The threatened air traffic controllers strike this morning could ground half the nation's regularly scheduled commercial flights, standing passengers and jamming bus and train terminals, according to some government predictions.

But airline spokesmen in the Washington area said yesterday they will adopt a wait-and-see attitude, attempting to accommodate as many passengers as possible if the strike materializes.

The Federal Aviation Administration has a three-step contingency plan to place for 7 a.m., when the strike is scheduled to begin. Each step depends on the severity of the walkout.

The first step, according to FAA spokesman Dennis Feldman, calls for the agency to implement "flow control," slowing regular schedules by all airlines. The extent of the slowing, again, will depend on how many controllers report for work.

The second step will go into effect at the nation's 23 busiest airports, including National Airport, if the strike forces the FAA to curtail regular air traffic by 25 percent, Feldman said. In that case, the FAA will direct all airlines to cancel a certain percentage of their scheduled flights.

If the striek forces a 50 percent cutback, then the FAA would issue another directive for more extreme measures.

The third step -- called the "comprehensive step" by the FAA -- will establish a system of priorities for all air travel, with national defense and medical emergency flights coming first followed by scheduled flights of 500 miles or more,

None of the major airlines serving Washington would disclose specific contingency plans for flight cuts at the three area airports, saying that those decisions will be made this morning as the strike's impact becomes apparent. Most airlines said they were booking flights as normal yesterday and were not advising passengers to take special precautions.

There was little indication of worry at National Aiport yesterday. William . Doran, a Fairfax County businessman, said he was not letting the threatened strike get in his way.

So he changed his American Airlines flight so he could reach Oklahome City last night instead of this morning. "I just though I'd avoid getting hung up," he said.

Doran was the exception at National, where thousands of passengers -- the usual crowd for a Sunday afternoon -- milled about, few of them apparently troubled by the strike threat. "People are becoming more accustomed to this sort of thing, I guess," said Eastern ticket agent Elizabeth Jackson. "Last month, [when the controllers first threatened to strike] the terminal was so crowded you could hardly walk."

Jackson said Eastern had extra ticket and gate agents working overtime yesterday because of fears of a sharp influx of passengers. But at 3 p.m. crowds were about normal for a Sunday afternoon, she said.

In contrast to last month, the airlines and the FAA took a more optimistic attitude toward the propects for flight operations. Government officials noted that both the FAA and the airlines have had more time to prepare for the strike.

A spokesman for Eastern Airlines, one of the major carriers at National, said the airlines would "try to operate our schedule -- including the [New York] shuttle -- as published."

Officials at USAir and United also predicted they would meet passenger demands today.

Train and bus officials also were getting ready for an expected increase in business, reporting that reservations were running 20 percent above normal yesterday.

"We worked out all our plans in June the first time this came up," said Jung Ha Lee, a spokesman for Amtrak. "This time we'll simply implement them."

Amtrak's plans include adding cars to most Washington trains, beefing up station and train personnel, and stocking more food on those trains. b

But Lee added that Amtrak is running at capacity now and could oly accommodate air-strike overflow passengers by adding some cars currently being repaired. Although these cars would meet safety regulations, he said, many have broken windows, toilets and chairs.

"The reality is that we don't have enought equipment to deal with the overflow," Lee said.

Spokesmen for Greyhound and Trailways bus lines said, however, that their contingency plans will easily accommodate stranded plane travelers. Both companies are ready to add 30 buses on routes under 500 miles, bringing their operations to Christmas and Thanksgiving levels."

As six weeks ago, Greyhound announced yesterday that all supervisors' vacations had been postponed. All telephone operators and other staff were put on overtime shifts. "It's the same plan as last time," said Camille Ghanayen, a Greyhound spokeswoman. "If they go this time, we're ready for them."

But at Union Station and at the bus terminals, it was just like any other Sunday, according to many employes. Union Statin passenger service aide Ben Hall predicted that passengers would wait until the 11th hours before looking for other types of transportation. "They remember that last time they canceled the strike during the night," he said.