(Bloomberg) -- A United Airlines Holdings Inc. 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 Thursday morning en route to Denver International when the crew heard an “abnormal noise,” according to a Federal Aviation Administration statement.

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, and the FAA said it would investigate.

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 aircraft in the latest United incident 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.

The plane lost several pieces of sheet metal when it took off that were recovered from the runway, and other debris fell off when it landed, according to a recording of conversations between the airport control tower and the plane posted on LiveATC.net.

The pilot of a nearby JetBlue Airways Corp. plane initially reported the debris to the tower, which had to divert some planes to other runways. The United pilot told the tower that one of the engines was slightly exceeding normal exhaust gas temperatures and he was concerned they had hit a bird or blew a tire on takeoff. 

“We are declaring an emergency,” the United pilot said. “Everything is running fine, the engine is within parameters.”

There were 124 passengers and five crew members on the flight, which taxied to an airport gate after landing.

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.

Chief Executive Officer Scott Kirby has said that the Chicago-based carrier was “embracing” the FAA evaluation as an opportunity to improve what it considers to be an already high level of safety.

United’s shares fell 1.4% in New York trading.

(Updates with details of pilot conversations, debris on runway beginning in sixth paragraph)

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