A US Airways flight crew stubbornly refused air traffic control clearance to take off into a foggy night in Providence, R.I., early this month, possibly saving dozens of lives because a United Airlines jet had wandered back onto the same runway after landing, government sources said.

Moments before the US Airways crew decided to sit tight, a Federal Express cargo jet took off ahead of them on the same runway, and passed directly over the lost United airliner. It was unclear how close those two planes came, but sources close to the investigation said the roar of the FedEx plane can be heard clearly on a radio tape as the United crew expressed concern to the control tower about their location.

Investigators for the National Transportation Safety Board have launched an investigation of the Dec. 6 incident, one of a series of runway "incursions" that have raised concerns in the aviation safety community. While some of these incidents are rules violations that cause no immediate safety threat, an increasing number involve close encounters between airliners full of passengers.

In one particularly close call, on Nov. 22 at Los Angeles, former senator Bob Dole and his wife, Elizabeth Hanford Dole, were aboard a United Airlines flight bound for Dulles International Airport that was forced to lift up abruptly as it was rolling toward takeoff in order to miss an Aeromexico plane that had blundered onto the runway directly ahead of it. Investigators said the United Boeing 757 passed only 60 feet above the Aeromexico MD-80.

"I do remember something a bit unusual," Dole said in a statement from his office. "If it was an abrupt takeoff, then I am thankful for the skill and professionalism of the United pilots. If there are safety issues involved, I certainly hope they are looked at as quickly and thoroughly as possible."

Another passenger on Dole's flight, who did not want to be named, said he noticed a "more abrupt" takeoff than normal and a "very steep rise." But the pilot never said anything about it to the passengers.

The Providence incident involved Metrojet Flight 2998 to Baltimore (Metrojet being the low-cost carrier of US Airways), United Flight 1448 from Chicago and a FedEx cargo plane.

Air traffic controllers at Providence, like those in many small- and medium-sized airports, lack radar that can detect planes on the ground. Sources said visibility that night was minimal and it was not possible to see one end of the runway from the other. Nor was it possible to see the runways from the control tower. However, the weather was still above minimum requirements for landing and takeoff.

The United Boeing 757 landed on Runway 5 Right and was given instructions to turn left onto a taxiway and later to turn left on another taxiway to the terminal area. The crew, apparently confused in the dark and fog, turned left too early and ended up on the original runway.

The United crew realized something was wrong and called the tower but reported an incorrect location, apparently believing they were on Cross-Runway 16, clear of the active runway. At about that point, the FedEx cargo plane flew directly overhead. One source close to the investigation said it is possible that the roar of the FedEx engines drowned out the transmission saying the United crew was on a runway.

"Somebody just took off," an increasingly concerned United pilot told the tower. It is unclear whether the FedEx or United crew saw the other plane through the fog, or if the controller realized the closeness of the encounter.

The controller then began quizzing the United pilots. At least four times the United crew reported their position as Runway 23 Right at Runway 16. In truth they were at Runway 23 Left at Runway 16, directly in the path of the US Airways plane, but out of sight 4,000 feet down the runway.

Runways are numbered according to the direction of takeoff and landing. Runway 5 Right and 23 Left are the same runway, with takeoffs from 5 Right going roughly northeast and from 23 Left going roughly southwest.

Apparently satisfied that the United plane was out of the way, the controller gave the US Airways plane clearance to take off. The US Airways crew, having listened to the radio conversation between the controller and the United crew, refused.

Normally, pilots routinely accept instructions from controllers, who have far more information about runway and airspace than the pilots. It is a relationship of mutual trust. However, under federal rules, a pilot has the right to reject any controller instruction the pilot considers unsafe.

A source who interviewed both pilots extensively said they held back because of the "tone of uncertainty" in the United pilots' voices. Both pilots said they immediately rejected any idea of takeoff, the source said, with the captain telling the co-pilot, "No, don't take that," just as the co-pilot said, "I don't want to do that."

"Where's United?" the crew asked the controller. The controller assured them that United was clear, the US Airways crew told an investigator. The crew told the investigator that the controller grew frustrated, telling them again that the United plane was out of the way, but the US Airways crew was adamant.

Again, the controller began quizzing the United crew about their location. It was then that the United crew determined their true location. Once the plane was safely taxiing toward the terminal, the controller told the US Airways crew to taxi onto the active runway and prepare for departure.

Again, the US Airways crew refused, saying, "We'd like to have United at a gate."

US Airways declined to name its crew or to discuss the details of the event.

"We are aware of the incident, and we are proud of the actions of our crew," a spokesman said. "But since this is under National Transportation Safety Board investigation, there will be no further comment."

Edmond L. Soliday, United's vice president for safety, quality assurance and security, said United followed its usual procedure in such incidents, taking the crew out of regular service for a no-penalty safety debriefing and for simulator training. He said that after a line check, in which a senior pilot flew with the crew until it was clear the crew could handle the job, the crew was put back into regular service on Thursday.

The Federal Aviation Administration would not name the controller or comment on the investigation, saying the safety board was the lead agency in the probe.

Jim Hall, chairman of the safety board, has been outspoken in the past few weeks about runway incursions. He has called for a national summit on the problem that would include the FAA and others in the aviation community.

"Whatever we've been doing, it's not effective," Hall said. If some solution is not found, there's going to be "a tremendous loss of life," he added.

The FAA has now consolidated all its runway incursion programs under one director with authority to cross organizational boundaries that have often defeated programs in the past.

After a decade of delays, the FAA says it is now pushing ahead with radar software--called the Airport Movement Area Safety System, or AMASS--that is supposed to warn controllers of impending runway accidents. However, officials have said the system may never live up to its initial promise. Other surface detection radar programs soon to be under way will attempt to produce a relatively inexpensive system for smaller airports such as Providence.

After rising steadily for years, the number of incursions seems to have flattened out and may be decreasing. In the first nine months of this year, there were 248 runway incursions, compared with 236 in the same period in 1998. John Mayrhofer, the FAA's new director of the runway safety program, said incursions have decreased sharply in the past three months from the same months in 1998--25 percent down in November, 23 percent down in October and 18 percent down in September. "I'd like to think this is due to a lot of the work we have done," he said, but numbers over such a short period "may not be statistically significant."

Much of the FAA program has concentrated on education and training for pilots and controllers, on surface-detection radar and on airport signage. Critics, including Hall, have called for faster, simpler solutions such as stop boards.

Mayrhofer said many airports are trying innovative solutions, and "we could paper the room with all the ideas and initiatives." But the FAA has to be careful, he said.

"There are no simple solutions," he said. "Every solution has other tentacles to it that involves other problems."