Two Boeing 737 MAX 8 aircraft are parked at a Boeing production facility in Renton, Washington, U.S., March 11, 2019. The United States will mandate that Boeing Co implement design changes by April that have been in the works for months for the 737 MAX 8 fleet after a fatal crash in October but said the plane was airworthy and did not need to be grounded after a second crash on Sunday. REUTERS/David Ryder

Boeing 737 MAX aircraft are parked at a Boeing production facility in Renton, Washington, U.S., March 11, 2019. An Ethiopian Airlines 737 MAX 8 bound for Nairobi crashed minutes after take-off on 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. REUTERS/David Ryder

A Boeing 737 MAX 8 aircraft bearing the logo of China Southern Airlines is parked at a Boeing production facility in Renton, Washington, U.S. March 11, 2019. Boeing confirmed the Federal Aviation Administration's announcement late Monday that it will deploy a software upgrade across the 737 MAX 8 fleet "in the coming weeks" as pressure mounted. Two U.S. senators called the fleet's immediate grounding and a rising number of airlines said they would voluntarily ground their fleets. REUTERS/David Ryder

A Boeing 737 MAX 8 aircraft bearing the logo of China Southern Airlines is parked at a Boeing production facility in Renton, Washington, U.S., March 11, 2019. The company confirmed it had for several months "been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer." REUTERS/David Ryder

Boeing 737 MAX aircraft, including a 737 MAX 8 aircraft bearing the logo of China Southern Airlines (3rd L), are parked at a Boeing production facility in Renton, Washington, U.S. March 11, 2019. Boeing did not reference Sunday's Ethiopian Airlines crash in connection to the software upgrade. The statement did express the company's condolences to the relatives of the 157 people who died, however. The FAA said the changes will "provide reduced reliance on procedures associated with required pilot memory items." REUTERS/David Ryder

Boeing 737 MAX aircraft are parked at a Boeing production facility in Renton, Washington, U.S. March 11, 2019. The FAA also said Boeing will "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. REUTERS/David Ryder