Back to home page737 MAX - MCAS

Home > History and Variants > 737 Max > MAX Differences > MCAS


Illustrated technical information covering Vol 2 Over 800 multi-choice systems questions Study notes and technical information Close up photos of internal and external components A compilation of links to major 737 news stories with a downloadable archive Illustrated history and description of all variants of 737 Detailed tech specs of every series of 737 Databases and reports of all the major 737 accidents & incidents General flightdeck views of each generation of 737's Description & news reports of Advanced Blended Winglets Press reports of orders and deliveries A collection of my favourite photographs that I have taken of or from the 737 Details about 737 production methods A compilation of links to other sites with useful 737 content History and Development of the Boeing 737 - MAX A quick concise overview of the pages on this site


This page describes the new 737 MAX Maneuvering Characteristics Augmentation System (MCAS)


The 737 MAX was produced with several differences from the NG. Many of these differences were obvious such as the new LEAP engines or the larger flight display screens. Some were less obvious but well documented such as the FBW spoiler system. It also now appears that some differences were almost hidden, certainly from the flight crew. MCAS is one such difference.

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

On the face of it, MCAS seemed like a reasonable solution to the low speed handling certification problem. However, two accidents occurred to the MAX which were attributed to MCAS - Lion Air MAX-8 PK-LQP on 29 October 2018 and Ethiopian MAX-8 ET-AVJ on 10 March 2019, both of which crashed shortly after take-off following erroneous data from a single AoA probe. It is believed that the MCAS used this erroneous AoA data to command nose down stabiliser trim which was not counteracted successfully by the crew until impact.

The MCAS design at the time of these accidents would trim the Stabilizer down for up to 9.26 seconds (2.5 deg nose down) then pause for 5 seconds and repeat if the conditions (high AoA, flaps up and autopilot disengaged) continued to be met. If the pilots used electric pitch trim, it would only pause MCAS for 5s; to deactivate it you have to switch off the STAB TRIM CUTOUT switches.

As details emerged about the similarities between the two accidents, the 737 MAX was grounded by many countries unilaterally. The FAA followed suit on 13 March. Boeing issued the following statement: “After consultation with the US FAA, the US NTSB, and aviation authorities and its customers around the world, Boeing has determined—out of an abundance of caution and in order to reassure the flying public of the aircraft’s safety—to recommend to the FAA the temporary suspension of operations of the entire global fleet of 371 737 MAX aircraft. Boeing makes this recommendation and supports the decision by the FAA.”.

See more details about the book

All of the information, photographs & schematics from this website and much more is now available in a 374 page printed book or in electronic format.

*** Updated 18 Apr 2019 ***

The 737 Tech Site on Facebook The 737 Tech Site on Twitter The 737 Tech Site on Instagram

14CFR §25.203 Stall characteristics.

(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.

Technical Description of MCAS (Pre-Modifications)

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance longitudinal stability characteristics with flaps UP and at elevated Angles of Attack (AoA). The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control Computer (FCC) using input data from sensors and other airplane systems.

The MCAS function becomes active when the AoA exceeds a threshold based on airspeed and altitude. MCAS will activate for up to 9.26 seconds before pausing for 5 seconds. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers (for the same AoA above the activation threshold).

After AoA falls below the hysteresis threshold (0.5 degrees below the activation angle), MCAS commands nose up stabilizer to return the aircraft to the trim state that existed before the MCAS activation.

The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.


To summarise; MCAS will trim the Stabilizer down for up to 9.26 seconds (2.5 deg nose down) and pause for 5 seconds and repeat if the conditions (high angle of attack, flaps up and autopilot disengaged) continue to be met. MCAS will turn the trim wheel. Using electric pitch trim will only pause MCAS for 5s; to deactivate it you need to switch off the STAB TRIM CUTOUT switches.

The AoA source

Since MCAS is an FCC function, the AoA source for MCAS is that of the FCC in use; ie FCC 1 uses the Captains AoA probe and FCC 2 uses the F/Os AoA probe. When the 737 is powered up the FCC used is FCC 1 for that flight, this changes for each subsequent flight until the aircraft is powered down. Therefore the AOA sensor that is used for MCAS changes with each flight post power-up.

