A small airliner and a helicopter, both carrying sightseers, collided in the often crowded skies over the Grand Canyon yesterday and plunged thousands of feet to the floor of the Arizona canyon, killing 26 people. The National Park Service said there were no survivors.

The plane, a deHavilland DHC6 Twin Otter operated by Grand Canyon Airlines on a flight from Las Vegas, carried 18 passengers, most of them foreign tourists, and a crew of two. The helicopter, a Bell 206 operated by Helitech and apparently based in the small canyon-rim village of Tusayan, Ariz., carried a pilot and five passengers.

The victims' identities were not available last night. Park Service officials told the Associated Press that those on the plane included 11 from the Netherlands, two from Switzerland, one from South Africa and six from the United States, including the two pilots. Four of the helicopter passengers were identified as Americans, the Park Service said last night.

Both craft were following "visual flight rules": They were not monitored by Federal Aviation Administration air traffic control radar, and it was the pilots' responsibility to "see and avoid" other aircraft.

John Guthrie, deputy superintendent of Grand Canyon National Park, told The Washington Post that the helicopter "appears to be in two pieces," while the plane crashed in one piece. That would indicate that the plane struck the helicopter but would not indicate why. The weather was clear when the collision occurred at about 9:30 a.m. PST.

"What this does is underscore the issue of aircraft safety here," Guthrie said. "There are many planes in the sky" over the canyon. The aircraft came down in the Crystal Rapids area of the canyon's Colorado River, about 15 miles northwest of Grand Canyon Village.

Guthrie said recovering the bodies would require the use of helicopters and could take two or three days. Some reports last night indicated that the Bell helicopter carried four passengers, not five.

The canyon, with its wondrous vistas, has become a summertime aviation freeway with as many as 600 flights a day below the rim. Environmental groups, focusing on the noise from low-flying aircraft that shatters a profound silence, have urged the Park Service to restrict flights. Three weeks ago, park officials issued six alternatives for aircraft management; a final regulation is scheduled in the fall.

The FAA, in the July-August 1985 issue of General Aviation News, wrote that "about 10 accidents a year occur within Grand Canyon. Cable strikes, low-altitude stalls, getting sealed in the valley by rapidly descending clouds and trying to out-climb granite walls are typical accident-resulting scenarios."

Ten people were killed May 31, 1984, when a Las Vegas Airlines sightseeing flight crashed in the canyon. The National Transportation Safety Board asked the FAA then whether it should "develop and publish standards for operating procedures, including route selection, flight schedule and altitude selection for sightseeing flights in the canyon."

FAA Chief Donald D. Engen responded that the FAA was increasing nonregulatory efforts to police traffic and working toward a voluntary agreement with tour operators. Grand Canyon Airlines general manager Ronald Warren was in Washington yesterday to discuss that agreement and other issues with FAA officials.

Industry sources said yesterday that many but not all tour-flight operators in the Grand Canyon area follow prescribed flight routes and procedures to increase safety. Grand Canyon Airlines has been in business since 1926 and was operating as a scheduled airline. It could not be learned last night how long Helitech has been in operation or the nature of its operating license.

Guthrie said the Park Service learned of the accident after a column of smoke was reported and a ranger was dispatched to the area. At the Park Service's request, the FAA restricted flights over the canyon so airborne search and rescue efforts would not have to compete with sightseers.

James Mahoney, at a Forest Service fire lookout 10 miles from the crash site, told United Press International that he did not see the collision but saw smoke billowing into the sky. He said he knew the aircraft had been flying below the rim "because of the position of the smoke."

FAA sources said yesterday that they did not know if air traffic control radar antennas at scattered western sites had recorded the flights. Radar tapes will be studied by crash investigators. But aircraft below the rim would not be trackable on radar.

The Grand Canyon airport has an FAA air traffic control tower, but, like many towers serving smaller communities, it is not equipped with radar, the FAA said. The safety board dispatched an investigating team from Washington.

The canyon is about nine miles wide at the point where most sightseeing flights occur. The entire District of Columbia would just about fit between the north and south rims at that point. The canyon is roughly a mile deep from the south rim to the river.

Nearly all the air-tour operators advertise that their flights go below the rim into the canyon. Some helicopter flights fly within a few hundred feet of the canyon floor; fixed-wing planes generally stay above Tonto Platform, a plateau about 1,500 feet below the south rim.

The Grand Canyon is a 277-mile-long stretch of the Colorado River. The enormous gulch was carved into the colorful rock of northern Arizona by water and wind over millions of years.

The FAA owes its birth in part to the congressional reaction that followed a 1956 collision over the Grand Canyon of a United Airlines DC7 and a Trans World Airlines Lockheed Constellation, both en route from Los Angeles to Chicago. A total of 128 people died in what was then the world's worst aviation accident. Both pilots were attempting to give their passengers a better view of the canyon.