A Southwest Airlines jet returned to Denver Sunday morning after the engine cover fell off and struck the wing flap during takeoff, according to the Federal Aviation Administration. The Boeing 737 landed safely, and the passengers headed to Houston were being put onto another aircraft, Southwest Airlines said in a statement.

“We apologize for the inconvenience of their delay, but place our highest priority on ultimate Safety for our Customers and Employees. Our Maintenance teams are reviewing the aircraft,” the statement reads.

It’s the second mishap this week for the airline, with a flight from Texas canceled Thursday after a report of an engine fire. The Lubbock, Texas, fire department confirmed online a fire in one of the two engines that needed extinguishing.

The FAA is investigating both incidents.

Both planes were Boeing 737-800s, an older model than the 737 Max.