Maneuvering Characteristics Augmentation System

The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law (software) written into the Boeing 737 MAX flight control system which attempts to mimic pitching behavior similar to the Boeing 737 NG, especially in high angle of attack (AoA) flight. When it detects that the aircraft is operating in manual flight, with the flaps up, at a high angle of attack, it adjusts the horizontal stabilizer trim to add positive force feedback (a "nose heavy" feel) to the pilot, through the control column.

The activation logic of MCAS has been shown to be vulnerable to erroneous angle of attack data, as analyses have shown following the Lion Air Flight 610 and Ethiopian Airlines Flight 302 crashes. Flaws found in the MCAS implementation, in crew training, and in operational procedures resulted in a worldwide grounding of the airliner, pending accident investigations, certification inquiry, and necessary changes to the MCAS software, cockpit instruments, and crew manuals and training.

MCAS also exists in another form on the Boeing KC-46 Air Force tanker, where it "similarly moves the stabilizer in a wind-up turn".[1]

The Wall Street Journal reported in May 2019 that Boeing failed to share information about the issue for "about a year" before the crash in Indonesia.[2]



The Maneuvering Control Augmentation System (MCAS) is a flight control law[3] 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.[4][5][6] 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.[7]

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 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."[8]

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,[9] in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining.[10][11][8]

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.[12]

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.[13] Following the Lion Air accident, Boeing issued an Operations Manual Bulletin (OMB)[14] which outlined 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[15] and training, pilots were unaware of the existence of MCAS due to its omission from the crew manual and no coverage in training.[13]

In addition, the system acts on only one of two available AoA sensors, a single point of failure that goes against aviation requirements of robustness and integrity, for example using redundancy.[13][16][17][10]

An AOA "disagree" light illuminates when the two sensors read different values. Airlines could optionally add a software gauge indicator to the multifunction cockpit displays. However, scrutinization has revealed that the two safety features were interdependent, and were inoperable in both doomed flights.

The older 737 cockpit has separate on/off control switches for independent electrically-assisted and automatic trim systems. On the 737 MAX, a combined switch is provided and the pilot cannot turn off the MCAS without also disabling electrically-assisted trim. A manual trim wheel is provided, but is not powerful enough to adjust the stabilizer in all flight conditions. Activating the powered trim system can be necessary and this also activates the MCAS.[18]


The MCAS design parameters originally envisioned automated corrective actions to be taken in cases of high AoA and G-forces beyond normal flight conditions. Test pilots routinely push aircraft to such extremes, as the FAA requires airplanes to perform as expected. Before the MCAS, test pilot Ray Craig determined the plane did not fly smoothly, in part due to the larger engines. Craig would have preferred an aerodynamic solution, but Boeing decided to implement a control law in software.

According to a news report from the Wall Street Journal, engineers who had worked on the KC-46A Pegasus tanker, which includes an MCAS function, suggested MCAS to the design team.[19]

With the MCAS implemented, new test pilot Ed Wilson said the "MAX wasn’t handling well when nearing stalls at low speeds” and recommended MCAS to apply across a broader range of flight conditions. This required the MCAS to function under normal G-forces and, at stalling speeds, deflect the vertical trim more rapidly and to a greater extent—but now it reads a single AoA sensor.

The FAA did not conduct a safety analysis on the changes. It had already approved the previous version of MCAS, and the agency's rules did not require it to take a second look because the changes did not affect how the plane operated in extreme situations. [20]

Safety engineering and human factors

As with any other equipment on board an aircraft, the FAA approves a functional "design assurance level" corresponding to the consequences of a failure, using the SAE International standards ARP4754 and ARP4761. MCAS was designated a "hazardous failure" system. This classification corresponds to failures causing "a large reduction in safety margins" or "serious or fatal injury to a relatively small number of the occupants", but nothing "catastrophic".[21]

The MCAS was designed with the assumption, approved by FAA, that pilots would react to an unexpected activation within three seconds.[1]

MCAS on the Boeing KC-46

MCAS exists in a different form on the Boeing KC-46 Pegasus, an aerial refueling tanker that is fundamentally a Boeing 767-2C. The system takes input on dual redundant angle of attack sensors; it will disengage with stick input by the pilot. The Air Force stated that "The KC-46 has protections that ensure pilot manual inputs have override priority" and that it "does not fly the models of aircraft involved in the recent accidents" and that it is "reviewing our procedures and training as part of our normal and ongoing review process."[22]


The MCAS has been under scrutiny following the fatal crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302 soon after takeoff. The Boeing 737 MAX global fleet has been grounded by all airlines and operators, and a number of functional issues have been raised.[23][24][25]

The use of a single AOA sensor at any one time creates a single point of failure and leads to the possibility that a single malfunctioning sensor could produce erroneous data, cause the MCAS system to pitch the nose downward and force the aircraft into a dive.[16][10]

The MCAS deflects the horizontal stabilizer four times farther than was stated in the initial safety analysis document..[23] Due to the amount of trim the system applies to the horizontal stabilizer, aerodynamic forces resist pilot control effort to raise the nose. As long as the faulty AOA readings persist, a human pilot "can quickly become exhausted trying to pull the column back".[17] In addition, switches for the horizontal stabilizer trim assist now serve a shared purpose of turning off the MCAS. In simulator sessions, pilots were stunned by the substantial effort needed to manually crank the trim wheel out of its nose down setting.[26][18][27]

