Washington: A United Airlines plane returned to a Connecticut airport after losing part of a liner from inside the engine’s cover, another incident for a carrier already under scrutiny over a series of flight mishaps this year.

The Airbus SE A320 aircraft departed Bradley International Airport in Hartford on Thursday morning en route to Denver International when the crew heard an “abnormal noise,” according to a Federal Aviation Administration statement. The agency said it will investigate the incident.

United said in a separate statement that a piece of a “sound-dampening outer liner” underneath the cowling was found on the runway after landing. There were no injuries.

The aircraft is 22 years old.

While the cause of the incident wasn’t immediately known, issues with older planes are often isolated and not a sign of systemic problems or manufacturing defects.

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The airline’s safety procedures have been under review by the FAA since a series of incidents including a piece of fuselage coming loose in flight and a wheel falling off a plane after takeoff.

The US aviation industry more broadly has been under heightened scrutiny since January, when a panel broke off of an Alaska Airlines flight.

The FAA’s review has limited certification activities for United, effectively restricting the carrier’s growth. While the examination is ongoing, United said last month that it could begin the process of adding new planes and routes again.