The Montgomery County Board of Appeals handed residents of downtown Bethesda a major victory this week when it voted to deny a request to establish a private helicopter landing pad atop the 16-story Clark Building.

In a three-page ruling, the five-member board unanimously decided that Clark Enterprises Inc., the country's seventh-largest construction company, "has not carried its burden of proof" that a helicopter would not adversely affect "the use, peaceful enjoyment and economic value of the surrounding property."

The board also determined that the firm failed to prove that a helicopter "will produce no objectionable noise or vibrations."

Hundreds of homeowners living near the new Clark Building on Wisconsin Avenue and Old Georgetown Road -- part of the multimillion-dollar development near the Bethesda Metro station -- have fought the "helistop" plan for more than a year, contending that the facility would be noisy and unsafe.

"It's wonderful. This comes as a real surprise," said Kathy Levy, a member of the Battery Park Citizens Association.

Clark Enterprises had sought a special exception that would have enabled its chief operating officer, millionaire developer James Clark, to commute to Bethesda from his Easton, Md., home.

The corporation also had planned to use the helicopter to fly Clark and other corporate officials to construction sites in the area.

It had proposed three incoming and three outgoing flights on weekdays, between 8 a.m. and 6 p.m., following a flight pattern along the Capital Beltway and Wisconsin Avenue at an altitude of about 1,000 feet.

Robert H. Metz, a lawyer representing Clark Enterprises, could not be reached for comment.

However, at public hearings before the board of appeals and the Montgomery County Planning Board -- which last month conditionally recommend approval to the appeals board -- Metz testified that measures to ensure safety and minimal noise levels complied with Federal Aviation Administration and county helistop regulations.

Clark Enterprises had planned to buy what Metz called the "safest" helicopter available, a twin-engine Bell 222. He also said that the chopper would fly only over major highway arteries that already diffuse high-level traffic noises, such as the Capital Beltway and Wisconsin Avenue.

But the board wrote that "accidents happen, and that possibility must be taken into account." Board members also said that the firm's identification of possible emergency landing sites in the area was "deficient."

A major concern among opponents was that there are three schools within several blocks of the Clark Building, and they contended that the playing fields of one those schools would become the most likely landing spots if a helicopter develops trouble.

The Bethesda Naval Medical Center, located north of the business district on Rockville Pike, lies near the chopper's proposed flight path and also opposed the helistop. At the planning board hearing, Cdr. James Smith, a center spokesman, testified that a helicopter could pose a threat to the 7,000 to 8,000 patients and staff members at the sprawling, 243-acre center.

Helicopters are permitted to land at the center and at nearby Suburban Hospital, but only for medical emergencies. Board members said that "citizens apparently do not oppose them, possibly because of the humanitarian, emergency and public welfare character of these flights."

Rooftop helistops have been permitted in the county since the late 1960s, but only one has been installed -- atop the Grammax Office Building in downtown Silver Spring.

Until last November, when the regulations were amended, helistops were banned in high-density central business districts such as Bethesda's, county planners said.