activism was on behalf of Mr. Mandela’s cause. A young Barack Obama offered a few words at an anti-apartheid rally in the early 1980s. Mr. Obama has written that he later drew inspiration from Mr. Mandela’s single term as president.

But Mr. Mandela was largely gone from the public stage by the time Mr. Obama entered the Oval Office. Mr. Mandela’s age and failing health prevented all but a fleeting meeting between the two men.

Mr. Mandela was also out of office by the time Mr. Bush became president. In 2002, the American president awarded Mr. Mandela the Presidential Medal of Freedom, though the former South African leader was unable to attend the ceremony. Mr. Bush hosted Mr. Mandela at the White House in 2005 and later visited him in South Africa. The two men discussed how to stem the AIDS crisis in Africa, a major interest of Mr. Bush’s.

In his statement upon Mr. Mandela’s death, Mr. Bush called him “one of the great forces for freedom and equality of our time” and said, “This good man will be missed.”