Microsoft Says Delta Was Largely Responsible for Flight Cancellations Amid Tech Outage

Microsoft criticized Delta Air Lines on Tuesday for overstating the technology company’s role in a costly disruption that led the airline to cancel thousands of flights last month.

In a letter between lawyers representing the two companies, which was reviewed by The New York Times, Microsoft said Delta had sought to deflect from its own shortcomings by blaming the technology giant for its problems.

Delta’s comments, both publicly and in legal correspondence, are “incomplete, false, misleading and damaging,” Microsoft said in the letter, which was addressed to David Boies, a prominent lawyer whose firm is representing Delta.

“The truth is very different from the false picture you and Delta have sought to paint,” wrote Mark Cheffo, a partner at the Dechert law firm who is representing Microsoft.

In mid-July, a cybersecurity company, CrowdStrike, issued a flawed software update to customers who run Microsoft Windows, effectively shutting down various computer systems of lots of businesses, including several airlines. Most carriers bounced back relatively quickly, but Delta struggled for days, ultimately canceling about 5,000 flights over four days, or more than a third of its schedule.

Delta’s chief executive, Ed Bastian, has said the episode had cost the airline about $500 million. Last week, he told employees that he had hired Mr. Boies’s firm, Boies Schiller Flexner, to pursue legal claims against Microsoft and CrowdStrike, which also rebutted Delta’s claims this week.

In its letter on Tuesday, Mr. Cheffo said that Microsoft “empathizes” with Delta and its customers and that while the technology company was not at fault, it had offered to help the airline for no charge after the outage. Microsoft repeated that offer over five days, from July 19 to July 23, but was turned down each time, it said.

The technology company’s chief executive, Satya Nadella, also reached out to Mr. Bastian on July 24 but never received a response, according to the letter. (In its letter, CrowdStrike said Delta had rejected or ignored its offers for help, too.)

Microsoft said Delta had probably turned it down because the airline was struggling the most with a computer system that it uses to track and schedule the airline’s crew and that was being serviced by other companies, including IBM. It also accused Delta of using outdated information technology.

“Our preliminary review suggests that Delta, unlike its competitors, apparently has not modernized its I.T. infrastructure, either for the benefit of its customers or for its pilots and flight attendants,” Microsoft said in the letter.

The airline rebutted that suggestion in a statement on Tuesday.

“Delta has a long track record of investing in safe, reliable and elevated service for our customers and employees,” the company said. “Since 2016, Delta has invested billions of dollars in I.T. capital expenditures, in addition to the billions spent annually in I.T. operating costs.”

Microsoft demanded that Delta preserve various documents related to the disruption, saying Microsoft would “vigorously defend itself” if the airline filed a lawsuit.