Down, down, down, deep into the craggy narrows of the magnificent chasm went the hiker, working his way with infinite care along a serpentine path carved into the sheer vermillion wall of rock that drops more than a mile from rim to river. The sun was setting by the time he reached the canyon floor, where he set up camp and sat down to soak up the splendid scenery and the profound, engulfing silence of the Grand Canyon.

But at sunrise the next morning, the canyon's ancient stillness was shattered. All day long the tall cliffs echoed with the high-pitched whine of small planes, the roaring throb of turboprops, the "WHUP-whup-WHUP-whup" of an acrobatic fleet of helicopters.

"You spend two or three days getting to a remote campsite -- and you might as well be on the main runway at O'Hare airport in Chicago ," recalled the hiker, Bruce Babbitt, a Grand Canyon authority who is also governor of Arizona. "The planes can be as loud in the heart of Grand Canyon as the downtown rush hour in Phoenix."

After complaining for years about noise from the air tour companies that operate some 50,000 sightseeing flights through Grand Canyon National Park each year, Babbitt and other environmentalists have launched a campaign to ban or limit the aerial traffic.

Air tour firms are fighting back vigorously, pointing out that their $40 million-per-year industry provides a key source of income for local communities and lets hundreds of thousands of people, including the handicapped and aged, see this awesome gulch each year.

Smack in the middle is the National Park Service. "The whole controversy is another example of the age-old conlict between natural preservation and human enjoyment of the resource," says Richard Marks, the park superintendent. "That's the question you face every day when you're managing a national park -- where do you set that balance?"

The Park Service has done little to control overflights in the past. But this spring the park will issue a comprehensive plan governing air tours. The Grand Canyon plan should serve as a model for some 50 other national parks and monuments where sightseeing planes and helicopters are becoming a common, and controversial, aspect of the landscape.

Park planners say they will not permit the current situation -- more than 20 flights per hour over the South Rim some summer days -- to continue. Nor will they ban all air tours. Between those extremes, the park could propose limits on the time and number of flights, on flight routes and minimum altitude, or on types of aircraft.

It's not certain, though, that the park rules will be effective. "I keep asking, how am I going to enforce this?" Marks says. "I don't have an Air Force."

Hovering over the dispute are congressional delegations representing the area; they could second-guess any regulations the park managers devise.

That precedent was set in 1980, when the Park Service issued a rule banning motors on boats within the canyon. "Our feeling was, people take a boat trip through Grand Canyon to get away from the world, so why would you want to hear a motor?" says Marks. But the ink was barely dry on that rule when Sens. Orrin G. Hatch (R-Utah) and Dennis DeConcini (D-Ariz.) passed legislation killing the park regulations.

About 3 million people come each year to the Grand Canyon, called by Theodore Roosevelt "the one great sight which every American should see." The big majority comes by bus or car to overlook sites along the rims. A small percentage cruises through by boat along the Colorado River. Another small group hikes down into the interior.

The air tour industry -- based at Grand Canyon airport, Las Vegas, Phoenix, and St. George, Utah -- carries about 400,000 people per year over and into the chasm, according to the Grand Canyon Flight Operators Association.

Those who pay from $50 to $200 for an air tour include people whose health or age precludes any other visit. But most flyers, the association says, are simply busy people who won't take more than an hour or so to see the majestic kingdom of rock that nature took a billion years to build.

Ronald Warren, president of the Air Tour group, says planes and helicopters are the "cleanest" way to see the canyon. "The buses pump out their exhaust over the rim and the backpackers drop litter and build campfires," he says. "Environmentally, we are preferable."

Environmental groups respond that the air traffic could cause damage to geological formations or archeological remains; the Park Service says it has no evidence of this. Environmentalists say the flight noise endangers dwindling species like the peregrine falcon and big horn sheep; the service is studying this question.

Mainly, though, the anti-airplane position is based on deeply felt but intangible concerns of emotion and aesthetics.

Rob Smith of the Sierra Club, a leader in the battle for silence, said environmentalists "have pretty much lost our argument" that the Grand Canyon should be free of motors. "But we'd still like to establish that silence is a resource -- that 'seeing' Grand Canyon includes hearing the silence," he said.

The British author J.B. Priestley heard the utter silence of the majestic depths when he hiked the canyon in 1937.

"There was in this immensity," Priestley wrote later, "a silence so profound that soon all noises from the life about us on the rim were lost in it -- as if our ears had been captured forever, drowned in these deeps of quiet."