In its time, half a century ago, the flight across the Pacific aboard a Pan American Airways "Clipper" flying boat was as audacious and extravagant as space flight today.

The Pacific route that symbolized Pan American's pioneering role in transocean transportation began in San Francisco and island-hopped to Manila in seven days and 60 hours flying time.

That era will soon draw to a close for Pan Am, which announced yesterday that it plans to sell its Pacific routes to United Airlines for $750 million.

Before the Pacific routes could be opened half a century ago, an oceanic list of technological, logistical, financial and political problems had to be solved. The "Clipper" flights, which began in 1935 for mail service and in 1936 for passengers, were a tribute to the pioneering genius of Juan Trippe, Pan American's founder and chief executive for its first 40 years. In the early 1930s, as the inspiration for trans-Pacific travel took shape in Trippe's mind, there was no commercial airliner capable of spanning the 2,402 miles of the first leg, between San Francisco and Hawaii. The land-based airliners of the day could only cover one-quarter of that distance.

The route lacked airfields and installations. Weather information was scanty. Important developments in fuel and aircraft technology were required. There was no radar, no inertial guidance systems.

"It involved a tremendous amount of planning, a fair amount of courage and a determination to succeed, all of which were possessed by Pan American and its leaders," said R. E. G. Davies, curator of air transportation at the Smithsonian Institution and an author of three books on the subject.

Trippe hired Charles A. Lindbergh, the hero of the 1927 nonstop trans-Atlantic flight, as technical adviser to scout a route across the Pacific and to chart weather information.

He challenged the aircraft industry to produce the planes for the flights, and three companies responded -- Sikorsky, Martin and Boeing.

The Martin 130, built at Middle River, Md., near Baltimore, was the flying boat that inaugurated the Pacific route in 1935. It took advantage of a breakthrough in propeller design, a massive wing, huge fuel tanks within a double-bottomed hull and new concepts in streamlining to achieve the distance requirements. The three Martin 130s Pan Am bought cost an average of $417,201 -- five times the cost of land-based airliners.

Advances in radio navigation were augmented with crude but useful methods for keeping on course -- such as dropping "paint bombs" on the ocean's surface to monitor the aircraft's drift in the air, noted P. St. John Turner, in a history of Pan American.

Trippe, who already had forged a profitable position in Latin American air transportation, used his Washington political connections to help negotiate a contract with the Post Office to carry mail for $2 a mile, a vital subsidy, said Davies.

The 16 passengers who made the trip paid $799 to go from San Francisco to Manila, a fare equivalent to $10,000 today.

"They were rich people -- oil magnates, film stars and top-level politicians. The man on the street would not be found on a long-distance air route," said Davies. Hot food, wash rooms with hot water and sleeping berths comforted the passengers. The flights left San Francisco once a week.

"When Trippe created Pan Am, he had a vision and wouldn't let anything stand in his way," Davies said.

World War II brought an abrupt end to the era of the flying boats. The Japanese conquests of the Philippines, Guam and Wake Island cut off the Pacific route. And the development of long-distance bombers and the proliferation of military airfields established a faster, simpler way of travel for Pan Am. But at the end of the war, Pan Am also was confronted with new competitors.

To Davies, the events that compelled Pan Am to surrender its Pacific routes are a tragedy.

"An airline that did so much to further U.S. commercial aviation -- playing almost an ambassadorial role -- should not have been reduced to this situation," he said.

"They did the pioneering of the world's ocean air routes and here they are, forced to sell their birthright."