The US Federal Aviation Administration (FAA) ordered US airline operators to temporarily ground some Boeing 737 Max 9 aircraft for inspections.
The order came after an Alaska Airlines flight had to make an emergency landing when a window and piece of fuselage blew out in midair on Friday.
The hole in the aircraft was ripped open some 20 minutes after takeoff, causing the cabin to depressurize.
The FAA said it was requiring immediate inspections of MAX 9 planes operated by US airlines or flown in the country by foreign carrier, affecting about 171 planes worldwide.
Early on Sunday, United Airlines announced it had temporarily suspended services on all Boeing 737 MAX 9 aircraft.
"We are working with the FAA to clarify the inspection process and the requirements for returning all MAX 9 aircraft to service," the airline said in a statement.
Earlier, United Airlines said that around 33 of its 79 MAX 9 planes had been inspected as required by the FAA.
Alaska and United cancel flights
Alaska Airlines canceled around 140 flights on Sunday, according to the FlightAware flight tracking firm, and United is expected to cancel 60 flights.
Outside of the US, Alaska Airlines said there were no Max 9 planes registered in the UK that were affected by the temporary suspension of some of the aircraft.
The impact of the grounding of the planes was therefore minimal in the UK.
Meanwhile, the European Union Aviation Safety Agency adopted the FAA directive.
However, it noted that no EU member state airlines "currently operate aircraft in the affected configuration."
Side panel blows out mid-flight on Alaska Airlines
Alaska Airlines grounded its whole fleet of Boeing 737 Max 9 after Friday's incident, but had returned some of its grounded planes to service.
The hole in the aircraft was ripped open some 20 minutes after take off, causing the cabin to depressurize.
Oxygen masks were released and the plane safely landed soon after, with over 170 passengers and six crew members unharmed.
"Following tonight's event on Flight 1282, we have decided to take the precautionary step of temporarily grounding our fleet of 65 Boeing 737-9 aircraft," Alaska Airlines CEO Ben Minicucci said in a statement.
Union hails grounding, lawmakers demand investigation
The president of the Association of Flight Attendants-CWA, Sara Nelson, said the union "supports the FAA's quick and decisive action to ground certain 737 MAX 9 Fleet that do not meet the inspection cycles specified in the Emergency Airworthiness Directive."
Republican lawmaker Ted Cruz, who heads the congressional committee overseeing the FAA, called for a thorough investigation.
"The NTSB (National Transporation Safety Board) and FAA must thoroughly investigate this incident to address an alarming breach of safety," he said.
Difficult start in the skies for the latest 737s
The Boeing 737-9 MAX just received its certification last October, FFA online records show. It has been on 145 flights since going into commercial service on Nov 11.
Boeing said it was gathering more information and had a technical team ready to support the investigation. Alaska Airlines' Minicucci said the carrier was "working with Boeing and regulators to understand what occurred."
The twin-engine, single-aisle MAX is the newest version of most-flown commercial series of aircraft in the world, Boeing 737s. In service since May 2017, it is frequently used on US domestic flights.
Two MAX 8 aircrafts crashed in 2018 and 2019, killing 346 people and prompting a worldwide grounding of all MAX 8 and MAX 9 planes that lasted nearly two years.
In 2018, a Lion Air Boeing 737 Max plane in Indonesia crashed, killing 189 people. A year later, the Ethiopian Airlines Boeing 737 Max crashed soon after takeoff from Addis Ababa, killing 157 people.
