A Hungarian American businessman who made a fortune in airplane leasing is planning to give the National Air and Space Museum $60 million -- the largest individual donation in the Smithsonian's 153-year history.

Steven F. Udvar-Hazy, 53, the president of the world's largest aircraft leasing company, International Lease Financial Corp., is giving the money to the new Air & Space annex at Dulles International Airport. When completed in 2003, the annex will be about four times the size of the Mall museum and commodious enough to hold the Space Shuttle Enterprise, an SR-71 Blackbird reconnaissance plane, an F-4 Phantom fighter and a Pershing II missile.

Udvar-Hazy, a certified jet pilot, was traveling in Asia yesterday and was unreachable, according to an administrator at the Beverly Hills, Calif., headquarters of his company.

I. Michael Heyman, the outgoing secretary of the Smithsonian, applauded Udvar-Hazy's generosity. "This wonderful gift is a great beginning for the campaign to build the new Dulles Center. It is yet another indication of the support of the American people for the Smithsonian Institution."

Nearly 10 million people a year visit Air & Space, making it the world's busiest museum. The $60 million gift is part of a $130 million capital campaign to pay for the Dulles annex, which will house the biggest and most recent spacecraft and airplanes in the museum's collection. Udvar-Hazy's unprecedented boost for Air & Space comes less than three months after the museum's 75-year-old director, Donald D. Engen, was killed when a glider he was flying plunged into the Nevada desert.

According to Forbes magazine, Udvar-Hazy is the 120th richest person in America. He was born in Budapest in 1946. His family fled to the United States in 1958 after the Soviet invasion of Hungary. Airplanes have been his passion since he was a boy. He once told a reporter, "While my friends were playing baseball, I used to go out to Kennedy Airport and make notes of who was flying what."

In 1968, he earned a bachelor's degree in economics from the University of California at Los Angeles and started his first company, Airlines Systems Research Consultants. In 1973, Udvar-Hazy co-founded the leasing company. By 1983, according to published reports, it was worth $220 million. In 1990 the company became a wholly owned subsidiary of American International Group (AIG), the leading U.S.-based international insurance organization. The leasing company now owns 400 aircraft, valued at more than $17 billion, according to industry reports.

International Lease Finance buys airplanes and then leases them to airlines. Udvar-Hazy is credited by some aviation business sources with keeping airline manufacturers afloat when the airlines were in an economic slump in the late '80s and early '90s. In June, Udvar-Hazy announced at the Paris Air Show that he intended to place an order for up to 100 of Boeing's newest 737 passenger jets. The list price of the 737-800 is roughly $35 million.

"Steve is one of the most creative guys and commercially astute people in the aviation business," said Clark Onstad, a longtime aviation executive.

The Smithsonian is the largest museum and research complex in the world and its 16 museums and the National Zoo attracted more than 28 million visits last year. In recent years, as public money has tightened, the Smithsonian has been aggressively seeking new sources of private money for multimillion-dollar restoration projects and new initiatives.

The Smithsonian receives 72 percent of its funding from Congress. The federal appropriation in fiscal 1999 amounted to $412 million. The remainder of the $569.7 million budget for this year comes from private trust funds.

The museum's endowment, which provides the largest portion of the private money in most years, includes the original bequest of British scientist James Smithson. In addition to corporate and individual donations, the museum generates revenues from its retail operations, Smithsonian magazine and other businesses.

The Smithsonian's largest cash donation to date was from Kenneth E. Behring, a California entrepreneur, who gave $20 million two years ago to the National Museum of Natural History for renovation projects and education programs. The Smithsonian also received a windfall in Chinese art last month when a $50 million to $60 million collection assembled by Paul Singer, a Summit, N.J., psychologist, was donated to the Arthur M. Sackler Gallery. Other recent gifts have included $10 million from designer Ralph Lauren's company to restore the Star-Spangled Banner at the National Museum of American History and a $10 million pledge from the Mashantucket Pequot Nation for the National Museum of the American Indian.