MCAS (Maneuvering Characteristics Augmentation System) compensated for the MAX's engine placement. It relied on one of two angle-of-attack sensors. When the sensor gave faulty readings, MCAS repeatedly pushed the nose down. Pilots were not informed of MCAS or trained on override procedures.
346 deaths. 737 MAX grounded worldwide for 20 months. $20+ billion in total costs to Boeing. CEO Dennis Muilenburg resigned. Criminal charges against Boeing resulted in a $2.5 billion settlement. Fundamental restructuring of FAA certification processes.
The Incidents
On October 29, 2018, Lion Air Flight 610 crashed into the Java Sea 13 minutes after takeoff from Jakarta, killing all 189 people aboard. On March 10, 2019, Ethiopian Airlines Flight 302 crashed 6 minutes after takeoff from Addis Ababa, killing all 157 people aboard. Both were Boeing 737 MAX 8 aircraft. Both crashes followed the same pattern.
The Root Cause
The 737 MAX was designed with larger, more fuel-efficient engines mounted further forward and higher on the wing than previous 737 models. This changed the aircraft's handling characteristics — the nose tended to pitch up in certain conditions. Rather than redesigning the airframe, Boeing created MCAS (Maneuvering Characteristics Augmentation System) — software that automatically pushed the nose down to compensate.
MCAS relied on a single angle-of-attack (AoA) sensor. The 737 MAX has two AoA sensors, but MCAS only read from one. When that sensor gave faulty readings — as it did on both fatal flights — MCAS repeatedly activated, pushing the nose down while pilots fought to pull it up.
Pilots were not informed that MCAS existed. It was not mentioned in the flight manual. No simulator training was provided. Boeing had marketed the MAX as requiring no additional pilot training compared to the previous 737 NG — because additional training would have reduced the aircraft's competitive advantage.
The two AoA sensors had a "disagree" indicator that would have alerted pilots when the sensors showed different readings. Boeing sold this indicator as an optional upgrade.
Why It Matters
Boeing priced safety as a feature. A sensor disagree indicator — the most basic check that the inputs to the system that controls whether the plane climbs or dives are consistent — was sold as an optional extra. The system that overrode the pilots relied on a single sensor in an aircraft with two sensors. The pilots weren't told the system existed. Every one of these decisions reduced cost or preserved competitive advantage. 346 people died.