Boeing and the FAA decided that the AOA display and an AOA disagree light, which signals if the sensors give different readings, were not critical features for safe operation.[28] Boeing charged extra for the addition of the AoA indicator to the primary display.[29][30] In November 2017, Boeing engineers discovered that the standard AoA disagree light cannot independently function without the optional AoA indicator software, affecting 80% of the global fleet which had not ordered the option.[31][32] The software remedy was scheduled to coincide with the roll out of the elongated 737 MAX 10 in 2020, only to be accelerated by the Lion Air accident. Furthermore, the problem had not been disclosed to the FAA until 13 months after the fact. Although it is unclear whether the indicator could have changed the outcome for the ill-fated flights, American Airlines said the disagree indicator provided the assurance in continued operations of the airplane. "As it turned out, that wasn't true." [33]

Boeing CEO Dennis Muilenburg has stated that there was "no surprise, or gap, or unknown here or something that somehow slipped through a certification process."[34] On April 29, 2019 he stated the design of the aircraft was not flawed and reiterated that it was designed per Boeing's standards.[35] In a May 29 interview with CBS, Boeing admitted that it had botched the software implementation and lamented the poor communications.[36]

On September 26, the National Transportation Safety Board criticized Boeing’s inadequate testing of the 737 MAX, and pointed out that Boeing made erroneous assumptions on pilots’ response to alerts in 737 MAX, triggered by activation of MCAS due to a faulty signal from an angle-of-attack sensor.[37][38]

The Joint Authorities Technical Review (JATR), a team commissioned by the FAA for 737 MAX investigation, concluded that FAA failed to properly review MCAS. Boeing failed to provide adequate and updated technical information regarding the MCAS system to FAA during Boeing 737 Max certification process, and had not carried out a thorough verification by stress-testing of the MCAS system.[6][39]

On October 18, Boeing turned over a discussion from 2016 between two employees which revealed prior issues with the MCAS system.[40]

Certification inquiry on 737 MAX

The day after the Ethiopian Airlines crash, a U.S. federal grand jury issued a subpoena on behalf of the U.S. Justice Department for documents related to development of the 737 MAX.[41][42][43][44] On March 19, 2019, the U.S. Department of Transportation requested the Office of Inspector General to conduct an audit on the 737 MAX certification process.[45] The FBI has joined the criminal investigation into the certification as well.[46][47] FBI agents reportedly visited the homes of Boeing employees in "knock-and-talks".[48]

Certification procedures were criticized previously, following the 1979 grounding of the McDonnell Douglas DC-10 in the wake of the American Airlines Flight 191 accident. A safety panel was convened by the FAA under the auspices of the National Academy of Sciences to investigate both the design of the DC-10 and the regulatory system itself. In its report, published in June 1980, the safety panel highlighted the FAA's reliance on the manufacturer during the certification process and the fact that in most cases it performs only a "cursory review" of the manufacturer's information.[49][50] The New York Times noted that the panel found "critical deficiencies in the way the Government certifies the safety of American-built airliners".[51]

For Marian Pistik, head of asset management at International Airfinance Corporation, the case of the MAX is unprecedented due to allegations of wrongdoings. The groundings of the DC-10 and of the Dreamliner could not be directly compared to global B737 MAX grounding, : "there was no suspicion that Boeing or any OEM knew of the problem and tried to disguise it or…any suspicion of wrongdoing or not being compliant or forthcoming with the issues of the 737 Max."[52] In November 2019, the online media "The Air Current" reviewed historical parallels of public trust between the DC-10 and the MAX.[53]

U.S. Congress

In March 2019, Congress announced an investigation into the FAA approval process.[54] Members of Congress and government investigators expressed concern about FAA rules that allowed Boeing to extensively "self-certify" aircraft.[55][56] FAA acting Administrator Daniel Elwell said "We do not allow self-certification of any kind".[57]

In September, a Congress panel asked Boeing's CEO to make several employees available for interviews, to complement the documents and the senior management perspective already provided.[58] The same month, Boeing's board called for changes to improve safety.[59] Representative Peter DeFazio, chairman of the House Transportation and Infrastructure Committee, said Boeing declined his invitation to testify at a House hearing. "Next time, it won't just be an invitation, if necessary," he said.

House Transportation and Infrastructure Committee

In September 2019, the House Transportation and Infrastructure Committee announced that Boeing CEO, Dennis Muilenburg, will testify before Congress accompanied by John Hamilton, chief engineer of Boeing's Commercial Airplanes division and Jennifer Henderson, 737 chief pilot.[60] In October 2019, the House asked Boeing to allow a flight deck systems engineer who filed an internal ethics complaint to be interviewed.[61]

On October 18, Peter DeFazio said "The outrageous instant message chain between two Boeing employees suggests Boeing withheld damning information from the FAA". Boeing expressed regret over its ex-pilot's messages after their publication in media.[62] Boeing's media room released a statement about Forkner's meaning of the instant messages, obtained through his attorney because the company has not been able to talk to him directly. The transcript of the messages indicates, according to experts, a problem with the simulator rather than an MCAS erratic activation.[63][64][65][66]

On October 25, Peter DeFazio commented the Lion Air accident report, saying ”And I will be introducing legislation at the appropriate time to ensure that unairworthy commercial airliners no longer slip through our regulatory system".[67]

House hearing with aviation stakeholders

The Subcommittee on Aviation met on June 19, 2019, to hold an hearing titled, ‘‘Status of the Boeing 737 MAX: Stakeholder Perspectives.’’

"The hearing is intended to gather views and perspectives from aviation stakeholders regarding the Lion Air Flight 610 and Ethiopian Airlines Flight 302 accidents, the resulting international grounding of the Boeing 737 MAX aircraft, and actions needed to ensure the safety of the aircraft before returning them to service. The Subcommittee will hear testimony from Airlines for America, Allied Pilots Association, Association of Flight Attendants—CWA, Captain Chesley (‘‘Sully’’) Sullenberger, and Randy Babbitt."[68][69]

On July 17, representatives of crash victims' families, in testimony to the House Transportation and Infrastructure Committee - Aviation Subcommittee, called on regulators to re-certificate the MAX as a completely new aircraft. They also called for wider reforms to the certification process, and asked the committee to grant protective subpoenas so that whistle-blowers could testify even if they had agreed to a gag order as a condition of a settlement with Boeing.[70] In a July 31 senate hearing, the FAA defended its administrative actions following the Lion Air accident, noting that standard protocol in ongoing crash investigations limited the information that could be provided in the airworthiness directive[71][72].

