An Air India Boeing 777 aircraft had to turn back after a drop in oil pressure forced the pilots to turn off one of the jet’s engines, India’s aviation regulator said on Monday.

The aircraft, which was headed to India’s financial capital of Mumbai, landed safely back in Delhi and the incident will be investigated, the Directorate General of Civil Aviation (DGCA) said in a statement. Modern aircraft are designed to safely fly and land on a single engine, if required.

Air India has been under intense scrutiny this year after the June 12 crash of a Boeing Dreamliner killed 260 people. The DGCA has flagged multiple safety lapses at the airline, which was previously owned by the government till 2022.

An Air India investigation into why one of its planes conducted commercial flights without an airworthiness permit found “systemic failures”, with the airline admitting it needed to do better on compliance, Reuters reported earlier this month.

On Monday, pilots observed a low engine oil pressure on the B777-300ER aircraft’s right-hand engine during flaps retraction after take-off. The pressure dropped to zero shortly thereafter, and the crew shut down the engine and turned back as per procedure, the DGCA said.

“Air India sincerely regrets [the] inconvenience caused due to this unforeseen situation. The aircraft is undergoing the necessary checks,” an Air India spokesperson said in a statement.

The aircraft is 15 years old and has flown to locations such as Vienna, Vancouver and Chicago, according to Flightradar24. Boeing did not immediately respond to a request for comment on the incident.