There were other incidents. A week later, a crack appeared in the cockpit window of a 737 flying between Sapporo and Toyama in Japan.
And in March 2019, the 737 Max was grounded for 20 months after two similar crashes which claimed 346 lives.
In the latest incident, in which nobody was hurt, the aircraft returned to Denver.
The pilot told air traffic control that the crew had been alerted by passengers who said they had heard something hit the wing.
The series of incidents have hit the company’s reputation, with its share price plummeting by 27 per cent since the start of the year. Karine Jean-Pierre, the White House press secretary, declined to confirm that it was safe for Americans to fly on Boeing’s aircraft.
As America’s biggest exporter, a decline in faith in the company which is competing with Europe’s Airbus, is damaging for the US balance of payments.
Boeing referred comments to Southwest Airlines.
A Southwest spokesperson said: ‘Southwest Flight 3695 returned to Denver International Airport this morning and landed safely after experiencing a mechanical issue. We’re working now to get customers on their way to Houston on another aircraft. Our maintenance teams are reviewing the aircraft.”