House hearing with Boeing

On October 29, 2019, Muilenburg and Hamilton appeared at the House hearing under the title "Aviation Safety and the Future of Boeing’s 737 MAX", which was the first time that Boeing executives addressed Congress about the MAX accidents.[73][74] The hearing came on the heels of the removal of Dennis Muilenburg's title as chairman of the Boeing board a week before and was intended to examine issues associated with the design, development, certification, and operation of the Boeing 737 MAX following two accidents in the last year.[74] The committee first heard from Boeing on actions taken to improve safety and the company’s interaction with relevant federal regulators. The second panel was comprised of government officials and aviation experts discussing the status of Boeing 737 MAX and relevant safety recommendations."[73]

On October 30, the House made public a 2015 internal email discussion between Boeing employees raising concerns about MCAS design in the exact scenario blamed for the two crashes: "Are we vulnerable to single AOA sensor failures with the MCAS implementation?"[75] Committee members discussed another internal document, stating that a reaction longer than 10 seconds to an MCAS malfunction "found the failure to be catastrophic."[75][76] The hearings' key revelation was insider knowledge of vulnerabilities amid a hectic rate of production.[77]

After the testimony of Boeing's CEO, Peter DeFazio and Rick Larsen, leader of its aviation sub-panel, wrote a letter to other lawmakers on November 4, saying that unanswered questions remain : "Mr. Muilenburg left a lot of unanswered questions, and our investigation has a long way to go to get the answers everyone deserves [...] Mr. Muilenburg’s answers to our questions were consistent with a culture of concealment and opaqueness and reflected the immense pressure exerted on Boeing employees during the development and production of the 737 Max".[78]

House hearing with FAA

On December 11, 2019, during a hearing of the House Committee on Transportation titled "The Boeing 737 MAX: Examining the Federal Aviation Administration’s Oversight of the Aircraft’s Certification," an internal FAA review[79] dated December 3, 2018, was released, which predicted a high MAX accident rate, if it kept flying with MCAS unchanged.[80] The findings were first reported by The Wall Street Journal in July 2019,[81] The FAA assumed that the emergency airworthiness directive sufficed until Boeing delivered a fix. Over a question whether a mistake was made in this regard, the FAA's chief Stephen Dickson responded, "Obviously the result was not satisfactory", and in response to repeated questions he admitted the agency had made a mistake at some point in the process. He said the report found an unacceptable level of risk, but FAA started to take action with the emergency order.[82][83] Peter DeFazio said that the committee's investigation “has uncovered a broken safety culture within Boeing and an FAA that was unknowing, unable, or unwilling to step up, regulate and provide appropriate oversight of Boeing".[84]

But perhaps most chillingly, we have learned that shortly after the issuance of the airworthiness directive, the FAA performed an analysis that concluded that, if left uncorrected, the MCAS design flaw in the 737 MAX could result in as many as 15 future fatal crashes over the life of the fleet—and that was assuming that 99 out of 100 flight crews could comply with the airworthiness directive and successfully react to the cacophony of alarms and alerts recounted in the National Transportation Safety Board’s report on the Lion Air tragedy within 10 seconds. Such an assumption, we know now, was tragically wrong.

Despite its own calculations, the FAA rolled the dice on the safety of the traveling public and let the 737 MAX continue to fly until Boeing could overhaul its MCAS software. Tragically, the FAA’s analysis—which never saw the light of day beyond the closed doors of the FAA and Boeing—was correct. The next crash would occur just five months later, when Ethiopian Airlines flight 302 plummeted to earth in March 2019.[85]

Office of Special Counsel investigation

On April 2, 2019, after receiving reports from whistle-blowers regarding the training of FAA inspectors who reviewed the 737 MAX type certificate, the Senate Commerce Committee launched a second Congressional investigation; it focuses on FAA training of the inspectors.[86][87][88]

The FAA provided misleading statements to Congress about the training of its inspectors, most possibly those inspectors that oversaw the Max certification, according to the findings of an Office of Special Counsel investigation released in September. The Office of Special Counsel is an organization investigating whistleblower reports. Its report infers that safety inspectors "assigned to the 737 Max had not met qualification standards".[89]

The OSC sided with the whistleblower, pointing out that internal FAA reviews had reached the same conclusion. In a letter to President Trump, the OSC found that 16 of 22 FAA pilots conducting safety reviews, some of them assigned to the MAX two years ago, "lacked proper training and accreditation."[90]

Safety inspectors participate in Flight Standardization Boards, that ensure pilot competency by developing training and experience requirements. FAA policy requires both formal classroom training and on-the-job training for safety inspectors.[91]

Special Counsel Henry J. Kerner wrote in the letter to the President, "This information specifically concerns the 737 Max and casts serious doubt on the FAA's public statements regarding the competency of agency inspectors who approved pilot qualifications for this aircraft".[92]

In September, Daniel Elwell disputed the conclusions of the OSC, which found that aviation safety inspectors (ASIs) assigned to the 737 MAX certifications did not meet training requirements.[93][94] To clarify the facts, lawmakers asked the FAA to provide additional information :

We are particularly concerned about the Special Counsel's findings that inconsistencies in training requirements have resulted in the FAA relaxing safety inspector training requirements and thereby adopting "a position that encourages less qualified, accredited, and trained safety inspectors."  We request that the FAA provide documents confirming that all FAA employees serving on the FSB for the Boeing 737-MAX and the Gulfstream VII had the required foundational training in addition to any other specific training requirements.[95]

