An engine cover has fallen off a Boeing plane and struck the wing flap - shortly after take-off in the US.
Passengers filmed videos from their window seats as part of the 737-800 was ripped off.
The safety scare happened shortly after the Southwest Airlines flight left Denver on Sunday.
About 135 passengers and six crew members were on board the aircraft, which hit an altitude of 3,140m (10,300ft) before making an abrupt U-turn just 25 minutes later.
The plane landed safely and was towed back to the gate, with passengers reaching their destination of Houston in Texas on a different aircraft... about four hours late.
No one was injured - but the Federal Aviation Administration has now launched an investigation into what happened.
"We apologise 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," Southwest Airlines said in a statement.
Analysis: Safety crisis leaves Boeing in grave situation
Records show the plane had entered service in 2015, but the company declined to say when its engine last had maintenance.
Several engine issues on 737-800s belonging to Southwest have been reported recently - and this was the second incident in under a week.
Three days earlier, one of these planes aborted take-off from Texas after an engine fire, and the service was abruptly cancelled.
On 22 and 25 March, two other flights turned back after the crew reported engine problems.
The Boeing 737-800 is from an earlier range than the 737 MAX, which has also suffered a slew of embarrassing, high-profile issues.
Read more:
Problems Boeing has had with its 737 MAX aircraft
Back in January, a window and a chunk of fuselage blew out of a 737 MAX 9 jet in mid-air, and six crew members were injured during a terrifying descent back to safety.
That Alaska Airlines plane had been in service for just eight weeks, with US regulators subsequently grounding dozens of aircraft for urgent safety checks.
Last month, Boeing announced that its embattled chairman and chief executive were both leaving their roles - with the plane manufacturer vowing to "fix" the crisis.
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