The MAX probes on the LHS. The AoA vane is the bottom of the three probes.

AoA Display

Pre-accidents, there was an option for airlines to have an AoA indicator displayed on the PFDs - for a fee. As far as I know this option was only been taken by Southwest and American Airlines before the accidents. As part of the post-grounding MCAS upgrade, the optional AoA indicator will now be available free of charge and the AOA DISAGREE alert will now be standard on all MAX aircraft.

The AoA Disagree Alert will display "AOA DISAGREE" in amber at the bottom right of the PFD if the AoA vanes disagree by more than 10 degrees for more than 10 continuous seconds.

The QRH AoA Disagree Procedure is as follows:

Condition: The AOA DISAGREE alert indicates the left and right angle of attack vanes disagree.

1 Airspeed errors and the IAS DISAGREE alert may occur.

2 Altimeter errors and the ALT DISAGREE alert may occur.


Boeing has said in a statement on Monday 29 April that an error on their part meant that the AOA DISAGREE alert was only enabled on aircraft in which the customers had selected the optional AOA indicator. The alert was intended to be enabled on all MAX aircraft as standard.

“The disagree alert was intended to be a standard, stand-alone feature on Max airplanes,” the company said. “However, the disagree alert was not operable on all airplanes because the feature was not activated as intended.”

The error was discovered in 2017, Boeing says a Safety Review Board it had convened confirmed the company’s view that that the absence of a 737 AOA disagree alerts did not present a safety issue, and this was shared with the FAA.

Note that the MAX will have a software update (CDS BP?) to allow the disagree alert to function without relying on any optional systems as it does on the NG. This of course is in addition to the FCC update for MCAS.

This is the full statement:

"On every airplane delivered to our customers, including the MAX, all flight data and information needed to safely operate the aircraft is provided in the flight deck on the primary flight deck displays. This information is provided full-time in the pilots’ primary field of view, and it always has been.

Air speed, attitude, altitude, vertical speed, heading and engine power settings are the primary parameters the flight crews use to safely operate the airplane in normal flight. Stick shaker and the pitch limit indicator are the primary features used for the operation of the airplane at elevated angles of attack. All recommended pilot actions, checklists, and training are based upon these primary indicators. Neither the angle of attack indicator nor the AOA Disagree alert are necessary for the safe operation of the airplane. They provide supplemental information only, and have never been considered safety features on commercial jet transport airplanes. 

The Boeing design requirements for the 737 MAX included the AOA Disagree alert as a standard, standalone feature, in keeping with Boeing’s fundamental design philosophy of retaining commonality with the 737NG. In 2017, within several months after beginning 737 MAX deliveries, engineers at Boeing identified that the 737 MAX display system software did not correctly meet the AOA Disagree alert requirements. The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature on the MAX and the NG. Accordingly, the software activated the AOA Disagree alert only if an airline opted for the AOA indicator. 

When the discrepancy between the requirements and the software was identified, Boeing followed its standard process for determining the appropriate resolution of such issues. That review, which involved multiple company subject matter experts, determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation. Accordingly, the review concluded, the existing functionality was acceptable until the alert and the indicator could be delinked in the next planned display system software update. Senior company leadership was not involved in the review and first became aware of this issue in the aftermath of the Lion Air accident.

Approximately a week after the Lion Air accident, on November 6, 2018, Boeing issued an Operations Manual Bulletin (OMB), which was followed a day later by the FAA’s issuance of an Airworthiness Directive (AD). In identifying the AOA Disagree alert as one among a number of indications that could result from erroneous AOA, both the OMB and the AD described the AOA Disagree alert feature as available only if the AOA indicator option is installed.

