An Airbus A320 flew passengers between New Delhi, Bengaluru, Mumbai and Hyderabad on Nov 24 to 25 without the mandatory Airworthiness Review Certificate, or ARC, a key permit issued annually by the regulator after a plane passes safety and compliance checks.

Air India found engineers and pilots had failed to check the aircraft’s documents and that changes were needed to bolster compliance protocols.

“Critical information was not shared with all relevant stakeholders, and opportunities for timely intervention were missed,” said the internal investigation report, which was reviewed by Reuters.

“The incident highlights the need for urgent improvements in process discipline, communication, and compliance culture,” added the report, which was dated Dec 6.

The findings, with a cover letter signed by Chief Operations Officer Captain Basil Kwauk, have been submitted to Indian aviation authorities, but have not been made public.

The report is a stark admission from an airline that suffered its worst disaster when a Boeing Dreamliner crashed moments after take-off in June killing 260 people. Air India has also received warnings for running planes without checking emergency equipment, not changing engine parts on time and forging records to show compliance, as well as other audit lapses.

Air India last week called the Airbus November incident “regrettable” and said that some people had been suspended, while India’s civil aviation watchdog DGCA ordered the aircraft to be grounded and called for an investigation.

Air India, which is owned by India’s Tata Group and Singapore Airlines, said in a statement to Reuters it had proactively reported the incident to the DGCA and had “implemented immediate measures to prevent similar instances”, adding it would continue to strengthen its compliance systems.