Federal safety investigators are trying to determine if an Eastern Airlines jet was allowed to land dangerously close behind Air Florida Flight 90 as it took off from National Airport in a snowstorm on Jan. 13. The Air Florida Boeing 737 crashed into the Potomac after half a minute in the air, killing 78 people.

At issue is how close Eastern Flight 1451, approaching from the south, drew to the Air Florida plane, and whether the Eastern jet actually touched down before the Air Florida flight lifted off from the northern end of the same runway.

Investigators said there is no evidence that the Eastern jet played a role in the crash.

But its proximity could raise serious questions about the adequacy of air traffic control rules for snowstorms and other times of low visibility, and fuel ongoing debate over the qualifications of supervisors and military air traffic controllers who replaced 11,500 civilian controllers whom the Federal Aviation Administration fired for striking last summer.

The FAA has stated repeatedly that the nation's traffic control system is safe despite the loss of the striking controllers. And late last year, the safety board itself ruled favorably on the post-strike system, saying that a detailed study of operations during the two months following the Aug. 3 strike showed that things were functioning smoothly.

Investigators have not yet determined how close the two airliners came, according to National Transportation Safety Board member Francis McAdams. But he said, "We're going to lay it out, what was the separation." The board, which is heading the investigation, plans to convene public hearings into the crash on March 1.

Using transcripts of conversations between the two Air Florida pilots and between pilots and the National Airport control tower, the flight data recorders from the two jets and radar readouts, investigators are trying to determine the two planes' precise positions on the afternoon of Jan. 13.

Transcripts of the Air Florida's cockpit voice recorder show that a tower controller cleared the plane to take off at 3:59 p.m. and 24 seconds. Four seconds later, the controller told the pilots: "No delay on departure if you will. Traffic's two and a half miles out for the runway." That was a reference to the approaching Eastern 1451.

At 11 seconds after 4 p.m., an Eastern pilot reported the plane was "over the lights," an apparent reference to lights that FAA officials say extend about a half-mile from the south end of the 6,870-foot runway. About 20 seconds later, Air Florida's pilot called out his go, no-go speed, indicating his accelerating plane had not yet lifted off.

It remains unclear if during that 20 seconds the Eastern airliner had touched down. If it had, the two jets were on the runway at the same time, a safety hazard that no controller or pilot wants to see. If not, the Eastern jet touched down only seconds after the Air Florida plane lifted off.

FAA sources argued that the tower handled the two planes in accordance with air traffic control regulations.

The relevant passage reads: "Separate a departing aircraft from an arriving aircraft on final approach by a minimum of two miles if separation will increase to a minimum of three miles within one minute after takeoff."

In practice, this is interpreted to mean that there must be no less than two miles between an arriving and departing plane when the latter starts its takeoff roll, FAA sources said. When takeoff clearance was delivered to Air Florida Flight 90, the sources said, radar showed the planes were 2 1/4 miles apart, meaning that the controllers did their job correctly.

However, until a departing plane accelerates to the speed of the arriving plane behind it, distance between the two planes will continue to close. Starting with a 2 1/4-mile separation, minimum separation between the Eastern and Air Florida planes eventually had to be considerably less than two miles--especially since the Air Florida plane apparently took 17 seconds longer than normal to reach liftoff speed.

But separation of less than two miles in itself would not be a violation, the FAA sources argued, as long as the departing plane was moving.

FAA sources confirmed that the man handling the two jetliners was a supervisor. The FAA has assigned many supervisors to controller positions since the strike began and has argued that they are fully qualified, if not more qualified than the strikers they replaced.

The sources were unable to say when the supervisor last had an "over the shoulder" evaluation of his proficiency, which is normally required every six months. This requirement was suspended by the tower chief after the strike began on grounds there was not sufficient time to conduct them. The supervisor had maintained other qualification requirements, however, the sources said.

The Professional Air Traffic Controllers Organization (PATCO), the strikers' union, has argued that controlling planes depends to a large degree on good judgment and constant practice. Given the falling snow and poor visibility on Jan. 13, the supervisor erred in placing planes so close together, even if he was staying within legal limits, it is argued.