INDIA, Dec. 5 -- IndiGo CEO Pieter Elbers issued an apology to staff and passengers as the airline continued to face massive operational turmoil, with 550 flights cancelled nationwide on Thursday alone.

His internal email, which quickly circulated on social media, acknowledged that the carrier had failed to deliver the reliable service it promises to the 3.8 lakh passengers it serves daily.

"We could not live up to that promise these past days, and we have publicly apologised for that," Elbers wrote, calling the last few days extremely challenging for both customers and employees.

Elbers explained that IndiGo's severe disruption resulted from an accumulation of operational setbacks-minor tech glitches, sudden changes in schedules, adve...