The solar airplane that will attempt to fly across the country has made its final test flight, spending most of the day soaring over San Francisco Bay on a day that couldn't have been any prettier.

Tuesday's flight began at sunrise as pilot and Solar Impulse co-founder Bertrand Piccard departed Moffett Field in Mountain View. He spent much of the day aloft, giving people from Silicon Valley to Sausalito a chance to see the gentle giant of an aircraft. The plane, which has the utterly unromantic name of HB-SIA, has a wingspan of 208 feet, just a few feet less than the 747 that transported it to the United States from its home in Switzerland. Despite the massive wingspan, it weighs just 3,527 pounds.

Piccard first headed west from Moffett Field, making his way at around 45 miles per hour up the coast from Half Moon Bay towards San Francisco. By 2 p.m. Solar Impulse was cruising around 3,500 feet over the entrance to San Francisco Bay, with multiple local icons in view, including the Golden Gate Bridge partially shrouded in fog.

HB-SIA features four brushless electric motors, each good for 10 horsepower. Flying under solar power alone, the engines provide an average of 8 horsepower, enough to keep the propellers spinning at 400 rpm and the airplane cruising at a leisurely 43 mph.

The test flight was the third flight the team has made in advance of its flight across America scheduled to begin next week. The airplane has made several flights in Europe (including a night flight), and one to Africa, before being transported to the U.S. in February. For the "Across America" flight, the solar powered airplane will take a southerly route across the United States before making its way north, ending the trip in New York City some time in June or July.