Investigation Summary
The accident centered on the Maneuvering Characteristics Augmentation System (MCAS), designed to make the 737 MAX handle like previous 737 models at high angles of attack. On the accident flight, a miscalibrated AOA sensor (which had been replaced just before the flight) provided data indicating the plane was stalling when it was not. MCAS activated, commanding significant nose-down stabilizer trim. Because MCAS was designed to reset if it still perceived a high AOA, it provided multiple cycles of nose-down input. The crew, facing a 'stick shaker' stall warning and conflicting airspeed/altitude data, did not recognize the runaway trim condition in time to use the stabilizer cutout switches.
Final Conclusions
The Indonesian KNKT concluded that the crash was caused by a combination of factors: a miscalibrated replacement AOA sensor, the design of MCAS which relied on a single sensor without redundancy, and the lack of information provided to pilots about the system's operation. Maintenance issues regarding the sensor's installation and the crew's inability to manage the multiple alerts also contributed to the loss of control.
Video Analysis
The Insane Design Flaw of the Boeing 737 MAX — An analysis of the Lion Air 610 and Ethiopian 302 crashes, focusing on the MCAS system and the certification failures of the Boeing 737 MAX.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
MCAS
Maneuvering Characteristics Augmentation System; a flight control law implemented to improve aircraft handling at high AOA.
AOA Sensor
Angle of Attack sensor; measures the angle between the wing's chord line and the oncoming airflow.
Stabilizer Trim Cutout Switches
Switches located on the center pedestal that disconnect power to the electric trim motor.
Interesting Facts
- 01The aircraft was a nearly brand-new Boeing 737 MAX 8, delivered only two months prior.
- 02The AOA sensor used by the flight computer had been replaced the day before the accident following similar issues on a previous flight.
- 03MCAS was designed to activate only in manual flight with flaps up and at high angles of attack.
- 04The flight crew of the previous night's flight (JT043) encountered the same issue but successfully disabled the trim system using the cutout switches.
- 05Boeing did not include information about MCAS in the flight crew operations manual (FCOM) at the time of the accident.
Safety Actions & Advisories
Global Grounding
The Boeing 737 MAX fleet was grounded worldwide following the subsequent crash of Ethiopian Airlines Flight 302.
MCAS Software Update
Boeing redesigned MCAS to compare inputs from two AOA sensors and limit the system's authority so it cannot override pilot input.
Enhanced Pilot Training
Mandatory simulator training was introduced for all 737 MAX pilots, specifically covering MCAS operation and runaway trim procedures.