Inspector General of the U.S. Department of Transport

At the request of Peter DeFazio, and Chair of the Subcommittee on Aviation Rick Larsen, the U.S. Department of Transportation (DOT) Inspector General opened an investigation into FAA approval of the Boeing 737 MAX aircraft series, focusing on potential failures in the safety-review and certification process.[96]

A report released on October 23 says that the FAA faces a "significant oversight challenge" to ensure that manufacturers carrying out delegated certification activities "maintain high standards and comply with FAA safety regulations", and that it plans to introduce a "new process that represents a significant change in its approach" by March 2020.[97][98]

Joint Authorities Technical Review

On April 19, a "Boeing 737 MAX Flight Control System Joint Authorities Technical Review" (JATR) team was commissioned by the FAA to investigate how it approved MCAS, whether changes need to be made in the FAA's regulatory process and whether the design of MCAS complies with regulations.[99] On June 1, Ali Bahrami, FAA Associate Administrator for Aviation Safety, chartered the JATR to include representatives from FAA, NASA and the nine civil aviation authorities of Australia, Brazil, Canada, China, Europe (EASA), Indonesia, Japan, Singapore and UAE.

On September 27, the JATR chair Christopher A. Hart said that FAA's process for certifying new airplanes is not broken, but needs improvements rather than a complete overhaul of the entire system. He added "This will be the safest airplane out there by the time it has to go through all the hoops and hurdles".[100]

According to the JATR final report, the FAA failed to properly review MCAS. About the nature of MCAS, "the JATR team considers that the STS/MCAS and EFS functions could be considered as stall identification systems or stall protection systems, depending on the natural (unaugmented) stall characteristics of the aircraft".[101] The report recommends that FAA reviews the jet's stalling characteristics without MCAS and associated system to determine the plane's safety and consequently if a broader design review was needed.[102]

"The JATR team identified specific areas related to the evolution of the design of the MCAS where the certification deliverables were not updated during the certification program to reflect the changes to this function within the flight control system. In addition, the design assumptions were not adequately reviewed, updated, or validated; possible flight deck effects were not evaluated; the SSA and functional hazard assessment (FHA) were not consistently updated; and potential crew workload effects resulting from MCAS design changes were not identified."[101] Nor has Boeing carried out a thorough verification by stress-testing of the MCAS.[103]

The JATR also found that Boeing exerted "undue pressures" on Boeing ODA engineering unit members (who had FAA authority to approve design changes).[101][104]


The updates proposed by Boeing focus mostly on MCAS software.[3] In particular, there have been no public statements regarding reverting the functionality of the stabilizer trim cutout switches to pre-MAX configuration. A veteran software engineer and experienced pilot suggested that software changes may not be enough to counter the 737 MAX's engine placement.[105] Seattle Times notes that while the new software fix Boeing has proposed "will likely prevent this situation recurring, if the preliminary investigation confirms that the Ethiopian pilots did cut off the automatic flight-control system, this is still a nightmarish outcome for Boeing and the FAA. It would suggest the emergency procedure laid out by Boeing and passed along by the FAA after the Lion Air crash is wholly inadequate and failed the Ethiopian flight crew."[106]

Maneuvering Characteristics Augmentation System

The Maneuvering Characteristics Augmentation System (MCAS) flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up because of the aerodynamic effect of its larger, heavier, and more powerful CFM LEAP-1B engines and nacelles.[107] The stated goal of MCAS, according to Boeing, was to provide consistent aircraft handling characteristics at elevated angles of attack in certain unusual flight conditions only and hence make the 737 MAX perform similarly to its immediate predecessor, the 737 Next Generation.[108]

737-200 JT8D engine with original cowling design
737-800 (Next Generation) CFM56 engine with ovoid inlet
737 MAX 9 CFM LEAP-1B engine with 787-derived engine chevrons

MCAS and the accidents

Investigators determined that MCAS was triggered by falsely high angle of attack (AoA) inputs, as if the plane had pitched up excessively. On both flights, shortly after takeoff, MCAS repeatedly actuated the horizontal stabilizer trim motor to push down the airplane nose.[109][110][111][112] Satellite data for the flights, ET 302 and JT 610, showed that the planes struggled to gain altitude.[113] Pilots reported difficulty controlling the airplane and asked to return to the airport.[114][115]

On March 6, 2019, four days prior to the Ethiopian crash, Boeing and the FAA declined to comment regarding their safety analysis of Maneuvering Characteristics Augmentation System (MCAS) for a story in The Seattle Times.

On March 11, 2019, after China had grounded the aircraft,[116] Boeing published some details of new system requirements for the MCAS software and for the cockpit displays, which it began implementing in the wake of the prior accident five months earlier:[109]

  • If the two AOA sensors disagree with the flaps retracted, MCAS will not activate and an indicator will alert the pilots.
  • If MCAS is activated in non-normal conditions, it will only "provide one input for each elevated AOA event."
  • Flight crew will be able to counteract MCAS by pulling back on the column.

