Delta Air Lines Inc apologised for cancelling thousands of flights during the busiest travel weekend of the summer as many of its systems failed following a catastrophic CrowdStrike Holdings Inc IT outage.

The Atlanta-based carrier stood out as the airline most affected by Friday’s technology meltdown, with US carriers by far the hardest hit over the weekend. The issues continued early on Monday, with Delta cancelling more than 600 flights, or about 18% of its domestic schedule, according to flight-tracking website FlightAware.

Delta chief executive officer Ed Bastian blamed the impact on a “significant number” of functions that rely on Microsoft Corp’s Windows operating system, including its crew-tracking tools, rendering the airline unable to process large volumes of changes to flight personnel.

“I want to apologise to every one of you who have been impacted by these events,” Bastian said in a statement, adding, “we understand how difficult it can be when your travels are disrupted.”

All-in, the chaos led to more than 3,500 flights being cancelled through Saturday, Bastian said, with more cancellations rippling through as the airline struggled to regain its footing. The disruption occurring during the profitable peak summer period hurt Delta, with flights exceeding 90% capacity, making it harder to rebook customers. The carrier still was trying to manage crew tracking issues Monday, a spokesman said in an email.

Delta cancelled more than 1,200 flights on each of Friday, Saturday and Sunday, according to FlightAware. The carrier accounted for the bulk of Sunday’s cancellations, while United Airlines Holdings Inc also axed several hundred flights. 

American Airlines Group Inc, which was among the first companies on Friday to quickly get back to normal, cancelled fewer than 100 on Sunday. As of Monday, FlightAware showed about 50 cancellations combined for American and United as their operations came back on track.

Delta’s shares fell 3.3% at 9.46am on Monday in New York, while United and American each slid about 1%.

US Secretary of Transportation Pete Buttigieg criticised Delta’s handling of the chaos, citing its “continued disruptions and unacceptable customer service conditions”.

“Delta must provide prompt refunds to consumers who choose not to take rebooking, free rebooking for those who do, and timely reimbursements for food and hotel stays to consumers affected by these delays and cancellations,” Buttigieg posted on X.

Since it erupted on Friday, the global travel chaos has affected airlines and airports to varying degrees depending on the systems impacted and the vendors used.

Some self-check in airport systems were rendered useless, passengers couldn’t access ticket reservations, or airlines were not able to use cockpit communications systems. At some airports, airlines resorted to checking in passengers manually with hand-written boarding passes.

A faulty software update by cybersecurity firm CrowdStrike was responsible for 8.5 million computers worldwide seizing up, affecting everything from medical appointments to investment banks and train operators.

After Microsoft, CrowdStrike is the second-biggest maker of “modern endpoint protection” software and controls 18% of the US$12.6 billion market, according to research firm IDC.

  - Bloomberg