Mutual of Omaha, the nation's largest issuer of air travel insurance, reports that the number of policies sold during the first two weeks of the air controllers' strike was up 25 to 30 percent over the preceeding two weeks. The percentage is slightly higher at the Washington area's three airports, according to Mutual.

The Phobia Center of Washington, which offers a Fearful Flyers Program, reports a 50 percent increase in phone queries from nervous travelers.

Area travel agents report they have been swamped by callers wondering about the schedule reliability and safety of air travel. And though agents say they are able to allay most callers' concerns, they add that some customers have chosen to cancel their flights and switch to other forms of transportation.

Those reports obviously are far from conclusive, but they do indicate that the walkout and warnings by the striking air traffic controllers apparently have heightened public concern about the safety of air travel.

Airlines, airports, pilots and the Federal Aviation Administration continue to provide assurances that the skies are safe and delays are minimal, but passenger levels remain down and some who are flying, such as Bill Ruh, are uneasy.

"Frankly, I'm a little nervous about flying right now," said Ruh, after purchasing $80,000 worth of Mutual of Omaha flight insurance for his afternoon flight to Minneapolis yesterday at National Airport. "I just think there is more of a chance of a crash during a strike.

"They are making a big issue about safety everywhere and I am thinking more about it," said Ruh, whose only previous purchase of flight insurance came five years ago.

"It's not that I'm buying insurance because I expect the worst to happen," he said. "But I don't want to be the first one to prove wrong everybody who's saying how safe the skies are."

A former Pan Am stewardess from Alexandria was a first-time customer at National's flight insurance counter. "I'd prefer not to be flying right now, but the weekend plans were too good to pass up," she said, preferring not to be identified because of her former job. "I'm not certain the air controller situation is well in hand. The people in the towers are overworking and the military people are not used to it. I bought the minimum amount of insurance," she said. "I'm an optimist."

Ed Chamberlain of Mutual of Omaha said the company was not expecting the increase. "We do attribute it in part to the strike," he said, "But August is a high volume month for us, anyway."

August for the airlines has not been high volume. The Air Transport Association reports that the nation's air carriers are getting 100,000 to 150,000 fewer passengers daily as compared to the normal August plane loads. "There is no question that people are concerned," says spokesman Daniel Henkin, "but you always have your white knucklers."

A recent study by Boeing Aircraft counted 25 million adult Americans who are afraid to fly and found that one out of 10 simply tolerates the experience in a state of fear. It is difficult to estimate by how much their ranks may have swollen during the strike, but The Phobia Program of Washington says the strike-spawned Fearful Flyers are a new breed.

According to John Lau, a manager at the center, nervous flyers calling before the strike were ready to try to combat their flying problem. The center will not say how many callers it now is getting, but Lau said the recent callers "are ambivalent about whether they want to tackle their problem right now during the strike and want some reassurance."

"Many people are simply not going to fly now," said Lau. "More are certainly afraid now. We hear most often from those who can't avoid flying . . . The major concern is safety now, whereas in the past, people who have a flying phobia have had the major concerns of claustrophobia and loss of control."

Lau said treatment of those with flying phobias is more difficult now, "with the air control situation and the controllers union running around and talking about the possibility of mid-air collisions. Phobic thinking is 'what if' thinking. All this news is feeding right into those concerns for people with flying phobias. It has complicated the issue and made treatment a lot more difficult because it's harder to demonstrate to them that their fear is unreasonable."

Travel agents also have been doing a lot of reassuring by phone to customers who are concerned not only about safety but about inconveniences and delays.

"Sure we have had some cancellations of flights, just like every other travel agency," said Jim Kerr, a travel agent at Ask Mr. Foster. "And some customers, if their des--tination is close enough, have decided to drive or take the train."

Some of those who have decided to fly may be employing a time-honored technique to deal with their anxieties. Western Airlines, for example, reports that despite a 5 percent ridership drop-off during the strike, champagne consumption is at year-ago levels.