US airline regulators have said it will investigate after an engine cover on a Boeing 737-800 fell off during takeoff in Denver, Colorado, and struck the wing flap.
Southwest flight 3695 returned safely to Denver International airport around 8.15am local time (3.15pm Irish time) and was towed to the gate.
The Boeing BA.N aircraft with 135 passengers and six crew members aboard had been headed to William P. Hobby Airport in Houston, Texas. No one was injured.
Southwest said maintenance teams were reviewing the aircraft.
The plane entered service in June 2015, according to Federal Aviation Administration (FAA) records.
Boeing referred questions to Southwest for information about the airline’s airplane and fleet operations.
The airline declined to say when the plane’s engine had last had maintenance.
In the aftermath of that incident, the FAA grounded the MAX 9 for several weeks, barred Boeing from increasing the MAX production rate and ordered it to develop a comprehensive plan to address “systemic quality-control issues” within 90 days.
Boeing production has fallen below the maximum 38 MAX planes per month the FAA is allowing.
The US justice department has opened a criminal investigation into the MAX 9 incident.
Read more: New Boeing boss says planemaker faces pivotal moment
The 737-800 is an earlier generation of the best-selling 737 from the current MAX.
The FAA is investigating several other recent Southwest Boeing engine issues.
A Southwest 737 flight on Thursday aborted takeoff and taxied back to the gate at Lubbock airport in Texas after the crew reported engine issues.
The FAA is also investigating a 25 March Southwest 737 flight that returned to the Austin airport in Texas after the crew reported a possible engine issue.
A 22 March Southwest 737-800 flight returned to Fort Lauderdale airport after the crew reported an engine issue. It is also being reviewed by the FAA.
