LOS ANGELES, FEB. 5 -- The cockpit crew of an airplane waiting to cross the runway where a USAir 737 slammed into a SkyWest commuter plane inadvertently broke radio contact with the control tower several minutes before the crash, delaying operations until a controller reestablished contact on another frequency, federal investigators said today.

The problem was one of several facing the controller in charge of runway 24L-6R Friday evening after she had allowed the SkyWest flight to position itself for takeoff on the runway. After reestablishing contact with Wings West Flight 5006 and getting it across the runway to the terminal, she cleared USAir Flight 1493 to land, apparently forgetting that SkyWest Flight 5569 was still on the runway. The accident killed 34 people.

Jim Burnett of the National Transportation Safety Board said investigators have determined that the 737 hit the smaller Fairchild Metroliner six seconds after touchdown at a speed of 117 knots, about 134 mph. He said the smaller plane was so crushed that fire fighters did not realize a second plane was involved until they spotted a propeller in the wreckage, something a jetliner would not have.

The accidental radio break by a third plane adds a new dimension to the safety board's investigation of the crash and its growing concern over crew fatigue in transportation accidents. Burnett said the Wings West crew members blamed their lapse partly on fatigue, saying they were in the seventh hour of an 11-hour workday.

The "lapse in communication" added to the controller's workload, Burnett said.

The first officer of the Wings West flight, also a Fairchild Metroliner turboprop, told investigators he routinely switched channels to talk to his company. Normally, he would continue to monitor the tower frequency while talking on the Wings West frequency, but he said that on Friday evening he inadvertently disconnected the tower frequency.

The unidentified controller, working with a second controller, then "had to spend some time trying to reach Wings West," Burnett said.

The SkyWest plane had been told to taxi into takeoff position and hold until the Wings West plane on a taxiway on the other end of the runway had crossed. The Wings West flight had just landed on parallel Runway 24R.

A controller not involved in the investigation, who asked not to be identified, said that after losing radio contact with one plane most controllers would not have allowed other planes on the runway until reestablishing contact. Under these circumstances, most controllers would just have frozen the runway and taxiway traffic in place, he said.

"There would be a real tendency to freeze the situation where it's at until you know," the controller said.

Burnett also said today:

For medical reasons, investigators will be unable for a few days to interview the controller who cleared both planes on the runway. He declined to elaborate.

Tower controllers interviewed today said they did not feel that their view of the intersection where the crash took place was obstructed, as a safety board investigator had said.

The cockpit voice recorder on the Boeing 737 would be only partly helpful in the investigation. While it clearly recorded radio transmissions, it had "a problem" recording conversations in the cockpit, he said. Board technicians will attempt to enhance the quality of the tape.