As surely as chestnut trees bloom in the spring, so do members of Congress find they have important and pressing business to accomplish in Paris.

This year's group of about 30 members and more than 100 of their spouses, staff aides and backup personnel came to the biannual Paris International Air Show extensively briefed on the pitfalls to avoid while in France. Pitfall No. 1, obviously enough, is to avoid creating the impression with voters back home that they are actually enjoying themselves at public expense.

For a reporter, this means that it's easy enough to grab a senatorial quote on the beauty of the U.S. space shuttle, which has been on display in Europe for the first time. But ask what the members are up to in the evening, and your nervous interlocutor is liable to spot a close friend at the other end of the room.

First stop on a journey of inquiry into congressional goings-on in Paris is the U.S. Pavilion at the air show. The electricity has gone out and Senate Armed Services Committee Chairman John Tower (R-Tex.) is stranded in the restaurant on the top floor. He is anxious to make clear that, as the president's personal representative at the air show, he is quite separate from the other members of Congress.

"No, I don't have anything to do with them . . . I came here on a commercial flight." Asked how his colleagues arrived, he replies mysteriously, "I think you'll have to ask them that question."

The electricity is finally restored, and Tower ("My name is Tower but I don't") reaches up to the microphone to declare the U.S. Pavilion "open for business." The year, 1983, he notes, is the bicentenary of the occasion man "first slipped the surly bonds of Earth" in a hot-air balloon. There is much quoting of Antoine de Saint-Exupery ("that great French writer on aviation") about the human yearning to build cathedrals in the sky, as Northrop's new F20 Tigershark performs noisy aerobatics overhead.

Everybody retires to the "Challenger Lounge" for a champagne reception, where drinks are being served by a coattailed English butler with a top hat who bears a striking resemblance to John Gielgud. Most of the members of Congress are in their hotel rooms recovering from the effects of jet lag after flying in from Washington.

One member who didn't go straight to bed hoves into sight. He's Rep. Norman Y. Mineta (D-Calif.), chairman of the House Public Works and Transportation subcommittee on aviation,, and he has strong opinions on why it's important to have a big American presence in Paris. Tens of thousands of jobs back home, he insists, depend on aerospace exports.

"This represents a $15 billion export industry for us--and we have to do everything we can to maintain our leadership position . . . I have no qualms about fighting back when other countries use unfair methods against us," he said.

Over at the U.S. Pavilion press office, questions about the congressional delegation are deflected with long answers about how the United States intends to conquer new export markets. The most a spokesman will reveal about the activities of the House and Senate members is that they will be meeting with "high government officials."

"These are very intense discussion times. Attending the air show represents a chance for congressmen to gain the kinds of knowledge they need for spending huge amounts of money. They are involved in discussions with countries friendly to the U.S. on subjects of mutual interest," said Donald Creed, press center director.

The plot thickens. A thin ray of light, however, is provided by a handout prepared by the press center for visiting congressional members. "Meet U.S. Pavilion exhibitors from your congressional district," it says. "Photographs will be taken of you with the exhibitor and copies of the photograph and background information about the 35th Paris Air Show will be sent immediately to your Washington congressional office for your own use."

Monday dawns and members of the congressional delegation are still keeping out of the public eye. But a phone call to the Tour D'Argent ("The restaurant which throughout the world symbolizes the luxury of la vie et la cuisine francaises," according to the guidebook Gault-Millau) uncovers the information that duckling No. 622,097 was served to Tower the previous night.

The senator is in good company. Tour D'Argent ducklings have been consumed by Edward VII of England (No. 328), Theodore Roosevelt (No. 33,642), Franklin D. Roosevelt (No. 112,151), Dwight D. Eisenhower (No. 222,580) and Japan's Emperor Hirohito (No. 533,211).

Ever discreet, Claude Terrail, the Tour's ebullient owner, refuses to reveal which other members of Congress have ordered his individually numbered ducklings. All he will say is that Americans attending the air show have been taking up roughly half the 150 places in his restaurant every night. Bookings for the most part were made between six and 10 months ago.

He adds: "We turn another 300 away every day. It's best to stop at 50 percent and take the other 50 percent from somewhere else. Otherwise it would spoil the ambience."

At the Lido, "the largest nightclub in the world with the most elaborate mechanical stage," public relations officer Josephine Gadot reports nightly visits from American parties attending the air show. There are similar random sightings of congressional members or their spouses and entourages at mealtimes at leading Paris hotels such as the Ritz, the Crillon, and the Plaza Athenee. In many cases, the tabs for receptions and meals are picked up by U.S. exhibitors at the air show.

When I dropped in at the Meridien Hotel, where the U.S. delegation has set up its headquarters, many of American representatives were getting ready to go out for the evening. It was a bad time to get hold of them, I was told. A staff aide at the control room for the House Science and Technology Committee said any appointments had to be made in advance by phone and invited me to vacate the premises.

The House Appropriations defense subcommittee people along the corridor were more helpful. The delegation, they said, had just come back from a day's visit to a U.S. Army base in West Germany where, among other things, members inspected the Army's revolutionary new M1 tank with its stabilized gun turret. Other official engagements included a visit to the air show, briefings by U.S. negotiators at the arms-limitation talks in Geneva and talks with the French ministry of defense.

George F. Allen, an appropriations committee staff member, bridles at suggestions that the congressional delegation has come to Paris to have fun.

"Believe me, this is not a plum. Before coming out here, the congressmen worked all day Thursday on the floor of the House voting in the MX missile debate. If anyone thinks it's fun staying up 30-odd hours at a stretch, then they're quite wrong."

Rep. William V. Chappell Jr. (D-Fla.), a defense subcommittee member, said the trip had given him and his colleagues "a unique opportunity" to see what kind of competition America is up against in the aerospace business.

"Here at the air show you can see in a very small space things that it would take you weeks and weeks to get to see otherwise. There are hundreds and hundreds of technologies on display out there . . . It's a shame more members of Congress don't take advantage of occasions like this," he said.

Chappell was also anxious to dispel the impression that the trip was being paid for by the government. The congressional party was flown over to Paris in a military plane, he acknowledged, but strictly on a "space-available" basis. Once they got to Paris, each member of Congress received a regular per diem of $96, but nothing extra for their spouses.

Noting that the price of a room at the Meridien runs about $120 a day, Allen chimed in: "It costs you $30 a day before you can turn out the lights and go to sleep."

Allen acknowledges that there are social events associated with the air show ("It would be silly of me not to") but defends them as being "extremely valuable."

"When wives of congressmen meet wives of another country's government leaders, a feeling develops that we're all in this world together. What's really a shame is that you can't bring American 7-year-old kids over and have them play with other 7-year-old kids," he said.

Seven-year-old kids, it seems, are not eligible for free rides to Paris on government planes--even on a "space-available" basis.