On March 17, a week after the crash, The Seattle Times published the following flaws as identified by aviation engineers:[116]

  • The analysis downplayed MCAS's capability of pushing down the plane nose to avert a stall;[116]
  • After the Lion Air crash, Boeing released a service bulletin informing airlines that the MCAS could deflect the tail in increments up to 2.5°, up from the 0.6° told to the FAA in the safety assessment;[116]
  • MCAS could reset itself after each pilot response to repeatedly pitch the aircraft down;[116]
  • MCAS failure was assessed as "hazardous", one level below "catastrophic", but even then it should not rely on a single sensor.[116]

On March 27, Daniel Elwell, the acting administrator of the Federal Aviation Administration (FAA), testified before the Senate Committee on Commerce, Science, and Transportation, saying that on January 21, "Boeing submitted a proposed MCAS software enhancement to the FAA for certification. ... the FAA has tested this enhancement to the 737 MAX flight control system in both the simulator and the aircraft. The testing, which was conducted by FAA flight test engineers and flight test pilots, included aerodynamic stall situations and recovery procedures."[117] After a series of delays, the updated MCAS software was released to the FAA in May 2019.[118][119] On May 16, Boeing announced that the completed software update was awaiting approval from the FAA.[120][121] The flight software underwent 360 hours of testing on 207 flights.[122] Boeing also updated existing crew procedures.[109] The implementation of MCAS has been found to disrupt autopilot operations.[123]

On April 4, 2019 Boeing publicly acknowledged that MCAS played a role in both accidents.[124]

Information and dispute about MCAS

On March 30, 2016, Mark Forkner, then MAX's chief technical pilot, requested senior FAA officials to remove MCAS from the pilot's manual. The officials had been briefed on the original version of MCAS but not that MCAS was being significantly overhauled.[125] Because Boeing offered Southwest Airlines a $1-million-per-plane rebate if training was ultimately required, pressure on Boeing executives and engineers increased. In 2017, as the airliner's five-year certification was nearly completed, Forkner wrote to an FAA official, "Delete MCAS".[126] Boeing also played down the scope of MCAS to regulators. The company "never disclosed the revamp of MCAS to FAA officials involved in determining pilot training needs".[125]

Boeing published a service bulletin on November 6, 2018, in which MCAS was anonymously referred to as a "pitch trim system." In reference to the Lion Air accident, Boeing said the system could be triggered by erroneous Angle of Attack information when the aircraft is under manual control, and reminded pilots of various indications and effects that can result from this erroneous information.[127][128] Only four days later, Boeing acknowledged the existence of MCAS in a message to operators on November 10, 2018.[129][130]

The FAA and Boeing both refuted media reports describing MCAS as an anti-stall system, which Boeing asserted it is distinctly not.[131][132][133] The aircraft had to perform well in a low-speed stall test.[134] The Joint Authorities Technical Review "considers that the STS/MCAS and EFS functions could be considered as stall identification systems or stall protection systems, depending on the natural (unaugmented) stall characteristics of the aircraft".

On May 15, 2019, during a senate hearing, FAA acting administrator Daniel Elwell defended their certification process of Boeing aircraft. However the FAA criticized Boeing for not mentioning the MCAS in the 737 MAX's manuals. Representative Rick Larsen responded saying that "the FAA needs to fix its credibility problem" and that the committee would assist them in doing so.[135][136]

Type rating

In the U.S., the MAX shares a common type rating with all the other Boeing 737 families.[137] The impetus for Boeing to build the 737 MAX was serious competition from the Airbus A320neo, which was a threat to win a major order for aircraft from American Airlines, a traditional customer for Boeing airplanes.[116] Boeing decided to update its venerable 737, first designed in the 1960s, rather than creating a brand-new airplane, which would have cost much more and taken years longer. Boeing's goal was to ensure the 737 MAX would not need a new type rating, which would require significant additional pilot training, adding unacceptably to the overall cost of the airplane for customers.

The 737 original and main certification was issued by the FAA in 1967. Like every new 737 model since then, the MAX has been approved partially with the original requirements and partially with more current regulations, enabling certain rules and requirements to be grandfathered in.[138] Chief executive Dai Whittingham of the independent trade group UK Flight Safety Committee disputed the idea that the MAX was just another 737, saying, "It is a different body and aircraft but certifiers gave it the same type rating."[139]

Crew manuals and simulator training

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.[116] The 1,600-page flight crew manual mentions the term MCAS once, in the glossary.[140] Top Boeing officials believed MCAS operated only far beyond normal flight envelopes, that it was unlikely to activate in normal flight.[66]

From November 2018 to March 2019, months between the accidents, the FAA Aviation Safety Reporting System received numerous U.S. pilot complaints of the aircraft's unexpected behaviors, and how the crew manual lacked any description of the system.[141] Most air regulatory agencies, including the FAA, Transport Canada and EASA, did not require specific training on MCAS. Brazil's national civil aviation agency "was one of the only civil aviation authorities to require specific training for the operation of the 737-8 Max".[142] Pilots of the 737 Next Generation received an hour-long iPad lesson to fly on the MAX.[143]

On May 17, after discovering 737 MAX flight simulators could not adequately replicate MCAS activation,[144] Boeing corrected the software to improve the force feedback of the manual trim wheel and to ensure realism.[145] This led to a debate on whether simulator training is a prerequisite prior to the aircraft's eventual return to service. On May 31, Boeing proposed that simulator training for pilots flying on the 737 MAX would not be mandatory.[146] Computer training is deemed sufficient by the FAA Flight Standardization Board, the US Airline Pilots Association and Southwest Airlines pilots, but Transport Canada and American Airlines urged use of simulators.[147][148] On June 19, in a testimony before the U.S. House Committee on Transportation and Infrastructure, Chesley Sullenberger advocated for simulator training. "Pilots need to have first-hand experience with the crash scenarios and its conflicting indications before flying with passengers and crew."[149][150] The "differences training" is the subject of worry by senior industry training experts.[151]

Textron-owned simulator maker TRU Simulation + Training anticipates a transition course but not mandatory simulator sessions in minimum standards being developed by Boeing and the FAA.[152]

On July 24, Boeing indicated that some regulatory agencies may mandate simulator training before return to service, and also expected some airlines to require simulator sessions even if these are not mandated.[153]

On October 20, 2019, in response to harsh reactions to the publication of Forkner's controversial messages about MCAS simulation during development, Boeing issued a statement about misinterpretations and how it informed the FAA of the expansion of MCAS to low speeds.[63]

Boeing 737 safety analysis

Aviation safety risk is defined in FAA AC 25-.309-1A, a document describing acceptable means for showing compliance with the airworthiness requirements of § 25.1309 of the Federal Aviation Regulations. A catastrophic failure must have an extremely improbable rate, defined as 1 in a billion flight hours or less (10 to the minus ninth).

