LOS ANGELES, FEB. 4 -- The runway collision between a USAir jetliner and a smaller commuter plane Friday night that killed 34 people here seems at first blush to have a simple cause: one air traffic controller under pressure made one mistake.

But as the National Transportation Safety Board's investigation of the accident unfolds, the reasons underlying the accident turn out to be far from simple. They involve not only frailties in human performance, but issues that have plagued the modern aviation era: congested airports, a strained air traffic control system and aging radar and communications equipment.

Runway 24L-6R, where the Boeing 737 slammed into the twin-engine SkyWest Fairchild-Metro- liner turboprop commuter plane, and a nearby runway reopened to traffic this morning. As an early fog lifted, a flow of jetliners arrived and departed, ending two days of major delays at the airport.

The remains of the 737 and the unrecognizable metal tangle of the Metroliner were moved to a hangar, out of sight of arriving passengers. Investigators this evening were to place another Metroliner on the runway and approach it in a helicopter along the same flight path as the 737 in an effort to answer a major question: Why did the 737 crew apparently not see the lights of the smaller plane?

Federal Aviation Administrator James B. Busey flew to Los Angeles late today to confer with officials and controllers. Also, the death toll from the crash rose by one, when a badly burned passenger, Richard D. Ronk of Mansfield, Ohio, died.

On the first day of the investigation, NTSB member Jim Burnett announced that air traffic control tapes showed that one controller allowed the smaller plane onto the runway to await takeoff clearance, then cleared the 737 to land on the same runway. While the board never draws conclusions early in an investigation, the evidence presented by Burnett indicated that the controller, who has not been identified, simply forgot she had allowed the smaller plane on the runway.

As the investigation continued, however, it was revealed that the controller was subjected to a sudden flood of distracting problems during the period leading up to the crash. She had experienced communications difficulties with an Aeromexico flight that required several repeats of instructions. She also had to take time to ask another controller to contact a plane that already had left her radio frequency.

Perhaps because of these distractions, she did not answer the USAir crew's first two requests for landing clearance. During this time, she asked two other aircraft if they were located on Runway 24L, but did not address that question to the SkyWest flight, which was there.

Records show that local and regional Federal Aviation Administration officials had recognized for at least two years that the tower position she occupied that night -- local controller 2 -- was overworked, and an assistant controller was needed to help with the workload. The recommendation by local FAA officials was first made following an Oct. 27, 1988, incident in which a takeoff was aborted because another plane was crossing the runway. And last July 3, a DC-10 almost landed on top of an Airbus.

Competence, initially, does not appear to be a problem. Twenty-six of the 34 controllers who work the tower at Los Angeles International Airport (LAX) are designated "full performance level (FPL)" controllers, the highest level of experience. All controllers in the tower the night of the crash were FPLs, including the controller who allowed both planes onto the runway. She had been a controller since 1982, and became a FPL controller Nov. 10, 1990. Last August, she received routine training in control of intersections, and in October, she received training in awareness of the location of aircraft.

Investigators privately appear impressed with her competence and dedication. In the minutes after the crash, tapes show she professionally and coolly handled initial emergency response duties.

Investigators also have determined that the controller's view of the area was obstructed, visually and electronically.

Burnett said four light poles near the tower obstruct the view of the intersection of Runway 24L and taxiway 45, where the smaller plane was holding. One appeared "dead in the middle of that intersection," he said.

Traffic on the ground is not controlled by radar, but at 12 airports, including Los Angeles, it is used as an aid to controllers. The Airport Surveillance Detection Equipment radars (ASDE-2) at LAX were not operational on the night of the crash; the tower log noted that the radars were down and repair had been requested.

Normally, controllers keep track of the location of aircraft partly by memory and partly by mechanical and paper aids, and issue orders by radio. At most major airports, "local" controllers clear planes onto runways and taxiways, working with "ground" controllers who track planes in other parts of the airport.

However, as aviation traffic expands, this method has shown the strain. The FAA recorded 212 runway "incursions" in 1989. This includes incidents in which aircraft or ground equipment is in the wrong place at the wrong time. Such incidents do not always lead to accidents, but the world's worst crash -- the 1977 collision between two 747s at Tenerife in the Canary Islands, which killed 582, was a result of a runway incursion. Last year, similar but less tragic runway collisions occurred in Atlanta and Detroit.

John K. Lauber, a safety board member, testified last year that a series of accidents "and the numerous close encounters that we hear about two or three times every week convince us that we are relying too heavily on perfect human performance combined with a bit of luck to assure the safety of landing and departing traffic."

Joseph Del Balzo, the FAA's executive director for system development, also testified that the government recognizes the problem and is working to solve it. In the short run, he said, the FAA is developing better runway and taxiway designs, improved lighting and signs, and better training and communication procedures.

In the long run, the FAA is designing the Airport Surface Transportation Automation program, which includes a new generation of ground radar, the ASDE-3. However, the ASDE-3, undergoing tests at Pittsburgh, has experienced numerous problems and may not be ready for deployment for years.

Steve Bell, president of the National Air Traffic Controllers Association, said controllers are handling increasing traffic loads with aging equipment. He said FAA management has tried to develop better labor-management relations in the years since the 1981 controllers strike, but when it comes to safety, "we very much have the same problem and the same conditions" facing the old Professional Air Traffic Controllers Organization (PATCO).

"We are a system of people who operate a system that is not fit for the 1990s," he said. "The people are doing the job, but the equipment is outmoded."

Staff writer Lou Cannon contributed to this report.