The nation's airlines are finding that some passengers seem almost eager to be left behind: * Swissair officials recently told travelers waiting for a Boston-Zurich flight that more seats were needed to accommodate everyone with reservations. Those willing to leave the plane to make room for others were offered a paid overnight hotel stay in Bean Town, a reservation on the next day's flight, and $ 100 spending money. Several passengers snatched at the offer.
* When a flight from Chicago to Hawaii was similarly overbooked a few months back, United Airlines offered volunteers $250 apiece in credit toward the purchase of a future ticket and a seat on the next flight out three hours later. The bearer of the offer faced an onslaught of two dozen eager takers, but was able to accommodate only one. ''The others were kind of disappointed,'' recalls one observer.
For years airlines have oversold flights as one way of protecting themselves against ''no shows'' who book multiple reservations but use only one. United Airlines alone, for instance, estimates that it will have 3.8 million ''no shows'' just this year. Delta has a ''no show'' rate as high as 30 percent on some flights. The airlines' overbooking rationale, accepted by the Civil Aeronautics Board (CAB), is that filled seats make for greater economy and cheaper fares.
Airlines are required by law to compensate anyone involuntarily bumped from his seat. Under CAB rules, which were temporarily waived because of the air controllers strike but will be reinstated Oct. 26, such passengers are entitled to a check for the amount of their flight if the airline can get them to their destination within two hours of their originally scheduled arrival (four hours for international flights). If more than two hours elapse, passengers are entitled to double the price of their ticket.
But most airlines now find that asking for volunteers, as opposed to involuntarily bumping people from flights, is both cheaper and increasingly popular with passengers. Far fewer people are being bumped than in past years.
''Well over half of those bumped now are volunteers - their numbers are way up,'' confirms Pat Kennedy of the CAB Bureau of Compliance and Consumer Protection. She points out that the volunteers are saving the airlines a good deal of money. ''For the first half of this year the average payment was only $ 120 compared to $232 for those bumped involuntarily.''
Adds American Airlines spokesman Al Becker: ''Our volunteer program has been so successful that it takes care of over 90 percent of all oversale situations. Today it's very rare that we have to deny a seat to someone who has a pressing need to travel.''
Recruiting of volunteers may take place anywhere from the boarding gate to on board the plane itself. Airlines say they find that the ''carrot'' that works best for both them and their customers is a travel voucher applying to a future ticket purchase. For many airlines, the idea evolved from an original program of cash payments.
''We didn't just hit on this all of a sudden - we came to it after trying a number of other methods,'' says United Airlines spokesman Joe Hopkins. ''We make it attractive to them - it's a pretty good piece of change.''
One factor that helps account for the increasing number of people who voluntarily vacate their seats is an increase in ''discretionary travel.'' Experts say about 6 percent of those boarding any flight usually can afford to be flexible in their travel plans. Another factor is the airlines' growing skill in persuading passengers to step aside. One airline even has a professional auctioneer as a customer service representative.
Fewer than one-tenth of 1 percent of all domestic airline passengers are bumped either willingly or against their wishes. But their numbers run well into the thousands, and those asked to take a later flight often remember it well.
Anyone involuntarily bumped - usually those paying the lowest fares or late arrivals at the boarding gate - can take an airline to court rather than accept the compensation offered. But court cases have been rare. Perhaps the most famous case was consumer advocate Ralph Nader's suit against Allegheny Airlines for bumping him from a 1972 flight to Hartford, Conn., where he was to give a speech.
But the amount awarded in that long-litigated case, including damages both to Mr. Nader and to the organization that had engaged him as a speaker, was only about one-fourth of the $208,000 a jury in the Chicago area recently awarded a retired Illinois Supreme Court Justice and his wife. They were bumped from a Delta Airlines flight to Florida five years ago and claimed they suffered ''humiliation, indignity, and outrage.''
Spokesmen for both the CAB and Delta Airlines say that the case could set a bad precedent. They stress that it is being appealed. Delta lawyers, who insist that the couple was offered a substitute flight that would have delayed them less than two hours, say it was really the industry practice of overbooking that was on trial.
Though the amount of the award, if allowed to stand, could induce other passengers to go to court, most airlines are clearly hoping that their current success in recruiting volunteers will keep the number of forcibly bumped passengers tiny.