By this measure, as of 2005, the Boeing 737 had an actual fatal accident rate of 1 in 80 million flight hours, missing the requirements by an order of magnitude. [154]

In the two years of the 737 MAX's commercial service prior to its grounding, the global fleet of nearly 400 aircraft flew 500,000 flights and suffered two hull loss incidents. As of March 11, 2019, the 737 MAX's accident rate was second behind the Concorde, with four accidents per million flights, compared to the Boeing 737 Next Generation's 0.2 accidents per million flights.[155]

MCAS and MAX safety risk analysis

On December 3, 2018, FAA conducted an internal safety review, using the TARAM system, over the plane’s lifetime; it predicted 15 fatal crashes of the roughly 4,800 MAX in service over the next 45 years, if the MAX kept flying with MCAS unchanged.[156] Boeing also did a similar risk analysis for the MAX after the Lion Air crash, with results matching those of the FAA.[157]

The report was refuted by MIT professor Arnold Barnett based on the loss of two aircraft out of only 400 delivered. He said there would be 24 crashes per year for a fleet size of 4,800, thus the FAA underestimated the risk by a factor of 72.[158]

Technology readiness

Boeing implemented the original version of MCAS on the KC-46 tanker, a plane derived from the Boeing 767. The tanker compares the data from both AoA sensors and allows pilots to retake control in the event of large differences; without cross-checking, the MAX flight control computer activates MCAS using just one AoA sensor. In addition, some familiar pilot actions for manually controlling the pitch on other 737 types do not deactivate the MCAS.[159][160]

Boeing presented MCAS to the FAA as being existing technology, avoiding deeper scrutiny. The U.S. House Transportation and Infrastructure Committee provided all 43 Boeing's presentation slides in the document titled “MCAS Development and Certification Overview.” at the request of the Seattle Times, which noted that MCAS was not evaluated as an individual system that was “new/novel on the MAX.” The FAA is required to be closely involved in the testing and certification of any new and novel features on an aircraft.[161] Aerospace reporter Dominic Gates summarized: "The justification given was a doubtful comparison with the 767 tanker".

Just before entering certification, the functional requirements for MCAS were still changing. Boeing modified MCAS so that it intervened more strongly and at lower airspeeds than originally planned. "Inadvertently, the door was now opened to serious system misbehavior during the busy and stressful moments right after takeoff", said Jenkins of the Wall Street Journal.[162]

The JATR found the technology unprecedented: "If the FAA technical staff had been fully aware of the details of the MCAS function, the JATR team believes the agency likely would have required an issue paper for using the stabilizer in a way that it had not previously been used. MCAS used the stabilizer to change the column force feel, not trim the aircraft. This is a case of using the control surface in a new way that the regulations never accounted for and should have required an issue paper for further analysis by the FAA. If an issue paper had been required, the JATR team believes it likely would have identified the potential for the stabilizer to overpower the elevator."[101]

in November 2019, Jim Marko, a manager of aircraft integration and safety assessment at Transport Canada aviation regulator’s National Aircraft Certification Branch questioned the readiness of MCAS. Because new problems kept emerging, he suggested to his peers at FAA, ANAC and EASA to consider the safety benefits of removing MCAS from the MAX.[163]

Runaway stabilizer and manual trim

In February 2016, the European Aviation Safety Agency (EASA) certified the MAX with the expectation that pilot procedures and training would clearly explain unusual situations in which the seldom used manual trim wheel would be required to trim the plane, i.e. adjust the angle of the nose; however, the original flight manual did not mention those situations.[164] The EASA certification document referred to simulations whereby the electric thumb switches were ineffective to properly trim the MAX under certain conditions. The EASA document said that after flight testing, because the thumb switches could not always control trim on their own, the FAA was concerned by whether the 737 MAX system complied with regulations.[165] The American Airlines flight manual contains a similar notice regarding the thumb switches but does not specify conditions where the manual wheel may be needed.[165]

Boeing's CEO Muilenburg, when asked about the non-disclosure of MCAS, cited the "runaway stabilizer trim" procedure as part of the training manual. He added that Boeing's bulletin pointed to that existing flight procedure. Boeing views the "runaway stabilizer trim" checklist as a memory item for pilots. Mike Sinnett, vice president and general manager for the Boeing New Mid-Market Airplane (NMA) since July 2019, repeatedly described the procedure as a "memory item".[166] However, some airlines view it as an item for the quick reference card.[167] The FAA issued a recommendation about memory items in an Advisory Circular, Standard Operating Procedures and Pilot Monitoring Duties for Flight Deck Crewmembers: "Memory items should be avoided whenever possible. If the procedure must include memory items, they should be clearly identified, emphasized in training, less than three items, and should not contain conditional decision steps."[168]

In November 2018, Boeing told airlines that MCAS could not be overcome by pulling back on the control column to stop a runaway trim as on previous generation 737s.[169] Nevertheless, confusion continued: the safety committee of a major U.S. airline misled its pilots by telling that the MCAS could be overcome by "applying opposite control-column input to activate the column cutout switches".[170] Former pilot and CBS aviation & safety expert "Sully" Sullenberger testified, "The logic was that when MCAS was activated, it had to be, and must not be prevented."[150]

In a legal complaint against Boeing, the Southwest Airlines Pilot Association states:[171]

An MCAS failure is not like a runaway stabilizer. A runaway stabilizer has continuous un-commanded movement of the tail, whereas MCAS is not continuous and pilots (theoretically) can counter the nose-down movement, after which MCAS would move the aircraft tail down again. Moreover, unlike runaway stabilizer, MCAS disables the control column response that 737 pilots have grown accustomed to and relied upon in earlier generations of 737 aircraft.

