The ban on the Boeing 737 MAX aircraft became worldwide on Wednesday after US President Donald Trump joined Canada and other countries in grounding the aircraft amid mounting global fears for the jets' airworthiness.
US authorities said new evidence showed similarities between Sunday's deadly crash of an Ethiopian Airlines Boeing 737 MAX 8 and a fatal accident in Indonesia in October.
The Federal Aviation Administration said findings from the crash site near Addis Ababa and "newly refined satellite data" warranted "further investigation of the possibility of a shared cause for the two incidents."
The FAA issued an emergency order grounding 737 MAX 8 and MAX 9 aircraft until further notice.
Trump told reporters at the White House the "safety of the American people and all peoples is our paramount concern."
Earlier on Wednesday, Canada had also joined the long list of countries to ban the plane from flying in their airspaces and many airlines have voluntarily taken it out of service.
Ethiopia said it would send the black boxes from Flight ET 302 to France for analysis which could provide crucial information about what happened.
"Hopefully they will come up with an answer but until they do the planes are grounded," Trump said of the planes.
FAA acting chief Daniel Elwell said the agency has been "working tirelessly" to find the cause of the accident but faced delays because the black box flight data recorders had been damaged.
The new information shows "the track of that airplane was close enough to the track of the Lion Air flight... to warrant the grounding of the airplanes so we could get more information from the black boxes and determine if there's a link between the two, and if there is, find a fix to that link," Elwell said on CNBC.
In Miami, a large hub for US air travel, passengers formed growing lines to reschedule after canceled flights.
Boeing chief Dennis Muilenburg said he supported the US decision "out of an abundance of caution" but continued to have "full confidence" in the safety of the plane.
The company continues its efforts "to understand the cause of the accidents in partnership with the investigators, deploy safety enhancements and help ensure this does not happen again," Muilenburg said in a statement.
The accounts of the recent crashes were echoed in concerns registered by US pilots on how the MAX 8 behaves.
Pilots concerns
At least four American pilots made reports following the Lion Air crash, all complaining the aircraft suddenly pitched downward shortly after takeoff, according to documents reviewed by AFP on the Aviation Safety Reporting System, a voluntary incident database maintained by NASA.
In two anonymous reports on flights just after the Lion Air crash, pilots disconnected the autopilot and corrected the plane's trajectory.
One said the flight crew reviewed the incident "at length... but can't think of any reason the aircraft would pitch nose-down so aggressively."
It was unclear if US transportation authorities review the database or investigate the incidents. However, the FAA said this week it had mandated that Boeing update its flight software and training on the aircraft.
Questions on the Lion Air crash have honed in on an automated stall prevention system, the MCAS, designed to automatically point the nose of the plane downward if it is in danger of stalling.
According to the flight data recorder, the pilots of Lion Air Flight 610 struggled to control the aircraft as the MCAS repeatedly pushed the plane's nose down following takeoff.
The Ethiopian Airlines pilots reported similar difficulties before their aircraft plunged into the ground as they tried to return to the airport.
Boeing was criticized after the Lion Air crash for allegedly failing to adequately inform 737 pilots about the functioning of the stall prevention system.
Ethiopian Airlines CEO Tewolde GebreMariam on Sunday said the captain on the flight, Yared Mulugeta Getachew, 29, was an experienced aviator with more than 8,000 flight hours.
Andrew Hunter, a defense industry expert at the Center for Strategic and International Studies in Washington, said that, while Boeing and the FAA had good track records on addressing safety concerns, sometimes the combination of automated systems and humans did not work smoothly.
"It is hard to get a system to work seamlessly with human beings," he told AFP.
"The fact the system was fighting the pilot was not an unintended consequence," because it should counteract a pilot error and correcting this is "challenging."
"I'm not sure the fix here was quite so easy," Hunter said.
Banned from the skies
In Ethiopia, distraught families wept and lit candles as they visited the deep black crater where the plane smashed into a field, killing 157 passengers and crew, an AFP correspondent said.
The Ethiopian Airlines 737 MAX 8 was less than four months old when it went down six minutes into a flight from Addis Ababa to Nairobi on Sunday, disintegrating on impact.
Families of the victims from Kenya, China, the United States and Canada, as well as diplomatic staff from embassies, were visiting the crash site.
A dozen airlines have grounded the plane, while Nigeria, Lebanon, Egypt, Serbia, Vietnam, New Zealand and Hong Kong on Wednesday also joined the list of countries to ban it from their airspace.
The European Union and major hubs such as the United Arab Emirates and Australia had already done so.
American Airlines said it was informed of the FAA decision earlier in the day and had 24 aircraft affected by the US ban, while Southwest Airlines said it was still confirming the move.
The MAX series is Boeing's fastest-selling model, with more than 5,000 orders placed to date from about 100 customers.
There are 74 of the planes registered in the United States, and 387 in use worldwide with 59 carriers, according to the FAA.
Low-cost airline Norwegian Air Shuttle has said it would demand financial compensation from Boeing as the implications of the mass grounding for the airline industry remained unclear.
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