The problems began early on Monday when a critical power control module at the airline's technology command centre malfunctioned, causing a surge to the transformer and a loss of power, the company said.
While power was stabilised and restored quickly, after the malfunction "critical systems and network equipment didn't switch over to backups. Other systems did," said Delta COO Gil West. The systems that failed to switch over suffered from "instability", he continued, affecting the performance of a customer service system used to process check-ins, conduct boarding, and dispatch aircraft.
Because airlines are so tightly scheduled, the delays and cancellations have had a major knock-on effect, because flight crews and planes are not where they should be. Additionally, those flight crews can only be on duty for a limited time before rest periods are required by law, with crews working in three or four day rotations. "Multiplied across tens of thousands of pilots and flight attendants and thousands of scheduled flights, rebuilding rotations is a time-consuming process," said West.
"We are sorry our technology failure inconvenienced so many customers and are grateful for the faith and patience so many have shown in Delta," he added.
Delta cancelled around 1,000 flights on Monday and about 775 flights on Tuesday as the airline worked to establish normal operations. The company said it expected to cancel about 90 flights on Wednesday morning, and return to normal operations later.
Delta CEO Ed Bastian said over the past three years the company had invested "hundreds of millions of dollars in technology infrastructure upgrades and systems, including backup systems to prevent what happened yesterday from occurring".
He said in a video message: "I'm sorry that it happened and I don't have the final analysis of what caused the outage. We did have a redundant backup power source in place. Unfortunately some of our core systems and key systems did not kick-over to the back-up power source when we lost power and, as a consequence of that, it caused our entire system effectively to crash and we had to reboot and start the operation up from scratch."
Update: Delta said it planned around 150 cancellations for Wednesday and said the systems that allow airport customer service agents to check-in and board customers, and dispatch aircraft are functioning normally. It said the bulk of delays and cancellations were the result of flight crews displaced or running up against their maximum allowed duty period following the outage.