The businesswoman in the aisle seat had planned to read on the plane from Houston to Tucson last week, but the airline industry's new version of musical chairs imposed other priorities.

A family of four boarded. As is increasingly the case, their assigned seats were not together and the plane, a United Express CRJ-700 regional jet with 66 seats, was nearly full. So a little girl, probably age 6, slid into the window seat next to the businesswoman.

And because the child was chatty and a bit uneasy about traveling many rows away from her parents, the woman spent the two-and-a-half-hour flight not reading, but entertaining the girl with talk and word games. By the end of the flight, the girl was playing happily on the woman's iPad.

When the flight landed in Tucson, my wife, who had been observing from a nearby seat, said to the woman, "I'll bet you didn't expect that." The reply came in four words, each punctuated with a period. "No. I. Did. Not."

That's a tiny slice of the new reality in domestic air travel, with airplanes flying more full than ever, and airlines shuffling their fleets to eliminate smaller jets and focus more on flying bigger planes in major markets that provide the most revenue.

In that process, the cramped 50-seat regional jets that were the backbone of service for midsize and smaller markets are disappearing. In some markets, that means service is being eliminated. In others, though, it means reduced schedules on fewer routes, served by slightly bigger planes like the Bombardier CRJ-700, which is a stretched version of the older 50-seat CRJ-200.

The CRJ-700 holds up to 78 passengers in an all-coach cabin, but can be reconfigured (as the one on United Flight 5176 had been), to accommodate six first-class, 28 "premium economy" and 32 basic coach seats. By the way, whether it carries 66 or 78 passengers, the bigger plane still has only one lavatory, just like the smaller 50-seater it replaced. And its overhead bins and under-seat storage spaces remain tighter than those on full-size airplanes.

A full-plane era

A new era has dawned in domestic flying, as airlines cut routes and capacity and fly planes more full than ever. United Airlines, for example, reported that its domestic load factor, the proportion of seats occupied by paying passengers, was 86.5 percent in the second quarter, which is around the industry average. By comparison, domestic carriers were reporting load factors of only about 78 percent at the same time five years ago, before the recession.

With planes so crowded, getting the seat you want can be a source of growing anxiety and expense. Business travelers now often face the option of spending an extra $15 to $50 or so to book an aisle seat or other preferable seat. Families traveling together on leisure trips are finding it more difficult (or expensive) to obtain seats together because so much of the coach cabin is now reserved for those who pay extra. And airlines really love that new revenue. "Outstanding Economy Plus and premium-cabin upsale-sales drove second-quarter ancillary revenue up 11 percent," Jim Compton, United's chief revenue officer, told stock market analysts in a conference call last week.

Airlines say they will continue to charge extra fees for better coach seats, while shrinking capacity as necessary to maintain higher revenues. "We remain committed to capacity discipline to generate sustained and sufficient profitability," Compton said. He added that United's "regional" capacity (the flights operated by its United Express contractors) was 3 percent lower in this year's second quarter, compared with the same period last year.

Airlines profit, passengers don't

Michael Boyd, the airline forecaster who heads the Boyd Group International, said this week that culling the inefficient 50-seat regional jets from the market makes obvious economic sense to airlines. For passengers, "it depends on where you are," he said. Some midsize airports will lose even more service, but others may gain as airlines consolidate regional networks. Generally, "there will be fewer points served," he said.

Airlines realize that passengers are already avoiding some smaller airports where service has been reduced, and instead driving an hour or more to bigger airports where the schedules are better. To me, this indicates that the airlines believe that passengers simply will "make do" — just as that businesswoman did when she put down her book and instead helped take care of a stranger's child last week.

The new realities include higher fares, fewer schedule choices and more cramped seating — unless you pay extra for the less-awful seats that airlines now so eagerly peddle. Welcome aboard!

"We grew up with air transportation growing and growing," Boyd said. "And now, it's not going to do that anymore."