Delta Air Lines’ operational meltdown and its handling of passengers has prompted warnings from U.S. Transportation Secretary Pete Buttigieg as the mess stretched into its fourth day on Monday, with trips disrupted for hundreds of thousands of people worldwide.

Atlanta-based Delta has canceled more than 5,000 Delta and Delta Connection flights since the initial global technology outage early Friday morning, according to FlightAware.com. Frustrated passengers have been sleeping overnight at Hartsfield-Jackson and other airports, and waiting for hours in long lines that stretch down the terminal.

Passengers slept at Hartsfield-Jackson early in the morning of Monday, July 22, 2024, amid a Delta Air Lines operational meltdown. (John Spink / John.Spink@ajc.com)

Credit: John Spink/AJC

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Credit: John Spink/AJC

While some other airlines including United have had mass flight cancellations since the outage — which stemmed from security updates related to Microsoft 365 apps and services from cybersecurity firm CrowdStrike — Delta has had far more flight cancellations than any other U.S. airline.

It has yet to recover. By about 4:15 p.m. Monday, Delta had canceled more than more than 825 of its mainline flights for the day, FlightAware data showed. By comparison, American had canceled about 54 and United had canceled about 21.

Buttigieg spoke with Delta CEO Ed Bastian on Sunday afternoon and reminded him of Delta’s obligations to its customers and the DOT’s enforcement authority, according to the DOT.

“We continue to receive reports of unacceptable disruptions and customer service conditions at Delta Air Lines, including hundreds of complaints filed with our department,” Buttigieg said in a written statement. “I have made clear to Delta that we expect the airline to provide prompt refunds to consumers who choose not to be rebooked, and free rebooking and timely reimbursements for food and overnight hotel stays to consumers affected by the delays and cancellations, as well as adequate customer service assistance to all of their passengers.

The DOT says Delta is legally obligated to care for its passengers and adhere to its commitments to customers, and to inform customers that they are entitled to a refund if a flight is canceled and they opt against a rebooking.

That also prompted Buttigieg to post on the social media platform X about reports of some airlines offering only flight credits to passengers for canceled flights.

No one should be stranded at an airport overnight or stuck on hold for hours waiting to talk to a customer service agent,” Buttigieg said in his statement Sunday. “I will ensure that our department supports Delta passengers by enforcing all applicable passenger protections.”

The DOT said the cancellations and delays are considered “controllable,” triggering a higher degree of commitments for handling passengers whose flights are disrupted.

Bastian told employees in a video message Monday that Buttigieg “wanted to remind me of our commitments to our customers, (to) which I said: You do not need to remind me — I know, because we do our very best, particularly in tough times taking care of our customers.”

He said Delta employees were “working around the clock with every potential resource, idea, opportunity to get our operation back on track.”

In a letter to customers Sunday, Bastian apologized to all who were affected by the disruptions.

Bastian also explained a reason behind why Delta’s flights have been so severely disrupted by the outage.

He wrote that Delta has “a significant number of applications that use (Microsoft systems), and in particular one of our crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown.”

“Our teams have been working around the clock to recover and restore full functionality,” Bastian wrote Sunday.

Delta has extended a travel waiver to allow customers who have flights booked through Tuesday to change their travel plans without paying a higher fare with certain conditions.

Bastian encouraged customers to “take advantage of that flexibility if possible.”

Some travel experts have advised travelers to avoid flying Delta while the airline’s operation remains dysfunctional and beleaguered with a backlog of thousands of stranded passengers trying to get home or to their destinations.

Air travelers at Terminal North storm ticket counters Friday, July 19, 2024 as a massive outage is affecting Microsoft users around the globe, disrupting airlines, railways, banks, stock exchanges and other businesses. (John Spink/AJC)

Credit: John Spink

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Credit: John Spink

Bastian also said the airline continues to “offer meal vouchers, hotel accommodations and transportation where available.” Many Delta customers have said they have not been offered vouchers.

Delta also Sunday said it is offering affected customers an apology and frequent flyer miles or a travel voucher as an “acknowledgment of the disruption,” along with guidance for how to file for reimbursement of unexpected expenses.

Along with the mass flight cancellations, Delta’s “Fly Delta” app has also been overwhelmed by demand and thus plagued by spotty service, while passengers have also encountered hours-long waits for customer service help.

Amid the meltdown, Delta executives were due to speak at a major convention of some of its largest and most important corporate customers that started Monday in Atlanta.

It’s yet to be seen how many of the corporate travel managers who were planning to head to Atlanta for the Global Business Travel Association convention at the Georgia World Congress Center will be unable to make it due to the thousands of Delta flight cancellations over the weekend and into Monday.

Delta sent a message Sunday to attendees apologizing to its customers traveling to the convention who had their travel plans disrupted.

The airline told attendees it was “working to resume normal operations.”

Massive multiday operational meltdowns can result in a financial hit amounting to tens or hundreds of millions of dollars of impact to airlines, which typically report such charges in financial reports after the quarter ends.

Southwest Airlines took a hit of more than $1.1 billion from its holiday meltdown that started in Dec. 21, 2022, and extended into January 2023. The financial impact included lost revenue, compensation to customers and reimbursement for expenses incurred when their trips were disrupted.

The DOT also announced a $140 million penalty against Dallas-based Southwest for violating consumer protection laws during and after that meltdown, including a $35 million fine and offsets for compensation to passengers.

A Delta computer outage in 2016 that forced 2,300 flight cancellations caused a $150 million hit to the airline’s finances.

An April 2017 meltdown of Delta operations — triggered by a day of thunderstorms at its Atlanta hub that led to 4,000 flight cancellations over several days — took a $125 million bite out of Delta’s profits, due to lost revenue and storm-related costs.

In the aftermath of that event, Delta pledged a “complete deep dive across the organization to find out lessons learned across all the different divisions.”