Thanksgiving will test whether airlines have done enough to insulate their operations from shocks that have roiled air travel in recent months.

The Transportation Security Administration said it expects Thanksgiving travel to come close to pre-pandemic levels with 20 million people likely to pass through U.S. airports over a 10-day period. Airlines including Delta Air Lines Inc. and United Airlines Holdings Inc. said they expect the Sunday after Thanksgiving to be the busiest day since the start of the Covid-19 pandemic.

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Thanksgiving will test whether airlines have done enough to insulate their operations from shocks that have roiled air travel in recent months.

The Transportation Security Administration said it expects Thanksgiving travel to come close to pre-pandemic levels with 20 million people likely to pass through U.S. airports over a 10-day period. Airlines including Delta Air Lines Inc. and United Airlines Holdings Inc. said they expect the Sunday after Thanksgiving to be the busiest day since the start of the Covid-19 pandemic.

The increased number of travelers puts pressure on the airlines to deliver a smooth experience after months of stumbles. Short-staffed airlines have been more easily knocked off course. They have struggled to recover from bad weather and other disruptions that they previously might have taken in stride, analysts and consultants said, resulting in dayslong cascades of cancellations and snarling travel for thousands of passengers.

“The network is brittle,” said Samuel Engel, senior vice president of aviation at consulting firm ICF. “With the slightest slip, the airlines are showing they have limited ability to accommodate disruption.”

Nine of the largest U.S. airlines canceled 1.9% of flights from June through the end of October this year—a slight uptick from the nearly 1.6% they canceled during the same period in 2019, according to figures from Cirium, an aviation data provider.

Videos shared online show crowds of people at airports across the country after American Airlines canceled hundreds of flights over the weekend due to bad weather and staff shortages. Screenshot: WSOC-TV via AP The Wall Street Journal Interactive Edition

Frustratingly for travelers, there have been more days of acute misery during that time frame. There were 147 days when these airlines canceled 5% or more of their flights, compared with 85 such days in 2019, according to an analysis of operational data by Brett Snyder, who writes the Cranky Flier blog and runs a concierge travel service.

Airlines have been taking steps to try to address operational problems in time for the holiday surge in demand.

Southwest Airlines Co. and Spirit Airlines Inc. tempered ambitious plans, scaling back the number of flights from what they had previously planned to offer and keeping more employees on call. American Airlines Group Inc. is paying hefty premiums to workers to make sure it can staff flights, including $1,000 bonuses for some workers and as much as triple pay for flight attendants with perfect attendance. Southwest is offering its frequent flier points to certain workers for shifts over the holidays, which they can redeem for travel.

“We’re well prepared in terms of staffing,” American Chief Executive Doug Parker said at an industry conference.

Even with recent cutbacks, airlines are planning busy schedules for the Thanksgiving holiday. Airlines are offering 19 million domestic seats the week of the holiday—up 46% from last year and within 9% of 2019 levels, according to data provider OAG.

Airlines were expected to have been able to ramp up quickly after the pandemic because of billions of dollars of federal aid that covered workers’ salaries until the end of September 2021, but it hasn’t worked out that way.

Fearing that the pandemic would outlast government funds, carriers encouraged thousands of employees to leave or take extended leaves of absence last year. Then demand surged in the spring, and bringing workers back and hiring new ones proved to be a slower, more complex process than carriers had anticipated, executives have said.

The missteps irked some lawmakers, including Sen. Maria Cantwell (D., Wash.), who is looking to hold a hearing on airline operational problems.

The results have been an exhausting stretch for workers who have remained on the job, and the poor operations have exacerbated disputes with labor unions at a time when many groups are negotiating new contracts, traditionally a period of heightened tensions.

Unions at some airlines said management teams haven’t gone far enough to fix underlying problems that could flare up again over the busy holiday season, particularly if bad weather or technical glitches once again trigger disruptions.

American’s pilots, for instance, rejected the airline’s offer of premium pay for holiday flying, saying the company’s management was throwing money at a problem without solving it.

American executives said in a letter to pilots following the union’s rejection that they were disappointed and would continue to look for ways to support pilots over the holidays.

The union that represents Southwest’s flight attendants said the frequent flier miles Southwest is offering aren’t enough. Southwest’s pilots said the airline’s management has been too quick to blame sick calls and absences for its problems.

“The truth is that mismanagement—primarily in long-term planning and short-term daily execution—feeds the dysfunction, not pilot sick calls,” the Southwest Airlines Pilots Association wrote to its members earlier this month. “To be perfectly blunt, the operation is a catastrophe waiting to happen.”

A Southwest spokesman declined to comment on the unions’ remarks, but said the airline has reduced flight schedules in November and December to bolster reliability and align with expected staffing, in addition to offering incentives for those who work shifts during the holidays.

A spokesman for Spirit Airlines, which canceled more than 2,800 flights in a 10-day meltdown in August, said the reductions it has made to flying have been working, boosting its place in on-time performance rankings last month.

Problems extend beyond staffing levels. Maintenance work also backed up during the pandemic. Supply-chain choke points make it harder to obtain parts, and retirements among the most experienced technicians mean complicated work is taking longer, resulting in fewer spare aircraft available, said ICF’s Mr. Engel.

In addition, tensions in airplane cabins have been running high this year, with a sharp rise in unruly—and sometimes violent—passenger behavior. Rates of such incidents have dropped off in recent months ahead of the holiday travel season.

Airline networks also changed during the pandemic. Carriers shifted to fly more to popular leisure destinations and thinned flight schedules in cities that had been business travel hot spots. Those changes put new stresses on airports that saw heavier traffic than they were used to handling, something Allegiant Travel Co. CEO Maurice Gallagher Jr. described last month as “leisure destination overload.”

Allegiant canceled nearly 4% of its flights from June through October and 63% were on time, according to Cirium. Mr. Gallagher said resources at the vacation destination airports that Allegiant focuses on have been strained by an influx of traffic from other airlines.

“At this point, the main thing is to make sure we don’t over-schedule the airline, and we have sufficient pilots, flight attendants, maintenance can do their job,” Mr. Gallagher told analysts and investors in October.

United Chief Executive Scott Kirby this week said some of his rivals didn’t leave themselves any margin for error in their zeal to fly full schedules. Executives tasked with generating revenue appear to have overruled those charged with keeping the operation running, he said.

“One hundred percent of the time when airlines have an operational meltdown, it is because the commercial team got out over their skis and didn’t listen to the operations team,” he said. “If you let them run wild, they’ll run the airline off a cliff.”

Write to Alison Sider at alison.sider@wsj.com