During the 1990s, most of the nation’s airlines prospered immoderately, achieved admirable new standards of safety and seemed to have at last perfected a business model that would serve them in the more competitive, post-regulatory age.

The only problem was that the 1990s was not a typical decade but a period that coincided with the longest economic expansion in U.S. history.

Still, at least until the events of last September, the air transport industry seemed to have established a tolerable standoff among its passengers, its labor force and its management in which mutual resentment was strong but not debilitating, economy-class food was, well, digestible, seating configurations were excruciating for all but premium-paying passengers and the threat of new entries into the market kept the established carriers marginally honest. There were always low-fare challengers like Southwest Airlines and upstarts like JetBlue Airways to breathe down their necks.

Even the four aircraft destroyed on Sept. 11 were property losses the carriers could have sustained were that day not followed by a collapse of confidence in air travel and unsustainable declines in revenues and profits.

Complicating the airlines’ problems is the post-Sept.11 atmosphere in which business travelers, or at least the employers who pay for the tickets, have suddenly become more resistant to plunking down the high cost of unrestricted fares. If economy-class passengers got the idea that they were being hauled and mauled like sacks of feed, they were. It has always been business-class travelers that airlines most covet. And the new resistance to unrestricted fares deprives the airlines of a major cash cow people to whom cost is no object if it meant getting to a particular meeting on time.

In the meantime, the airline industry, which prided itself on having survived deregulation by the government, lined up like everybody else to benefit from the $4 billion in emergency assistance that became available to the industry after Sept. 11. And this hasn’t been nearly enough to improve balance sheets to a point where carriers can see light at the end of their wind tunnels.

Uncertainty about fuel costs is another destabilizing factor. Airlines live, like everybody else, at a time when a rupture in the Middle East, a revolution in Venezuela or the likelihood of an invasion of Iraq could suddenly drive up their fuel costs. This may be the least of their worries because fuel typically amounts to as little as 10 percent of a carrier’s costs.

The biggest item is labor, which consumes about 40 percent. And if you had to name an industry in which organized labor has prospered in the past 30 years while the percentage of organized workers has been declining overall, it would be the airlines. Outside of overpaid union baseball players, it’s hard to imagine a sector where labor enjoys greater leverage. Airline pilots, the top of this heap, have vast power but so do the newly more militant ranks of flight attendants, who have every right to resent their once-glorified reputation as ornamental sky bunnies.

At this point, the major airlines are reduced to grasping at straws.

In March, they carried 46.5 million passengers, down from 51.8 million in the same month of 2001. This represents a decline of 10.2 percent. The bright side and you had to look hard for it was that this was the smallest percentage decline since September.

As the nuisance of air travel is multiplied by security concerns, passengers have something else to worry about, which is that they may land safely but dead or disabled. A growing number of lawsuits are being brought alleging that passengers were victims of deep-vein thrombosis, which is also known as economy-class syndrome. It typically results from long flights.

The best way to prevent this rare but sometimes fatal syndrome is for passengers to get up and roam about their aircraft. This, of course, puts them on a collision course with heavy beverage carts and with crew members who’d prefer that everybody just sit down and shut up instead of cramming the aisles with their legs and arms.

In any event, it’s clear that airline travel has ended one age and entered another in which flying will be a greater annoyance even if the carriers regain their profitability.