A National Airport controller told federal investigators yesterday that he had followed standard procedure in clearing Air Florida Flight 90 for takeoff as an Eastern Airlines jet closed in for a landing behind it on the same runway.

Testifying during the second day of hearings into why the Air Florida Boeing 737 crashed on Jan. 13 into the Potomac River 30 seconds after takeoff, killing 78 people, tower supervisor Stanley Gromelski discounted suggestions that he let the two jets come illegally close to each other.

Gromelski said he followed the standard interpretation by National controllers of so-called "separation" rules on the allowable distance between two planes. "That's what I was taught 20 years ago," he said. "I've been using it for 20 years and it's never failed."

Among the issues the National Transportation Safety Board wants to settle during its hearings is how close the two planes passed and whether the Eastern plane, which was approaching from the south to land on Runway 36 as Air Florida prepared to take off on it, touched down before Air Florida had lifted off from the northern end.

Investigators discount suggestions that the Eastern jet played a role in the accident, but say that two planes on the runway together or illegally close in the air would be a safety violation worthy of investigation in its own right.

Gromelski's performance has assumed larger importance because he is a supervisor who took up increased air traffic control duties after most of National's controllers joined a nationwide strike last August and were fired. Federal officials have said that despite lower staffing, the air traffic control system remains safe.

Tape transcripts show that Gromelski cleared Air Florida to take off seconds before 4 p.m. on Jan. 13, as heavy snowfall obscured visibility. Then he told the pilots, "No delay on departure, if you will. Traffic's two and a half miles out for the runway," a reference to the Eastern jet.

Yesterday, the pilot of the Eastern plane, Lawrence S. Jones, testified that when that message was picked up in his cockpit, his flight engineer remarked: "It looks like this is going to be close." Jones said he did not respond to the comment, as he considered 2 1/2 miles to be adequate separation, and continued his approach.

Federal Aviation Regulations state: "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."

Gromelski said that this commonly is interpreted to mean that the arriving plane must be at least two miles out when the departing plane is cleared for takeoff, though he conceded under questioning from Air Florida officials that this interpretation does not exist in writing.

Because the departing plane takes time to accelerate, separation might close to less than two miles, but that is not a violation, he said. Radar data indicates Air Florida and Eastern came within 0.9 miles of each other.

National Airport controllers often use a rule of thumb that if the arriving plane is near the Woodrow Wilson Bridge, there is time to safely send off another plane, according to Gromelski.

Gromelski said that the falling snow obscured his view of Flight 90 as it started its takeoff roll. Responding to questions, he acknowledged that an abnormally long time elapsed from takeoff clearance to the time that it came into his view.

Investigators believe that Flight 90 had trouble accelerating and lifted off at about 35 seconds after 4 p.m. A board report released Monday said that the Eastern jet's flight data recorder and radar data indicated that that plane touched down either at 28 seconds or 31 seconds after the hour. "It is possible that both aircraft could have been on Runway 36" at the same time, the report said. Yesterday, the board formally withdrew that report on the grounds that additional analysis remained to be done. Investigators said that flight data recorders give imprecise readouts, making it difficult to pin down times to the second.

In questioning Gromelski yesterday, investigators were not allowed to refer to the document. But the issue was clearly on their minds. "Did you see two aircraft on the runway at the same time?" one investigator asked. "No," Gromelski replied.

Investigators also cited a report by a Delta Airlines employe, Richard W. Cordell, that he saw Flight 90 as it rolled down the runway. "Immediately after I noticed the Air Florida 737, an Eastern 727 landed unbelievably close after AF737 the Air Florida jet . I felt it was too close for normal conditions--let alone very hard snow."

Among other witnesses who testified yesterday were:

Rudolph W. West, the ground controller who handled Flight 90 as it taxied for takeoff. Much questioning centered on ways in which planes could be moved quickly from the gate to takeoff. Air Florida had to wait in line and may have accumulated snow or ice on its wings, which damaged their ability to give lift.

Oran Hoover, copilot of a USAir jet that landed after Flight 90 departed and was asked to check for a 737 at the end of the runway. He testified that he saw two tracks far down the runway but no sign of the plane.