In May 2019, The Seattle Times reported that the two stabilizer cutoff switches on the MAX operate differently than on the earlier 737 NG. On previous aircraft, one cutoff switch deactivates the thumb buttons on the control yoke that pilots use to move the horizontal stabilizer; the other cutoff switch disables automatic control (as from autopilot) to move the stabilizer in the tail. On the MAX, both switches do the same thing: they cut off all electric power to the stabilizer, both from the yoke buttons and from an automatic system, like MCAS. With all power to the stabilizer cut, pilots have no choice but to use the mechanical trim wheel in the center console.[172] However, as pilots pull on the 737 controls to raise the nose of the aircraft, aerodynamic forces on the elevator create an opposing force, effectively paralyzing the jackscrew mechanism.[173] It becomes very difficult for pilots to hand crank the trim wheel.[173] The problem was encountered on earlier 737 versions, and a "roller coaster" emergency technique for handling the flight condition was documented in 1982 for the 737-200 but did not appear in training documentation for later versions (including the MAX).[173]

In its safety analysis for the 737 MAX, Boeing made the assumption that pilots trained on standard Boeing 737 safety procedures should be able to properly assess contradictory warnings and act effectively within four seconds.[174] This four-second rule, for a pilot's assessment of an emergency and its correction, a standard value used in safety assessment scenarios for the MAX, is deemed too short, and criticized for not being supported by empirical human factors studies.[175] The Lion Air accident investigation report found that on the fatal flight and on the previous one, crews responded in about 8 seconds. According to the report, Boeing reasoned that pilots could counter an erratic MCAS by pulling back on the control column alone, without using the cutout switches. However, MCAS could only be stopped by the cutoff switches.[176]

Angle of Attack (AoA) system

The Angle of Attack sensors measure an aircraft's pitch relative to oncoming winds. Though there are two sensors, only one of them is used at a time to trigger MCAS activation on the 737 MAX. Any fault in this sensor, perhaps due to physical damage,[150] is then a single point failure mode: the flight control system lacks any basis for recognizing its output as faulty and rejecting the data.

Reports of a single point of failure were not always acknowledged by Boeing. Addressing American Airlines pilots, Boeing vice-president Mike Sinnett contradicted reports that the MCAS had a single-point failure, because the pilots themselves are the backup. Reporter Useem said in The Atlantic it was "showing both a misunderstanding of the term and a sharp break from Boeing’s long-standing practice of having multiple backups for every flight system".[177]

Problems with the angle-of-attack sensor had been reported in over 200 incident reports submitted to the FAA; however, Boeing did not flight test a scenario in which it malfunctioned.[178]

Recognized civil aviation development practices, such as those of SAE International ARP4754 and ARP4761, require a safety process with quantitative assessments of availability, reliability, and integrity, validation of requirements, and verification of implementation. Such processes rely on engineering judgment and the application of these practices varies within the industry.[179][180] Redundancy is a technique that may be used to achieve the quantitative safety requirements.[181] The sensors themselves are under scrutiny. Sensors on the Lion air aircraft were supplied by United Technologies' Rosemount Aerospace.[182]

In September 2019, the European Union Aviation Safety Agency (EASA) said it prefers triple-redundant Angle of Attack sensors rather than the dual redundancy in Boeing's proposed upgrade to the MAX.[183] Installation of a third sensor could be expensive and take a long time. The change, if mandated, could be extended to thousands of older model 737s in service around the world.[183]

A former professor at Embry-Riddle Aeronautical University, Andrew Kornecki, who is an expert in redundancy systems, said operating with one or two sensors "would be fine if all the pilots were sufficiently trained in how to assess and handle the plane in the event of a problem". But, he would much prefer building the plane with three sensors, as Airbus does.[184]

AoA Disagree alert

When a significant difference is registered between a pair of Angle of Attack sensors, the message AoA Disagree is supposed to appear on the primary flight display of an aircraft[185] Thus, pilots get insight into sensor problems, and possible resulting control system problems; aircrew will also normally make a maintenance log entry.[109]

Boeing had charged extra for this optional safety feature.[186] For example, Air Canada, American Airlines and Westjet had purchased the disagree alert, while Air Canada and American Airlines also purchased, in addition, the AoA value indicator, and Lion Air had neither.[187][188]

In November 2017, after several months of MAX deliveries, Boeing discovered that the disagree alert depended on the presence of the visual indicator software, a paid option that was not selected by most airlines. Boeing had determined that the defect was not critical to aircraft safety or operation, and an internal safety review board (SRB) corroborated Boeing's prior assessment and its initial plan to update the aircraft in 2020. Boeing did not disclose the defect to the FAA until November 2018, in the wake of the Lion Air crash.[189] [190][191][192] Consequently, Southwest had announced to pilots that its entire fleet of MAX 8 aircraft will receive the optional upgrades.[193][194]

In March 2019, after the second accident of Ethiopian Airlines Flight 302, a Boeing representative told Inc. magazine, "Customers have been informed that AOA [angle of attack] disagree alert will become a standard feature on the 737 Max. It can be retrofitted on previously delivered airplanes."[195]

In May 2019, Boeing defended that "Neither the angle of attack indicator nor the AoA Disagree alert are necessary for the safe operation of the airplane." Boeing recognized that the defective software was not implemented to their specifications as a "standard, standalone feature." Boeing stated, "...MAX production aircraft will have an activated and operable AOA Disagree alert and an optional angle of attack indicator. All customers with previously delivered MAX airplanes will have the ability to activate the AOA Disagree alert."[190] Boeing CEO Muilenburg said the company's communication about the alert "was not consistent. And that's unacceptable."[196][190]

