[Boeing’s shares dropped as trading began on Monday.]

The business of building and selling jets is brutally competitive, and the 737 Max was Boeing’s answer to an update that Airbus, the giant European aircraft manufacturer, unveiled for its popular A320 jet that made it more fuel-efficient. The two companies are global leaders in the field, and they have jockeyed for dominance for years. About 10,000 total planes from Boeing’s 737 family are in service, compared with over 8,000 in Airbus’s A320 family. Many airlines rely on these kinds of planes as linchpins of their fleets. They are designed to efficiently serve short- and medium-haul routes (like New York to Miami or Los Angeles), and carry about 200 passengers.

Boeing’s response to its rival’s move was a more efficient engine, but the Max engine was bigger than the earlier versions. To address this engineering challenge, Boeing updated the software for the flight control system. After the Lion Air crash, some American aviation authorities said that the change had not been adequately explained to pilots.

But in light of the Indonesian disaster, pilots have since been informed by Boeing and regulatory agencies of the Max’s new system, and airlines have provided training classes on it. Whether Ethiopian Airlines, which, unlike Lion Air, has a strong safety reputation, carried out that training was not immediately known.

Boeing installed the system on the new 737s as part of the “control law” — commands issued by the plane’s flight-control computer that bypass the pilots. On the Lion Air flight, the swings up and down may have come about as pilots repeatedly tried to keep that system from pushing the nose of the aircraft down and putting it into a fatal dive. Whatever happened, the pilots lost their battle after about 12 minutes of flight.

While a malfunction of that system is a possibility in the Ethiopian flight, which lasted about six minutes and included a shorter series of swings, early information is still too sketchy to draw conclusions. And what is known so far does not rule out pilot error or the malfunction of a completely separate system.