A Boeing 737 Max 8 aircraft bearing the logo of China Southern Airlines is parked at a Boeing production facility in Renton, Washington, March 11, 2019.
A Boeing 737 Max 8 aircraft bearing the logo of China Southern Airlines is parked at a Boeing production facility in Renton, Washington, March 11, 2019.

The United States told international carriers on Monday that the Boeing 737 Max 8 is airworthy as regulators scrutinize two fatal crashes of the new model of aircraft since October, but said it will mandate forthcoming "design changes" from Boeing by April.

An Ethiopian Airlines 737 Max 8 bound for Nairobi crashed minutes after take-off Sunday, killing all 157 aboard and raising questions about the safety of the new variant of the industry workhorse, one of which also crashed in Indonesia in October, killing 189 people.

In a notice, the Federal Aviation Administration said it planned to require design changes by Boeing no later than April.

Boeing is working to complete "flight control system enhancements, which provide reduced reliance on procedures associated with required pilot memory items," the FAA said.

FILE - Workers service an Ethiopian Airlines Boeing 737-800 plane at the Bole International Airport in Ethiopia's capital Addis Ababa, Jan. 26, 2017.
Boeing Likely to Face New Questions After Another 737 Crash
Investigators rushed to the scene of a devastating plane crash in Ethiopia on Sunday, an accident that could renew safety questions about the newest version of Boeing’s popular 737 airliner.The Boeing 737 Max 8 operated by Ethiopian Airlines crashed shortly after taking off from the capital of Addis Ababa, killing all 157 people on board.The plane was new. The weather was clear. Yet something was wrong, and the pilots tried to return to the airport.
Rescuers work at the scene of an Ethiopian Airlines flight crash near Bishoftu, or Debre Zeit, south of Addis Ababa,  Ethiopia, Monday, March 11, 2019.
Black Boxes Recovered from Crashed Ethiopian Jet
Ethiopia is observing a day of mourning for the 157 people who died when the Boeing 737 MAX-8 jet headed for Nairobi went down just six minutes after takeoff from Addis Ababa
The Boeing logo appears above a trading post on the floor of the New York Stock Exchange before the opening bell, Monday, March 11, 2019.
Boeing Shares Plunge After Ethiopian Airlines Crash
Airlines in China, Indonesia, and Ethiopia grounded all 737 MAX 8 planes following Sunday's crash

The FAA also said Boeing "plans to update training requirements and flight crew manuals to go with the design change" to an automated protection system called the Maneuvering Characteristics Augmentation System or MCAS. The changes also include MCAS activation and angle of attack signal enhancements.

The FAA said in the notice made public that external reports are drawing similarities between the crashes in Ethiopia and Indonesia.

"However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions," according to the Continued Airworthiness Notification to the International Community for Boeing 737 Max 8 operators.

U.S. Transportation Secretary Elaine Chao told reporters that regulators would not hesitate to act if they find a safety issue.

"If the FAA identifies an issue that affects safety, the department will take immediate and appropriate action," Chao told reporters. "I want people to be assured that we take these incidents, these accidents very seriously."

FILE - Transportation Secretary Elaine Chao, right
FILE - Transportation Secretary Elaine Chao, right, speaks to reporters outside the West Wing of the White House, March 4, 2019.

Boeing's top executive told employees on Monday he was confident in the safety of the U.S. manufacturer's top-selling 737 Max aircraft.

Reuters and other media outlets have reported that Boeing has for months planned design changes after the Lion Air crash in Indonesia, but the FAA notice is the first public confirmation.

Canada's transport minister also said he will not hesitate to act once the cause of the crash is known.

FAA chief Dan Elwell on Monday said the notification basically "informs the international community where we are and [gives] sort of ... one answer to the whole community."

Some Boeing jets grounded

Senator Dianne Feinstein, a California Democrat, and Paul Hudson, the president of FlyersRights.org and a member of the FAA Aviation Rulemaking Advisory Committee, on Monday both said the plane should be grounded.

"The FAA's 'wait and see' attitude risks lives as well as the safety reputation of the U.S. aviation industry," Hudson said in a statement.

The National Transportation Safety Board and the FAA are both at the crash site in Ethiopia, Chao said.

Boeing's shares fell as much as 10 percent on the prospect that two such crashes in such a short time could reveal flaws in its new plane. Boeing, whose shares closed down 5.3 percent at $400.01 in the heaviest trading trade since July 2013, did not immediately comment Monday on the FAA notification, but said it was sending a team to Ethiopia to aid investigators.

Airplane parts lie on the ground at the scene of a
Airplane parts lie on the ground at the scene of an Ethiopian Airlines flight crash near Bishoftu, or Debre Zeit, south of Addis Ababa, Ethiopia, March 11, 2019.

The 737 line, which has flown for more than 50 years, is the world's best-selling modern passenger aircraft and viewed as one of the industry's most reliable.

China ordered its airlines to ground the jet, a move followed by Indonesia and Ethiopia. Other airlines, from North America to the Middle East, kept flying the 737 Max 8 on Monday after Boeing said it was safe.

Boeing's 737 Max is the newest version of a jet that has been a fixture of passenger travel for decades and the cash cow of the world's largest aircraft maker, competing against Airbus SE's A320neo family of single-aisle jetliners. The 737 family is considered one of the industry's most reliable aircraft.

Boeing 737 Max8 aircraft

The Max has a bigger and more efficient engine compared to earlier 737 models.

Boeing rolled out the fuel-efficient Max 8 in 2017 as an update to the already redesigned 50-year-old 737, and had delivered 350 Max jets out of the total order tally of 5,011 aircraft by the end of January.