ROMULUS, MICH., DEC. 3 -- Two Northwest Airlines passenger jets collided on a fog-covered runway at Detroit Metropolitan Airport today, igniting a fiery explosion that killed eight people and left one airliner a charred hulk. Twenty-one other people were injured, many with severe burns.

A source in Washington said that investigators were puzzled why a Pittsburgh-bound DC-9 was on the same runway as a Memphis-bound 727 that had been cleared for takeoff. Kevin Wheland, a Northwest Airlines spokesman, said the 727 was about to lift off at 2 p.m. when its right wing hit the DC-9's right side.

"The planes were headed for a head-on collision," said Bob Gibbons, another Northwest spokesman. The wing of the 727 struck the smaller DC-9 just behind the co-pilot's seat, ripping open the plane's roof and tearing one of its two engines from the fuselage. Jet fuel spewed from the engine and the DC-9 erupted in flames moments later.

Officials said all the injured were among the 33 passengers and four crew members on the DC-9, Northwest Flight 1482. Some of the victims were trapped in their seats, the Wayne County coroner's office said. None of the 146 passengers and 10 crew members aboard the 727, Northwest Flight 299, was reported injured.

A five-member National Transportation Safety Board investigating team from Washington arrived here tonight to begin piecing together the sequence of events that led to the fiery collision on the center of the airport's three main runways.

Neither Northwest nor airport officials could provide many details of the accident. Dan Kerber, deputy director of airport operations, said the 727, traveling north on its takeoff run, came to a stop about 1,500 feet north of an intersection on the runway after the crash. The DC-9, inexplicably traveling south along the same runway, came to rest facing south about 1,000 feet south of the intersection.

Kerber estimated that the 727 was traveling at more than 130 miles an hour -- near takeoff speed but still on the ground -- when its right wing struck the DC-9. He said one emergency escape chute on the left side of the DC-9 was opened.

Northwest officials said it was up to the airline's pilots to decide whether they had enough visibility through the fog to take off. Wayne County Executive Ed McNamara said that, according to the airport fire chief and other officials, there was "practically no visibility" at the time of the collision.

But Kerber said he was not aware of any pilots refusing to take off because of the weather. The airport reopened later in the evening.

Northwest spokesmen said they could not explain why the DC-9 pilot had maneuvered his plane onto one of the airport's main runways. They said the runway had served as a taxiway earlier in the day.

Tony Dresden, a spokesman for the National Air Traffic Controllers Association, said tonight that controllers lost track of the DC-9 in the dense fog. "We were relying on what he was telling us," Dresden said. "Whether he was lost, disoriented or what, we're not sure."

"When they {the ground controllers} discovered where he was, they told him to exit the runway immediately, but it was too late," Dresden said.

John Izzo, 41, a Westinghouse engineer from Pittsburgh who was aboard the DC-9, said there was no shouting or panic at the time of collision. He unstrapped himself and managed to crawl out on one of the jet's wings, he said.

"Before I jumped I looked back; I saw two people who were not going to make it," he said. "I didn't panic, but I jumped from the wing."

Izzo said he and others managed to pull injured passengers on the ground away from the plane, "which was sizzling like a kerosene can." He said he and a flight attendant, who was riding as a passenger, applied compresses made of snowballs to some of the injured and used a necktie to stanch another victim's bleeding.

"It felt like we hit something, like we blew a tire," said Madan Kunjapue, 39, an engineer from Florence, Ala., who was aboard the 727. "I looked out the window and I could see a smaller plane, and the roof was off the plane."

Firefighters were on the scene almost immediately, but the fired blazed for an hour and turned the silver-and-red DC-9 into a blackened hull. The 20 injured, including the cockpit crew of the DC-9, were taken to three Detroit area hospitals, officials said.

Wayne County Executive McNamara, whose government runs the airport in this Detroit suburb, credited the 727 pilot with avoiding a more serious collision. "Obviously, God was his co-pilot," McNamara told reporters.

Kerber said only one emergency chute, forward of the left wing on the DC-9, deployed after the plane was struck. Most of the fire was concentrated in the center of the plane and Kerber could not say if some passengers were trapped because of the failure of other chutes to deploy.

The accident was one of the worst U.S. ground accidents involving commercial aircraft. In the spring of 1986, the National Transportation Safety Board issued a special report on what it described as a troubling increase in runway accidents, a report triggered by an incident involving two Northwest DC-10 airliners.

Those two jumbo jets, carrying a total of about 400 passengers and crew members, nearly collided on a runway at the Minneapolis-St. Paul International Airport, renewing concerns about ground control and congestion at U.S. airports.

Safety experts often note that the world's worst aviation disaster, which killed 582 people, occurred on a runway. In that 1977 disaster, two jumbo jets collided in dense fog on Tenerife in the Canary Islands.

The report called for a number of changes in airport operations, some of which were opposed by the Federal Aviation Administration and the airline industry.

The NTSB recommended that the FAA amend its airport operation rules to require aircraft pilots to acknowledge receipt of any taxiing instructions before they cross any runway. At most major airports, pilots must receive permission from a ground control tower to leave their terminal. They then must receive specific permission to cross any runway and to take off.

Since few airports have a ground radar system, they rely on pilots to know the airport layout and to follow the numbered signs designating runways and taxiways. This often becomes difficult in bad weather when parts of the airport are not visible from the control tower.

Northwest, which is based in Minneapolis, has had its share of major operating troubles. A Northwest MD-80 passenger jet crashed shortly after takeoff at Detroit on Aug. 16, 1987, killing 156 people in the second worst air crash in U.S. history. In the past year, Northwest was the focus of much criticism after three of its pilots were convicted of flying while intoxicated.

Izzo, the Pittsburgh engineer who survived today's crash, was philosophical as he talked to reporters tonight. "Somebody told me today was not a good day for flying and I should have taken their advice," he said.

Walsh reported from Detroit, McAllister from Washington. Special correspondents Susan Fleming, Donna Pach and Louisa Calderon Hayes contributed to this report from Detroit.

Two Northwest Airlines jets, a DC-9 en route to Pittsburgh and a Boeing 727 en route to Memphis, collided in fog at Detroit's Metropolitan Airport yesterday. At least 19 people aboard the DC-9 were killed.

1. Flight 299, the Boeing 727 with 146 passengers on board, had started its takeoff roll.

2. Flight 1482, the DC-9 with 39 passengers and five crew members on board, was taxiing.

3. The accident occurred on Runway 3C at 1:45 p.m., about 15 minutes after both planes left gates at the airport's North Terminal.

4. The right wing of the 727 tore into the right side of the DC-9 at cabin level, just behind the copilot's seat. All known casualties were aboard the DC-9.