Boeing 737 MAX: MCAS's Single Sensor, a 20-Month Grounding and a Door Plug

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To compete with a fuel-efficient Airbus jet, Boeing fitted its low-slung 1960s 737 design with larger LEAP-1B engines mounted farther forward and higher, which could pitch the nose up during some maneuvers. Its fix was software, the Maneuvering Characteristics Augmentation System, or MCAS, which pushed the nose down automatically, and to preserve fleet commonality and avoid costly simulator training, Boeing had the FAA remove MCAS references from pilot manuals. MCAS relied on a single angle of attack sensor and moved the horizontal stabilizer in 10-second bursts, firing again five seconds after pilots countered it. The crashes of Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 in March 2019 killed 346 people.

A December 2018 FAA risk assessment predicted 15 more fatal crashes without a fix, yet the FAA, which delegated certification work to Boeing employees under Organization Designation Authorization, was among the last regulators to ground the jet after China moved first. 387 planes sat for 20 months, costing an estimated $20 billion directly and over $60 billion indirectly; CEO Dennis Muilenburg was ousted in 2019, and a 2021 Justice Department settlement totaled $2.5 billion, including a $500 million fund for victims' families. MCAS was rewritten to use two sensors and activate once per event, with simulator training required. Then, on January 5, 2024, a door plug blew out on Alaska Airlines Flight 1282 at 16,000 feet, inspections found loose bolts on other MAX 9s, and Boeing agreed to buy back fuselage maker Spirit AeroSystems for $4.7 billion.

• The 737 was designed to sit low so baggage handlers at small airports could load it without conveyors.
• To free a jammed manual trim wheel, pilots had to let the nose drop further in a roller coaster maneuver.
• Ethiopian investigators found the crew tried the manual trim wheels but could not overcome aerodynamic forces.
• A congressional report found deadlines and budgets were put ahead of safety and engineers who raised concerns were pressured to stay quiet.
• After the door plug blowout, the Justice Department opened a criminal probe into whether Boeing breached its 2021 deferred prosecution agreement.