page_title: Maneuvering Characteristics Augmentation System
section_title: Background
hierarchical_section_title: Maneuvering Characteristics Augmentation System / Description / Background
context_page_description: The Maneuvering Characteristics Augmentation System is a flight control law 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, 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.
context_section_description: The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law built into the Boeing 737 MAX's flight control computer, designed to help the aircraft emulate the handling characteristics of the earlier Boeing 737 Next Generation. According to an international Civil Aviation Authorities team review (JATR) commissioned by the FAA, MCAS may be a stall identification or protection system, depending on the natural (unaugmented) stall characteristics of the aircraft. Boeing considered MCAS part of the flight control system, and elected to not describe it in the flight manual or in training materials, based on the fundamental design philosophy of retaining commonality with the 737NG. Minimizing the functional differences between the Boeing 737 MAX and Next Generation aircraft variants allowed both variants to share the same type rating. Thus, airlines can save money by employing and training one pool of pilots to fly both variants of the Boeing 737 interchangeably.
  When activated, MCAS directly engages the horizontal stabilizer, thus is distinct from an anti-stall device, such as stick pusher, which physically moves the pilot's control column forward and engages the airplane's elevators when the airplane is approaching a stall.
  Boeing's former CEO Dennis Muilenburg said "[MCAS] has been reported or described as an anti-stall system, which it is not. It's a system that's designed to provide handling qualities for the pilot that meet pilot preferences."
  The 737 MAX's larger CFM LEAP-1B engines are fitted further forward and higher up than in previous models. The aerodynamic effect of its nacelles contributes to the aircraft's tendency to pitch up at high angles of attack (AOA). The MCAS is intended to compensate in such cases, modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining.
  The software code for the MCAS function and the computer for executing the software are built to Boeing's specifications by Collins Aerospace, formerly Rockwell Collins.
  As an automated corrective measure, the MCAS was given full authority to bring the aircraft nose down, and could not be overridden by pilot resistance against the control wheel as on previous versions of the 737. Following the Lion Air accident, Boeing issued an Operations Manual Bulletin (OMB) on November 6, 2019 to  outline the many indications and effects resulting from erroneous AOA data and provided instructions to turn off the motorized trim system for the remainder of the flight, and trim manually instead. Until Boeing supplemented the manuals and training, pilots were unaware of the existence of MCAS due to its omission from the crew manual and no coverage in training.  Boeing first publicly named and revealed the existence of MCAS on the 737 MAX in a message to airline operators and other aviation interests on November 10, 2018, twelve days after the Lion Air crash.
caption: The trim wheel and cutoff switches in a previous generation cockpit
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