Air traffic controllers cannot squeeze commuter-plane departures into short gaps between arriving jets on the same runway after dark, federal regulators said.

The nationwide change in the way airports operate could potentially increase delays during busy periods.Federal Aviation Administration spokesman John Clabes on Friday called the order for the temporary ban "fairly drastic and unusual."

The FAA order came four days after reviewing a near-crash at the Dallas-Fort Worth airport. An American Airlines MD-11 jumbo jet flew as low as 50 feet over a Lone Star Airlines' commuter plane that was preparing to depart Monday from the same runway. There were 68 people on the jet and 10 on the smaller plane.

Regulators said the mishap, the fourth nighttime close call on a runway nationally since September, closely mirrored the circumstances of a 1991 crash in Los Angeles that killed 34 people.

In that accident, a USAir jetliner cleared to land came down on a Skywest Airlines' commuter plane.

"Obviously, we've got a situation that needs to be corrected," Clabes said. "Since most of the problems with this procedure have happened at night, it's best to stop doing it."

The previous practice, which was an efficiency move, called for a smaller plane to taxi onto the runway immediately after a jet lands, but in front of another jet that still may be several miles from the airport.

The so-called "taxi into position and hold" procedure would normally enable controllers to maximize increasingly busy runways.

In the practice, the smaller plane is cleared to take off as soon as the first jet is clear of the runway.

Under the FAA's order, controllers nationwide will undergo more training during the next two weeks. The agency will also examine ways to make the process, which will still be used during the daytime, safer.

National Transportation Safety Board investigators determined that in the Dallas-Fort Worth mishap, a tower controller cleared both planes to be on the runway at the same time without following proper procedure.

A supervisor who noticed the impending conflict on a ground radar notified the controller.

The jet was unaware of the close call until after the 9 p.m. landing.