A Boeing 737 Max 8 aircraft from the fleet that was grounded after two deadly crashes made an emergency landing in Florida on Tuesday.
No passengers were aboard Southwest Airlines Flight 8701, which was being ferried from Orlando International Airport to Victorville, California, for short-term storage during the grounding, the airline said.
Just before 3 p.m. Tuesday, the two pilots aboard the flight reported "a performance issue with one of the engines shortly after takeoff," the airline said.
"The crew followed protocol and safely landed back at the airport."
The global fleet of 737 Max jets was grounded indefinitely on March 13 after two fatal crashes involving the aircraft in March and October.
It is not clear if Tuesday's emergency landing was related to suspected problems with the aircraft. An investigation into the crashes focuses on an automated anti-stall system and not engine problems.
The landing and the runway shutdown it caused did not impact other traffic, an airport spokeswoman said.
Southwest said the aircraft will be moved to its Orlando maintenance facility for a review. The Federal Aviation Administration is investigating.
The Trump administration said it grounded the fleet in light of information gathered at the site of the March 10 crash of an Ethiopian Airlines flight. Before then, administration officials had said the planes were safe, despite concerns raised after a Lion Air flight crashed in October.
The causes of the crashes have yet to be determined. But the focus has been on an automatic safety feature which may have forced the nose of each plane lower when it incorrectly believed the plane was in danger of going into a stall.
Boeing and the FAA said they are working on an upgrade of the planes' software to deal with the safety feature.
Southwest has 34 of Boeing's 737 Max jets. Its pilots were among those from three American carriers who tested recent software changes intended to decrease the chances of triggering the Maneuvering Characteristics Augmentation System, or MCAS.