Cape Canavarel, Fla. ? Aviation adventurer Steve Fossett says he isn’t sure he’ll succeed in his quest to break the 20-year-old record for the longest flight.

First, there’s the combustible danger of taking off with thousands of pounds of fuel attached to a very light aircraft. Then there’s the unpredictability of weather and worries about running out of fuel.

“It will be very close,” Fossett said Monday at a news conference accompanied by Richard Branson, whose company is sponsoring the flight.

Fossett was set to take off from the Kennedy Space Center in Florida early today on the 27,012-mile trip around the world, and then on to London, in a spindly experimental airplane that helped him break a different record last year.

Last March, Fossett became the first person to fly solo nonstop, without refueling, around the globe in 67 hours in the Virgin Atlantic Global Flyer experimental plane. During the trip, he lost 3,100 pounds of fuel from a leak but still landed with a reserve of 1,500 pounds of fuel. He started and ended that record-setting flight from Salina, Kan.

If it hadn’t been for the leak, “the plane would have been able to fly substantially further,” Fossett said. “We have designed this flight to use the full capability of this airplane, to fly further than any plane has ever flown.”

If Fossett completes this 3 1/2-day trip, he will surpass the previous airplane record of 24,987 miles set in 1986 as well as a balloon record of 25,361 miles set in 1999.

Fossett planned to spend Monday making any final changes to the route, which required permission from countries around the world, and evaluating weather conditions. He also will try to sleep since he will only be able to take five-minute catnaps during the 80-hour voyage.

“There will be no parties for me tonight,” he said.