U.S. safety regulators are investigating an engine failure on a Delta Air Lines Inc. jet that forced pilots to shut down the turbine and return to Atlanta shortly after takeoff.

The accident occurred Wednesday on a Boeing Co. 757-200 bound for Orlando, Florida, the National Transportation Safety Board said Thursday on Twitter. Delta Flight 1418, which had 121 passengers and six crew members, landed safely and there were no injuries. The engine failure occurred at about 18,000 feet, according to the Federal Aviation Administration.

The Delta incident is at least the fourth since August 2016 in which an engine failure has allowed debris to escape. The powerplants are designed with a hardened exterior so that even in the event of a blowout, fan blades and other components can’t get out and threaten fuel tanks, passengers and other sensitive aircraft structures.

Flight 1418 “experienced a maintenance issue in the right engine shortly after takeoff,” Anthony Black, a Delta spokesman, said in an interview. All maintenance on the engines is performed by Delta’s TechOps unit.

The Atlanta-based carrier is cooperating with the NTSB investigation, Black said.

Pratt Engines

“Once they have completed their investigation we will change the engine and the aircraft will be ready to be placed back into service,” he said. Delta isn’t certain how soon that will be. The 27-year-old aircraft had two PW2037 engines made by Pratt & Whitney, a unit of United Technologies Corp. Pratt didn’t immediately respond to a request for comment.

The NTSB didn’t provide further details.

The agency is also investigating several other cases in which engine failures allowed debris to escape. A woman died in an accident April 17 on a Southwest Airlines Co. flight. The Boeing 737 lost a fan blade, which careened in front of the protective shield and tore off the front of the engine. After debris broke a window, the woman was partially sucked out of the plane.

In August 2016, a Southwest Boeing 737-700 had to make an emergency landing in Pensacola, Florida, after a fan blade broke off and debris struck the fuselage, wing and tail, causing the plane to lose cabin pressure. A preliminary NTSB investigation found evidence of a crack “consistent” with metal fatigue in the blade of the CFM56 engine made by CFM International, a joint venture of General Electric Co. and Safran SA. No one was injured.

Two months later, an American Airlines Group Inc. flight was accelerating for takeoff at Chicago’s O’Hare airport when the right engine exploded, sending shrapnel through the casing and ignited leaking fuel. One wing of the Boeing 767-300 was engulfed in a fireball as passengers rushed to get off the plane. Twenty people were injured. The GE CF6-80 engine had an apparent manufacturing defect, the NTSB said later, causing a disk to break into at least four pieces.