From Delta to Southwest, the airlines in the best—and worst—shape going into the holidays

America’s major airlines are bringing back thousands of pilots, flight attendants, and other employees and thinning their flight schedules in an effort to ensure holiday travelers reach their destinations on time. Airlines can’t afford another one of the high-profile breakdowns that have plagued the industry this year, analysts say.

The latest culprit was American Airlines, which canceled more than 2,000 flights Halloween weekend. The company blamed bad weather and staff shortages. It was American’s second breakdown since late spring, when travel bounced back.

Southwest Airlines and Spirit Airlines have had their own meltdowns. Delays have gone up for every major air carrier during the COVID-19 pandemic.

“What’s at stake [this holiday season] is nothing short of the trust of the American traveling public,” Henry Harteveldt, an airline analyst with Atmosphere Research, told Fortune. “If you’re not trusted as an airline, you’re nothing.”

This year’s holiday travel is expected to approach 2019 levels, offering a potential windfall for airlines if they correctly balance staffing and equipment needs with travel demand.

Airlines have been recalling workers who were on extended leave or furloughed, and hiring new ones, including pilots and flight attendants. Many have pared flight schedules for the holidays and plan to have more employees and aircraft waiting to jump in, in case of disruptions. The four major U.S. airlines—American, Delta, Southwest, and United—have lighter schedules during the first half of December, giving them more flexibility with flight and cabin crews, both of which have federal limits on how many hours they can work.

Without a robust reserve, a problem can quickly spread across a carrier’s network, Southwest’s incoming CEO Robert Johnson said during the airline’s third-quarter earnings call last month. “It deteriorates faster as you eat up that margin that you’ve got, especially on the crew side.”

Johnson assured financial analysts and reporters on the call that Southwest is ready for the holidays. Like its competitors, the airline has been hiring thousands of employees across the company in recent months.

The industry slashed headcount in 2020 largely through buyouts, voluntary leave programs, and furloughs, as well as cutting contractors and vendors. American Airlines, for example, trimmed its payroll by $500 million last year, according to the company.

However, adding workers at airlines is not as simple as opening a spigot. It can take weeks to months to get new workers up to speed and on the job. Pilots take the longest, requiring background checks, psychological assessments, and medical exams, as well as classroom and cockpit training. The process typically takes two to three months.

Despite executives’ confident assurances that major disruptions are in the past, airlines can still be caught off guard. American and Southwest have added capacity faster than other major airlines. They also have suffered the biggest schedule meltdowns in the industry this year.

Ultimately, airlines have to contend with conditions beyond their control, Cowen airline analyst Helane Becker said. “If there is bad weather that causes delays and flight cancellations, we can expect a similar experience during the holidays as we’ve seen recently.”

“United and Delta have fewer flights and more people, so they seem to be better positioned than American, Spirit, and Southwest right now,” Becker told Fortune. “Depending on hiring between now and month’s end, we could see improvement across the board.”

However, even Delta flight crews are worn out, according to a letter from Jason Ambrosi, chairman of the Air Line Pilots Association unit at Delta, to union members.

“We have heard from pilots loud and clear that you are justifiably dissatisfied and frustrated with fatiguing, poor-quality rotations due to pilot staffing shortages across most categories,” the Atlanta-based pilot said in the Oct. 18 letter. Delta and ALPA declined to comment on the letter.

The COVID-19 vaccine mandate for federal contractors is another wild card. Most major airlines had been requiring pilots be vaccinated by Nov. 24, the day before Thanksgiving. (The Federal Aviation Administration says pilots cannot fly for 48 hours after getting the shot.) But after the Biden administration announced on Thursday that the vaccination deadline for federal contractors had been pushed back from Dec. 8 to Jan. 4, American Airlines pushed back its own deadline to January, and other airlines are expected to follow suit.

Executives at major airlines say most employees are vaccinated, but only United Airlines has already required workers get the vaccine or an exemption. Federal vaccine mandates also apply to TSA and other workers critical to airport operations.

Given the industry’s travails so far this year, adding the mandate to the holiday mix made “absolutely no sense,” analyst Harteveldt said. “There could not have been a worse time to impose a vaccine mandate.”