Airline regulators in the US have launched an investigation after an engine covering fell off a Boeing plane during takeoff, hitting the aircraft’s wind flap.

The Southwest Airlines flight was carrying 135 passengers and six crew members bound for Houston when it took off from Denver International Airport early on Sunday.

The Boeing 737-800 returned to Denver safely after only 25 minutes in the air and was towed to a gate. No one on board was injured.

Air traffic control recordings have revealed the incident.

“Let’s go ahead and declare an emergency for Southwest 3695, and we’d like an immediate return,” an air traffic control official can be heard saying.

“We’ve got a piece of the engine cowling hanging off apparently.”

A cowling is a removable engine cover.

Footage taken by some passengers showed what appeared to be part of the plane hanging off and flapping in the breeze.

“It all blew away,” one person can be heard saying in a video taken by passenger Lisa C., according to the US’s ABC News.

Lisa C. told the network that the engine cowling “peeled off within the first 10 minutes” of the flight.

“We all felt kind of a bump, a jolt, and I looked out the window, because I love window seats, and there it was,” she said.

The footage also captures passengers cheering as the plane lands safely back in Denver.

Southwest said travellers were transferred to a different plane, and scheduled to arrive in Houston later on Sunday.

“Our maintenance teams are reviewing the aircraft,” the airline said.

The US’s Federal Aviation Administration is also investigating the latest alarming incident involving a Boeing plane.

They include a mid-flight plunge that injured 50 people on a LATAM Airlines Boeing 787 on its way from Sydney to Auckland last month and the grounding of hundreds of Boeing aircraft after part of the fuselage tore off the left side of an Alaska Airlines flight in January.

Footage circulating online in the latest incident shows the ripped engine cover flapping in the wind with a torn Southwest logo.

According to FAA records, the plane in Sunday’s emergency was delivered in 2015.

US media reports that Southwest is one of Boeing’s bigger customers, with more than 200 737-800s. The average age of its fleet is about 12 years.