On June 7, the delayed escalation on the defective Angle-of-Attack Disagree alert on 737 MAX was investigated. The Chair of the House Committee on Transportation and Infrastructure and the Chair of the Aviation Subcommittee sent letters to Boeing, United Technologies Corp., and the FAA, requesting a timeline and supporting documents related to awareness of the defect, and when airlines were notified.[197]

Clint Balog, a professor at Embry-Riddle Aeronautical University, said after the Lion Air crash: “In retrospect, clearly it would have been wise to include the warning as standard equipment and fully inform and train operators on MCAS”.[198]

According to Bjorn Fehrm, Aeronautical and Economic Analyst at Leeham News and Analysis, "A major contributor to the ultimate loss of JT610 is the missing AOA DISAGREE display on the pilots’ displays."[199]

Visual AoA indicator

In 1996, the National Transportation Safety Board (NTSB) issued Safety Recommendation A-96-094.

TO THE FEDERAL AVIATION ADMINISTRATION (FAA): Require that all transport-category aircraft present pilots with angle-of-attack info in a visual format, and that all air carriers train their pilots to use the info to obtain maximum possible airplane climb performance.

The NTSB also stated about another accident in 1997, that "a display of angle of attack on the flight deck would have maintained the flightcrew's awareness of the stall condition and it would have provided direct indication of the pitch attitudes required for recovery throughout the attempted stall recovery sequence." The NTSB also believed that the accident may have been prevented if a direct indication of AoA was presented to the flightcrew (NTSB, 1997)."[200](p29)

Boeing published an article in Aero magazine about AoA systems, "Operational use of Angle of Attack on modern commercial jet planes":

Angle of attack (AOA) is an aerodynamic parameter that is key to understanding the limits of airplane performance. Recent accidents and incidents have resulted in new flight crew training programs, which in turn have raised interest in AOA in commercial aviation. Awareness of AOA is vitally important as the airplane nears stall. [...] The AOA indicator can be used to assist with unreliable airspeed indications as a result of blocked pitot or static ports and may provide additional situation and configuration awareness to the flight crew.[201]

Boeing announced a change in policy in the Frequently Asked Questions in a (FAQ) about the MAX corrective work, "With the software update, customers are not charged for the AOA disagree feature or their selection of the AOA indicator option."[202]

Flight computer architecture

In early April 2019, Boeing reported a problem with software affecting flaps and other flight-control hardware, unrelated to MCAS; classified as critical to flight safety, the FAA has ordered Boeing to fix the problem correspondingly.[203] In October 2019, the European Union Aviation Safety Agency (EASA) has suggested to conduct more testing on proposed revisions to flight-control computers due to its concerns about portions of proposed fixes to MCAS.[204] The necessary changes to improve redundancy between the two flight control computers have proved more complex and time-consuming than the fixes for the original MCAS issue, delaying any re-introduction to service beyond the date originally envisaged.[205]

Microprocessor stress testing

The MAX systems are integrated in the "e-cab" test flight deck, a simulator built for developing the MAX.[206][207] In June 2019, "in a special Boeing simulator that is designed for engineering reviews,"[208] FAA pilots performed a stress testing scenario  an abnormal condition identified through FMEA after the MCAS update was implemented[209]  for evaluating the effect of a fault in a microprocessor: as expected from the scenario, the horizontal stabilizer pointed the nose downward. Although the test pilot ultimately recovered control, the system was slow to respond to the proper runaway stabilizer checklist steps. Boeing initially classified this as a "major" hazard, and the FAA upgraded it to a much more severe "catastrophic" rating. Boeing stated that the issue can be fixed in software.[210] The software change will not be ready for evaluation until at least September 2019.[211] EASA director Patrick Ky said that retrofitting additional hardware is an option to be considered.[123]

Early news reports were inaccurate in attributing the problem to an 80286[212] microprocessor overwhelmed with data. The test scenario simulated an event toggling five bits in the flight control computer. The bits represent status flags such as whether MCAS is active, or whether the tail trim motor is energized. Engineers were able to simulate single event upsets and artificially induce MCAS activation by manipulating these signals. Such a fault occurs when memory bits change from 0 to 1 or vice versa, which is something that can be caused by cosmic rays striking the microprocessor.[213]

The failure scenario was known before the MAX entered service in 2017: it had been assessed in a safety analysis when the plane was certified. Boeing had concluded that pilots could perform a procedure to shut off the motor driving the stabilizer to overcome the nose-down movement.[214] The scenario also affects 737NG aircraft, though it presents less risk than on the MAX. On the NG, moving the yoke counters any uncommanded stabilizer input, but this function is bypassed on the MAX to avoid negating the purpose of MCAS.[215] Boeing also said that it agreed with additional requirements that the FAA required it to fulfill, and added that it was working toward resolving the safety risk. It will not offer the MAX for certification until all requirements have been satisfied.[210]

Computer redundancy

As of 2019, the two flight control computers of Boeing 737 never cross-checked each other's operations; i.e., each was a single non-redundant channel. This lack of robustness existed since the early implementation and persisted for decades.[213] The updated flight control system will use both flight control computers and compare their outputs. This switch to a fail-safe two-channel redundant system, with each computer using an independent set of sensors, is a radical change from the architecture used on 737s since the introduction on the older model 737-300 in the 1980s. Up to the MAX in its prior to groundings version, the system alternates between computers after each flight.[213] The two computers architecture allowed switching in flight if the operating computer failed, thus increasing availability. In the revised architecture, Boeing required the two computers to monitor each other so that each one can vet the other.[205]


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