When Boeing introduced an updated version of its original 737 aircraft, several faults were quickly uncovered after two tragic plane crashes. Critical safety issues riddled the original Boeing 737 MAX aircraft’s design, pertaining specifically to the functioning aspects of the maneuvering characteristics augmentation system (MCAS) system. The technical and human aspects of the system implementation and documentation contributed to the fatal accidents involving this aircraft model.
There were considerable mistakes in the technical design and the new model’s release. 737 MAX had larger engines, which caused their relocation and aircraft destabilization in steep angles like hard turns or takeoffs (Johnston & Harris, 2019). The angle of the plane’s wings against the air is also known as the angle of attack (AOA); when it exceeds a certain point, the aircraft may over-swing and stall (Johnston & Harris, 2019). The solution was the MCAS system, a flight control software designed to deliver consistent airplane control to compensate for the altered balance (Johnston & Harris, 2019). The initial system relied on a single AOA sensor to determine how the aircraft’s nose is angled relative to oncoming air (Boeing, n.d.). Relying on a single data source was the first critical technical error that severely compromised safety.
The second error was in the system’s actions once it detected that AOA was too high. To prevent stalling, it automatically commanded the plane trim system to correct the rear stabilizer and lower the nose and moved the pilot’s rudder down (Johnston & Harris, 2019). MCAS was only activated under the simultaneously occurring circumstances: the pilot controls the airplane manually; the airplane’s nose is at an unusually high angle; the pilot raised the wing flaps (Boeing, n.d.). MCAS would deactivate if the AOA dropped below the target threshold, the pilot overrode it with the manual trim setting, or the pilot activated the CUTOUT switch (Johnston & Harris, 2019). However, if the pilot overrode the system with trim controls, it would be reactivated within five seconds if the sensors were registering a higher-than-normal AOA – another critical error (Johnston & Harris, 2019). The only way to disable the automatic stabilizer trim control was to use the CUTOUT switch and handle the trim manually (Johnston & Harris, 2019). Thus, another vital flaw was the complicated system that resisted the switch to manual control.
Hence, Boeing designed the MCAS system to not turn off after the pilot manually engaged the controls. This design defeated the system’s original purpose, which was to prevent an inadvertent entrance into a stall-risk angle (Johnston & Harris, 2019). In the two accidents, a faulty AOA sensor reported incorrect information to the MCAS system, repeatedly triggering the system (Boeing, n.d.). As a result, the software repeatedly pushed the nose of faulty aircraft down whenever pilots tried to exit the dive, ultimately leading to a fatal crash (Johnston & Harris, 2019). The MCAS system thus became the suspected cause of two major fatal accidents.
There is then the question of why the pilots did not engage the CUTOUT switch to disable the system entirely. The 737 MAX aircraft was initially designed as a ‘minor upgrade’ to the original 737; hence, Boeing claimed that the pilots did not need to be relicensed, which was the human safety error (Johnston & Harris, 2019). In returning to the lack of additional training for the new model, Johnston and Harris (2019) speculate that the pilots likely had no idea that the system existed. Therefore, poor documentation and the lack of appropriate training were the last factors that exacerbated this model’s safety issues.
In conclusion, the safety issue in the Boeing 737 MAX design was multifaceted. Upon the engine’s size and positioning change, the MCAS system was introduced to handle the resulting balancing issues. However, the issue with the automated mode overriding the manual command after five seconds, its single-sensor reliance, and the lack of appropriate licensing and documentation all contributed to the system becoming a safety threat. Therefore, two recorded accidents resulted from the MCAS system relying on the faulty sensor and erroneously correcting the aircraft’s angle, unbeknownst to the pilots.
References
Boeing. (n.d.). Changes to the 737 MAX.
Johnston, P., & Harris, R. (2019). The Boeing 737 MAX Saga: Lessons for Software Organizations. Software Quality Professional, 21(3), 4–12.