US airline regulators have launched an investigation after an engine cowling on a Boeing plane fell off during takeoff and struck the wing flap, the latest in a series of safety problems facing the aircraft manufacturer. Southwest Airlines flight 3695 rose to about 3,140 metres (10,300ft) before returning safely 25 minutes after takeoff to Denver international airport at about 8.15am local time on Sunday. It was towed to the gate after landing. The Boeing aircraft with 135 passengers and six crew members onboard had been headed to Houston, Texas. No one was injured. The airline said maintenance teams were reviewing the aircraft. It was a 737-800, a model that the troubled 737 Max series was intended to replace. The incident will add to the problems facing Boeing as it prepares for a major shake-up in its leadership. Its chief executive, Dave Calhoun, last month announced he would step down at the end of this year, while the commercial planes boss, Stan Deal, also left with immediate effect. The board chair, Larry Kellner, was replaced by Steve Mollenkopf, who last week told shareholders the board “will leave no stone unturned in our efforts to get this company to where it needs to be”. Boeing’s safety record has come under renewed scrutiny this year after a door plug panel came off a new Alaska Airlines 737 Max 9 jet at about 5,000 metres (16,000ft) on 5 January. In the aftermath of that incident, the Federal Aviation Administration grounded the Max 9 for several weeks, barred Boeing from increasing its rate of production of the aircraft 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 a month the FAA is allowing, while the US justice department has opened a criminal investigation into the incident. Boeing has been trying to increase the rate of 737 Max production for several years since its bestselling plane was allowed to fly again after an extended ban. Regulators around the world banned the 737 Max from flying after 346 people died in two crashes in 2018 and 2019 because of a design flaw. The US plane-maker spent years trying to rebuild its reputation and reassuring airline customers that its culture had changed to prioritise safety over profits. The door plug incident seriously undermined those efforts. Despite the controversy over the safety problems, Boeing last week revealed it had increased Calhoun’s pay by 45% to the highest since he took over. Boeing awarded Calhoun $32.8m (£26m) in pay, after adding $30m in stock options on top of his $1.4m salary. However, he may not collect all of the options if he steps down at the end of the year as planned. The Southwest plane entered service in June 2015, according to FAA records. The airline declined to say when the plane’s engine had last undergone maintenance. Footage posted on X showed the ripped engine cover flapping in the wind with a torn Southwest logo. The FAA is investigating several other recent Southwest Boeing engine problems. A Southwest 737 flight on Thursday aborted takeoff and taxied back to the gate at an airport in Texas after the crew reported engine problems. The FAA is also investigating a Southwest 737 flight on 25 March that returned to Austin airport in Texas after the crew reported a possible engine problem. A Southwest 737-800 flight on 22 March returned to Fort Lauderdale airport in Florida after the crew reported an engine problem. It is also being reviewed by the FAA.