An engine cover on a Southwest Airlines Boeing 737-800 fell off on Sunday during takeoff in Denver and struck the wing flap, prompting the Federal Aviation Administration (FAA) to open an investigation.
No one was injured and Southwest Flight 3695 returned safely to Denver International Airport around 8:15am local time (1415 GMT) on Sunday and was towed to the gate after losing the engine cowling.
The Boeing, aircraft bound for Houston Hobby Airport with 135 passengers and six crew members aboard climbed to about 10,300 feet (3,140 m) before returning 25 minutes after takeoff.
Passengers arrived in Houston on another Southwest plane about four hours behind schedule. Southwest said maintenance teams are reviewing the aircraft.
The plane entered service in June 2015, according to FAA records. Boeing referred questions to Southwest.
The 737-800 is in the prior generation of the best-selling 737 known as the 737 NG, which in turn was replaced by the 737 MAX.
Southwest declined to say when the plane's engine had last had maintenance.
ABC News aired a video posted on social media platform X of the ripped engine cover flapping in the wind with a torn Southwest logo.
Boeing has come under intense criticism since a door plug panel tore off a new Alaska Airlines, opens new tab 737 MAX 9 jet at 16,000 feet on 5 January.
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 Justice Department has opened a criminal investigation into the MAX 9 incident.
In December, the FAA proposed mandating engine housing inspections and component replacements on Boeing 737 NG airplanes after a 2018 Southwest fatal fan blade incident.
The directives would require operators to inspect and replace certain components on the engine cowling by July 2028. The National Transportation Safety Board called on Boeing in 2019 to redesign the fan cowling structure after the incident.
The FAA is investigating several other recent engine issues on Southwest's fleet of Boeing planes.
A Southwest 737-800 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 March 25 Southwest 737 flight that returned to the Austin airport in Texas after the crew reported a possible engine issue.
A March 22 Southwest 737-800 flight returned to Fort Lauderdale airport after the crew reported an engine issue. It is also being reviewed by the FAA.
