Boeing's 737 MAX had a hidden flight-control flaw that killed 346 people in two crashes

The Boeing 737 MAX was the latest version of Boeing's best-selling airliner, rushed to market to compete with the fuel-efficient Airbus A320neo. To handle the plane's larger, repositioned engines, Boeing added a software system called MCAS, which could automatically push the nose down if it sensed a stall risk. To keep the MAX handling like earlier 737s — and avoid expensive simulator training — Boeing designed MCAS to rely on data from just one of the plane's two angle-of-attack sensors, and did not fully tell pilots the system existed or how to respond.

On October 29, 2018, Lion Air Flight 610 crashed into the sea off Indonesia minutes after takeoff, killing all 189 people on board. A faulty angle-of-attack sensor had fed MCAS erroneous data, repeatedly forcing the nose down while pilots struggled to understand and control the aircraft. Less than five months later, on March 10, 2019, Ethiopian Airlines Flight 302 crashed in nearly identical circumstances, killing all 157 on board. The two disasters killed 346 people in all.

After the second crash, regulators worldwide grounded the entire 737 MAX fleet; the US FAA, which had certified the plane, was among the last to act. Investigations found Boeing had concealed MCAS from operators and regulators, and that the FAA's certification had been inadequate. The MAX was grounded for twenty months, until November 2020, while Boeing redesigned the system to use both sensors and required new pilot training. In January 2021, Boeing agreed to pay more than $2.5 billion to resolve a US criminal investigation into the crashes.