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A second electronic system found on other Boeing jets also alerts pilots to unusual or hazardous situations during flight and lays out recommended steps to resolve them.

On 737s, a light typically indicates the problem and pilots have to flip through their paper manuals to find next steps. In the doomed Indonesia flight, as the Lion Air pilots struggled with MCAS for control, the pilots consulted the manual moments before the jet plummeted into the Java Sea, killing all 189 people aboard.

“Meanwhile, I’m flying the jet,” said Mr. Tajer, the American Airlines 737 captain. “Versus, pop, it’s on your screen. It tells you, This is the problem and here’s the checklist that’s recommended.”

Boeing decided against adding it to the Max because it could have prompted regulators to require new pilot training, according to two former Boeing employees involved in the decision.

The Max also runs on a complex web of cables and pulleys that, when pilots pull back on the controls, transfer that movement to the tail. By comparison, Airbus jets and Boeing’s more modern aircraft, such as the 777 and 787, are “fly-by-wire,” meaning pilots’ movement of the flight controls is fed to a computer that directs the plane. The design allows for far more automation, including systems that prevent the jet from entering dangerous situations, such as flying too fast or too low. Some 737 pilots said they preferred the cable-and-pulley system to fly-by-wire because they believed it gave them more control.

In the recent crashes, investigators believe the MCAS malfunctioned and moved a tail flap called the stabilizer, tilting the plane toward the ground. On the doomed Ethiopian Airlines flight, the pilots tried to combat the system by cutting power to the stabilizer’s motor, according to the preliminary crash report.

Once the power was cut, the pilots tried to regain control manually by turning a wheel next to their seat. The 737 is the last modern Boeing jet that uses a manual wheel as its backup system. But Boeing has long known that turning the wheel is difficult at high speeds, and may have required two pilots to work together.

In the final moments of the Ethiopian Airlines flight, the first officer said the method wasn’t working, according to the preliminary crash report. About 1 minute and 49 seconds later, the plane crashed, killing 157 people.