For more than 10 years, on and off, National Park Service officials who manage this magnificent natural wonder have been at work on a controversial regulatory decision, a proceeding that has spawned hundreds of hours of testimony and thousands of pages of comment.

Last Wednesday morning, the complexion of that proceeding was changed in a split second.

The midair collision of two sightseeing aircraft has prompted the Park Service and the Federal Aviation Administration to take another look at their pending "Aircraft Management Plan." This controversial plan is designed to set rules for sightseeing flights over and through the Grand Canyon. The new assessment seems likely to produce stricter controls over the flights than the government was contemplating before the crash. A final rule-making decision is expected by the end of this summer.

The accident, under investigation by the National Transportation Safety Board, was a collision between a helicopter and a twin-engine plane during routine morning sightseeing flights within the canyon. The craft were operating under "see and be seen" flight rules. All 25 people aboard the two craft were killed.

The disaster occurred as the Park Service was making the final decisions on a formal set of regulations governing the 50,000-plus sightseeing flights each year that make the sky over Grand Canyon some of the nation's most crowded airspace.

The overflights have spawned a major controversy, not because of safety, but on environmental and esthetic grounds. Conservation groups have complained that the constant throb and whine of aircraft engines destroy the profound silence that is one of the Grand Canyon's most remarkable features.

The issue has been debated for more than decade. But the argument heightened last fall when Park Service officials here began a formal regulatory proceeding to set rules governing overflights. The Grand Canyon aircraft plan is expected to serve as a model for restrictions at some 50 other national parks and monuments where sightseeing planes have sparked complaints from hikers and campers.

In the public hearings, and in the Park Service's "environmental assessment" published three weeks ago, the major issue has been noise. With the notable exception of one Interior Department official, Mike Martin, who warned that Grand Canyon has become a "hazardous flying environment," there was almost no discussion of safety in the thousands of public and official comments received.

As a result, the debate turned into a contest between two groups of canyon visitors: on one side were the serious hikers, who were offended by aircraft noise; on the other were the 250,000 people each year who pay $50 or more to take a flight through the canyon.

In those circumstances, the Park Service's preliminary proposals, issued last month, involved only minor restrictions on overflights. The agency offered six alternative plans for consideration. Only two would have prohibited planes from flying below the rim of the canyon, and even the strictest would have set aside only 11 percent of the national park as "nonflight" areas.

After Wednesday's crash, however, Park Service officials said that safety will become as important a consideration as silence when the government issues its ruling. And that could lead to tighter rules than anyone had expected.

"It's clear that we have an air-traffic problem, a safety problem, in the park," said Richard Marks, the park superintendent. "And we're going to have to address that when the final rules are issued ."

Since Maj. John Wesley Powell led the first expedition through the canyon in 1869, explorers have been giving all manner of exotic names to various rock formations inside the giant gulch: "Vishnu Temple," "Wotan's Throne," "Cheops Pyramid." The air tours swoop and glide through the rock spires of the canyon to show their customers each of the glamorously named natural features.

Wednesday's collision occurred just below a towering red butte known as "Confucius Temple."