A Boeing 737 Max gives a display during the opening day of the Farnborough International Airshow in Hampshire.

The incident was the second deadly crash of the new Boeing planes in less than five months. A Lion Air Boeing 737 MAX 8 plunged into the Java Sea shortly after taking off from Jakarta, Indonesia, in October, killing all 189 people on board.

Aviation officials in China, Indonesia, Singapore, Australia and Malaysia have ordered domestic airlines to ground their fleets of the popular Boeing single-aisle planes after the deadly crash Sunday of one operated by Ethiopian Airlines. The 149 passengers and eight crew members on board were killed when the plane crashed shortly after takeoff.

The Boeing 737 MAX, the type of plane involved in a deadly crash in Ethiopia over the weekend, is still airworthy, the Federal Aviation Administration said Monday, giving a vote of confidence to the type of jet that has been involved in two deadly crashes since October.

"External reports are drawing similarities between this accident and the Lion Air Flight 610 accident on October 29, 2018," the FAA said in its notice. "However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

The popular planes are a key revenue driver for Boeing, whose commercial airplane business brought in nearly 60 percent of its more than $100 billion in revenue last year.

U.S. airlines on Monday sought to calm concerns from travelers that the planes are safe.

While it is highly unusual to have two fatal crashes of new aircraft so close together, analysts have cautioned that it is too early to know the cause of the Ethiopian Airlines crash or whether it is at all linked to the crash of the Lion Air flight last year.

The crash of Lion Air Flight 610 in October drew scrutiny of the maneuvering characteristics augmentation system, known as MCAS, which is included on the new planes and a system that officials said they believed pushed the nose of the plane down repeatedly.

A nose-down position is the way to recover from a stall but can be catastrophic if the plane signals it is in a stall when it is not. Boeing issued a safety bulletin to pilots in November directing them how to handle if the nose of the plane is automatically pushed down.

The FAA said in its notice on Monday that it expects to mandate design enhancements to the automated system and signaling on board the Boeing planes by April 2019. Boeing is planning to update training requirements and manuals along with those changes, the FAA said.

Boeing confirmed that it was planning to changes to flight-control software for the planes' MCAS system and said the changes are "designed to make an already safe aircraft even safer."

The FAA's notice applied to both the Boeing 737 MAX 8 and the larger MAX 9 variant. There are 74 of these aircraft in U.S. fleets and 374 worldwide, the agency said.

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