page_title: Boeing 737 MAX groundings
section_title: Maneuvering Characteristics Augmentation System
hierarchical_section_title: Boeing 737 MAX groundings / Maneuvering Characteristics Augmentation System
caption_attribution_description: English: Photo of the root of the trimmable horizontal stabilizer on an Embraer ERJ-170. The markings UP and DOWN refer to "nose up" and "nose down", hence the orientation in reverse from what might be expected. العربية: صورة لقاعدة الموازن الأفقي على طائرة. العلامات فوق وتحت تشير إلى مقدمة الطائرة فوق و مقدمة الطائرة تحت، حيث اتجاه الموازن معاكس لتوجيه الطائرة
context_page_description: In March 2019, the Boeing 737 MAX passenger airliner was grounded worldwide after 346 people died in two crashes, Lion Air Flight 610 on October 29, 2018 and Ethiopian Airlines Flight 302 on March 10, 2019. Ethiopian Airlines immediately grounded its remaining MAX fleet. On March 11, the Civil Aviation Administration of China ordered the first nationwide grounding, followed by most other aviation authorities in quick succession. The U.S. Federal Aviation Administration publicly affirmed the airworthiness of the airplane on March 11, but grounded it on March 13 after receiving evidence of accident similarities. All 387 aircraft, which served 8,600 flights per week for 59 airlines, were barred from service by March 18, 2019. The groundings became the longest ever of a U.S. airliner.
  In November 2018 Boeing revealed the MAX had a new automated flight control, the Maneuvering Characteristics Augmentation System, that could repeatedly push the airplane nose down. Boeing had omitted any mention of the system from the aircraft manuals. A week after the first accident, Boeing and the FAA sent airlines urgent messages emphasizing a procedure to recover from a malfunction.
context_section_description: The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law (software mode) first deployed on the Boeing KC-46 Air Force tanker, where it "moves the stabilizer in a wind-up turn". MCAS adjusts the horizontal stabilizer trim to push the nose down when the aircraft is operating in manual flight, with flaps up, at an elevated angle of attack (AoA), so the pilot will not inadvertently pull the airplane up too steeply, potentially causing a stall. Contrary to descriptions in news reports, however, Boeing claims that MCAS is not an anti-stall system. 
  On the Boeing 737 MAX, MCAS was intended to mimic pitching behavior similar to aircraft in the previous generation of the series, the Boeing 737 NG. MCAS activated by input from only one of the airplane's two angle of attack sensors, making the system susceptible to a single point of failure. It could not be instinctively disabled by pulling on the control yoke. During aircraft certification, Boeing removed a description of MCAS in the MAX flight manuals, leaving pilots unaware of the system when the airplane entered service. 
  On November 10, 2018, Boeing publicly revealed MCAS in a discussion with airline operators and other aviation interests twelve days after Lion Air Flight 610 crashed. Yet, a recovery procedure highlighted by Boeing and the FAA failed to prevent the crash of Ethiopian Airlines Flight 302, which led to the global grounding of all 737 MAX aircraft pending investigations and software fixes.  
  In April 2019, Boeing admitted that MCAS played a role in both accidents. The investigations identified numerous defects with associated systems, including an AoA disagree message that should have prevented MCAS activation. The Wall Street Journal reported that Boeing failed to share information about that issue for "about a year" before the Lion Air crash in Indonesia.
caption: Movable horizontal stabilizer on an Embraer ERJ-170
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