Perched on foam cushions so he could see out the window of his Cessna 210, 11-year-old pilot John Kevin (Hawk) Hill ended a week-long cross-country trip with a flawless landing yesterday at National Airport.

Heralded by some as the youngest pilot to fly across the United States, John Kevin had just ended a journey that carried him from Los Angeles over the Rocky Mountains and Midwest cornfields to within view of the Capitol.

But as the sixth grader from Arlington, Tex., jumped from the single-engine, four-passenger Cessna just before 3 p.m. yesterday, his thumbs triumphantly toward the sky, he was clearly still in the clouds.

"It was just like a roller coaster," freckle-faced John Kevin said, explaining how he flew into thunderstorms on the final leg of his trip.

"It was real bumpy," he told reporters, aviation officials, throngs of well-wishers and his congressman, Rep. Richard K. Armey (R-Tex.). "The plane was going side to side and up and down."

Then, tossing his cap back on his blond head, John Kevin said the turbulence and the high-altitude nosebleeds "weren't that bad. It was easier than I thought."

John Kevin's flight instructor, Mark Fields, accompanied him on the plane because the Federal Aviation Administration forbids anyone under 16 to fly solo. Fields said he never took control of the plane. Also on board were a Fort Worth reporter and an ABC television cameraman.

Newly won fan Mike Woodruff, a fifth grader from Prince George's Melwood Elementary School, was clearly wowed. "I think it's pretty great."

Everett Langworthy, executive vice president of the National Aeronautic Association, presented John Kevin with a certificate that read: "In recognition of your accomplishment as being the youngest known pilot to fly an aircraft across the continental United States." The NAA did not declare the cross-country flight an official record.

A former American Airlines pilot, who heard about John Kevin's planned trip, recently claimed that years ago his daughter, then age 10, flew from California to New York. However, there are no known official records of that or other flights piloted by youngsters.

John Kevin, who hunts rattlesnakes and denies rumors that he has a girlfriend named Melissa, said he hopes his flight lands him in the Guinness Book of World Records.

Alex Reid, Guinness Book deputy editor, said that because pilot age requirements vary from country to country, he doubted John Kevin's feat would qualify for the record book.

"I gather this is creating quite a stir in your country," Reid said from his office in Middlesex, England. ". . . . Of course we'll give it every consideration."

Since John Kevin took a $20 plane ride over Dallas two years ago, he's been "hooked on" planes. He already has logged 260 airborne hours. The FAA requires a person to be 17 and to have flown 20 hours solo before being granted a private license.

Johnny Hill, a motor home salesman, said he gladly paid for his son's flight instructions. Asked if he was also a pilot, he said, "No way. I hate planes."

Yet when the tiny, blue and white Cessna came into view above National Airport, Johnny Hill looked up and shouted, "That's him right there. That's him."

Neighbors, local Texas banks, and fascinated strangers paid for the cross-country adventure, which cost about $5,000.

John Kevin stopped in Utah, Colorado, Missouri and Cincinnati along the route, meeting a governor, a few mayors, a boy who rode his bicycle three miles to meet him and an 87-year-old grandmother who shook his hand. This morning, a limousine was scheduled to pick him up at his hotel and take him to NBC's "Today" television studio.

John Kevin said he wasn't going to bask too long in this accomplishment. He said by the time he's 13 he wants to fly around the world. Then, he wants to be an astronaut. "I want to walk on the moon and bounce up and down on it," he said.