FARMINGDALE, N.Y. - Getting a message from New York to San Francisco these days is as easy as a click of a mouse. But nearly a century ago, sending a missive across the country was an ordeal that often placed mail carriers in mortal danger in experimental flying machines.

It's a piece of aviation history being celebrated in Wyoming and across the nation this week with a flight that will replicate those harrowing cross-country journeys.

A trio of pilots in vintage airplanes took off Wednesday from Long Island's Republic Airport on a six-day, 15-stop flight to San Francisco to mark the 90th anniversary of the U.S. Postal Service's involvement in airmail delivery. It includes several stops in Wyoming.

"This airmail anniversary flight of historic biplanes is a compelling display of pilot skills that too often seem forgotten in an era of autopilots, GPS and daily flights high above the weather," said Josh Stoff, curator of the Cradle of Aviation museum, which along with the American Airpower Museum chronicles Long Island's rich aviation history.

Historians are quick to point out that the post office's launch of airmail helped jump-start commercial aviation in America, showing that airplanes could fly safely across country on a regular basis.

The first-ever experimental airmail flight took place on Long Island in 1911, a three-mile journey between Garden City Estates and Mineola. The first regularly scheduled intercity U.S. airmail began on May 15, 1918, after Congress appropriated $100,000 to establish airmail routes.

Using planes on loan from the Army Signal Corps, pilots flew between Washington's Polo Grounds and Belmont Park on Long Island, stopping in Philadelphia on the way. The first transcontinental airmail flight took off on Feb. 22, 1921, from Mineola to San Francisco.

Among the initial airmail pilots was a young Charles Lindbergh, who flew for the post office in the early 1920s as part of a squadron that often had to fly exclusively in the daylight, following railroad tracks and dirt roads to locate destinations. Lindbergh took off from Long Island on his historic 1927 trans-Atlantic flight to Paris.

Addison Pemberton, the owner of a Spokane, Wash.-based manufacturing company, took off Wednesday aboard a 1928 Boeing 40C he recently restored carrying 300 envelopes with special cancellations from postal representatives to San Francisco.

Stops are expected to include Bellefonte, Pa.; Cleveland; Chicago; Iowa City, Iowa; Omaha, Neb.; Cheyenne; Rawlins; Rock Springs; Reno, Nev., and Hayward, Calif., before arriving in San Francisco on Monday. The Wyoming stops will all occur on Saturday.

Two other pilots will join him on the cross-country trip: Larry Tobin will fly a 1927 Stearman C3B biplane, and Ben Scott will fly a 1930 Stearman 4E Speedmail.

Pemberton intends to write a daily blog that will appear on the Smithsonian's Air and Space Museum Web site. He noted that the Boeing 40C was a workhorse in delivering mail decades ago, and recalls his father telling stories of air mail flights over the family farm in Iowa.

"My father used to say that that the mail flights were so regular you could set your watch by them," he said in a telephone interview.

The post office didn't stay in the airplane business for long.

By 1926, commercial airlines took over the flights and a year later, all airmail was carried under contract. Today, the Postal Service is the airline industry's largest customer, spending more than $3 billion for air transportation. Letters traveling coast to coast travel by air and typically take three days to deliver, according to the Postal Service.

"This is a piece of American history that is largely unknown," Pemberton said. "The post office is largely responsible for the growth of commercial aviation. These were the first people to show that airplanes could be capable of traveling coast to coast."