Thousands of airline passengers here and at airports around the country scrambled to book new flights yesterday after their travel plans were disrupted by the grounding of the controversial DC10.

The early afternoon decision by the Federal Aviation Administration literally caught some airline crews and their passengers in mid-air, although flights in progress were allowed to continue to their final destination.

Many passengers temporarily stranded by the announcement said they had mixed feelings about the precautions being taken.

"I'm just nervous about the people meeting me in Boston and the inconvenience this is causing them," said Alice O'Brien, whose 4 p.m. United flight from Dulles International Airport was cancelled.

"Even if the flights had not been canceled, I would have taken my chances and gone ahead and flown," said O'Brien, 79 a Boston native. Expressing sympathy for those killed in the DC10 crash in Chicago last Friday, she called it "a shame that it takes this disaster before people start to take action."

FAA officials and spokesmen for the airlines remained uncertain yesterday about the severity and length of the flight disruptions. Airlines affected by the grounding of some of their planes began substituting 707s and 727s for their DC10s.

National Airport was not directly affected by the groundings, according to airline representatives there,since DC10s do not use that facility. But all DC10 flights at Dulles, where Northwest, American and United airlines fly the planes, and at Baltimore - Washington International, where United and World airlines use the jets, were canceled.

FAA spokesman John Leyden called yesterday's grounding the first such action taken in the jet age, and he expressed concern for passengers who already were airborne by the time the grounding decision was made.

"I wouldn't want to be up in one now," Leyden said.

Dulles Airport has as many as 10 DC10s arriving at and departing from that facility daily, according to a spokesman, and five of them were canceled yesterday afternoon.

Among the DC10 flights canceled were one American Airlines flight, two United flights and two Northwest Orient flights, all but one of which were bound for or originating from the West Coast.

At Baltimore - Washington, two United DC10 flights that normally fly to Chicago and Seattle and Chicago and Denver respectively had already been scrapped from United's schedule because the nation's largest air carrier is still recovering from a recently concluded strike by its mechanics.

Of two scheduled incoming United flights, one landed at BWI from Seattle and Denver prior to the announcement. The second was canceled.

Passengers at both area airports had little trouble finding alternative flights yesterday and are not expected to experience long departure delays today and throughout the week, according to airline spokesmen.

Airlines at Dulles added some flights yesterday evening to handle inconvenienced travelers, and the affected arilines said they plan to substitute other aircraft so that passengers can be assured of same-day service in the event their flights are canceled. The airlines, following normal procedures, will attempt to notify all ticketed passengers of any flight changes or substitutions.

Those passengers who caught the last DC10 flights yesterday seemed little disturbed to hear of the grounding announcement upon arriving at their destinations.

One initially startled passenger, arriving at Dulles early yesterday evening on a United flight from San Francisco, recovered his composure and turned philosophical.

"What if we had flown on a DC10 last week?" said J. J. Ewing, a doctor from Danville, Calif. "Four hundred and fifty-five people were killed in traffic accidents last weekend, and 273 people were killed on a plane. Which one do you think is safer?"

Ewing and other travelers on the flight had been given an extra incentive to fly the DC10 because United was giving away half-price coupons on their "Welcome Back Day" to lure back post-strike passengers.

Art Shapiro, flying into Dulles on an American DC10 flight from California said he "half suspected" the flights would be grounded eventually. "I didn't think one bolt holds on the engine.The whole explanation has yet to come out."

But despite his discomfort over last week's crash, Shapiro had no special hesitation about getting on a DC10 flight yesterday.

"It could be a little stupidity or a lot of wine," he said.

Sheila Muddiman, a Pan Am flight attendant from Alexandria, flew into Dulles on an American DC10 from Los Angeles. "I did feel, oh, there is some danger, but I figured they must haved checked it out quite well." She said, adding, "I'm glad my airline doesn't fly any of those aircraft."

Some passengers and airline crew members also joked about or ridiculed the groundings.

"Here comes one now. . . it looks like it's still got an engine on it," said an unidentified United flight attendant as he watched an incoming DC10 flight.

Passenger Peggy Camppeau said her DC10 flight hit two air pockets with heavy turbulence "but we didn't hear any bolts rattle."

One United Airlines pilot who asked not to be identified called the DC10 "still the best plane I've ever flown. Smooth, responsive, dependable. I think they're really going overboard because of one, certainly tragic, but isolated, example."

An American Airlines mechanic at Dulles said a specialist on DC10s would be flown from Los Angeles to do the FAA-ordered inspections, but he said the airline is not yet sure what work must be done.

Lynn Stowell, a Department of Housing and Urban Development official from Seattle who philosophized that "in today's society one must be somewhat of a fatalist," said the safety of the DC10 was the furthest thing from his mind yesterday during his flight into Dulles.

"Why should I be thinking about the DC10?" he asked. "I was more concerned about whether the Seattle Sonics [basketball team] would be winning."

Dr. Kenneth Lynn and his wife, from Potomac, were among the passengers who had to reschedule their flights after the DC10d were grounded. But Lynn had not been concerned about the aircraft's safety adn even joked that he had "brought my own bolt with me."

Another physician, Dr. James King of Los Angeles, said he was "not thrilled by the fact that they waited so long to ground the planes. This could have been done much earlier."

Still, he said after deplaning at Dulles, "if you gotta fly, you gotta fly." CAPTION: Picture 1, Sheila Muddiman, a flight attendant from Alexandria, was among those deplaning from a DC10 at Dulles Airport. By Ken Fell - The Washington Post; Picture 2, Passengers crowd terminal at O'Hare International Airport in Chicago seeking alternative flights to those canceled when the FAA ordered all DC10s grounded. UPI.