Boeing discussed the status of the AOA Disagree alert with the FAA in the wake of the Lion Air accident. At that time, Boeing informed the FAA that Boeing engineers had identified the software issue in 2017 and had determined per Boeing’s standard process that the issue did not adversely impact airplane safety or operation. In December 2018, Boeing convened a Safety Review Board (SRB) to consider again whether the absence of the AOA Disagree alert from certain 737 MAX flight displays presented a safety issue. That SRB confirmed Boeing’s prior conclusion that it did not. Boeing shared this conclusion and the supporting SRB analysis with the FAA.

Boeing is issuing a display system software update, to implement the AOA Disagree alert as a standard, standalone feature before the MAX returns to service. When the MAX returns to service, all 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."


MAX Runaway Stabilizer procedure

For full details of the evolution of the runaway stabilizer procedure over the history of the 737, follow this link.

Following the Lion Air accident, on 7 Nov 2018 the FAA issue an Emergency AD (2018-23-51) and Boeing issue an Ops Manual Bulletin (TBC-19) for MAX Runaway Stabilizer procedure directing operators to “existing flight crew procedures" to address circumstances involving erroneous angle-of-attack sensor information.

FAA Emergency AD 2018-23-51 - SUMMARY: We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 737-8 and -9 airplanes. This emergency AD was sent previously to all known U.S. owners and operators of these airplanes. This AD requires revising certificate limitations and operating procedures of the airplane flight manual (AFM) to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions. This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer. We are issuing this AD to address the unsafe condition on these products..

This AD requires revising certificate limitations and operating procedures of the airplane flight manual (AFM) to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions.

Ops Manual Bulletin TBC-19

This is a copy of the bulletin, it has a different reference (MLI-15) because it is with a different airline; TBC = The Boeing Company


Stab Trim Cut-Out Switches

MAIN ELEC and AUTO PILOT Stab trim switches now either switch will cut-off both main electric and autopilot stab trim. They are renamed PRI and B/U (primary and backup). The switches are guarded in the NORMAL position (switch up) and CUTOFF is switch down.

737 MAX Stab Trim Cut Out Switches


Manual Trim Wheel Loads

The preliminary report into the Ethiopian accident, appears to show that after the crew had switched off the Stab trim cut-out switches they were unable to operate the trim wheel manually and subsequently switched them back on again to get electric trim which unfortunately also allowed MCAS to reactivate. The reason for why the trim wheel could not be operated manually is still unknown (hopefully only until the final report is issued) but it is probable that the control forces on the stab, and therefore the wheel, were too high due to the very high IAS at the time. It is now known that the FAA has doubts about the ability of some pilots (eg women) to have the level of strength required to operate the manual trim wheel at high IAS. This is applicable for all series of 737, not just the MAX.

On Friday 17 May, Boeing spokesman, Gordon Johndroe, said that Boeing has made corrections to 737 MAX simulator software and the company has provided additional information to device operators. He said the changes will ensure that the simulator experience is representative across different flight conditions and will improve the simulation of force loads on the manual trim wheel that helps control the airplane.

The comments came after the New York Times reported that Boeing recently discovered that the flight simulators airlines use to train pilots could not adequately replicate conditions that played a role in the 737 MAX crashes.

As a point of interest, the stab trim wheel on the MAX is 1 inch less in diameter than in all previous generations of 737. This would slightly reduce the force that the crew could turn the wheel as the handles are located near the circumference.


FAA Actions


On March 11, 2019, the FAA issued a Continued Airworthiness Notification to the International Community (CANIC), providing information regarding FAA continued operational safety activity related to the Boeing 737-8 and Boeing 737-9 (737 MAX) fleet. (available here)

Situation description: Following the accident of an Ethiopian Airlines Boeing Model 737-8 airplane on March 10, 2019, the National Transportation Safety Board (NTSB) as the accredited representative, and the FAA as Technical Advisors, are supporting the Ethiopian Accident Investigation Bureau. The FAA has dispatched personnel to support the investigative authorities in determining the circumstances of this event. All data will be closely examined during this investigation, and the FAA will take appropriate action if the data indicates the need to do so.

External reports are drawing similarities between this accident and the Lion Air Flight 610 accident on October 29, 2018. However, this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions.

Following the Lion Air Flight 610 accident, the FAA has completed these activities in support of continued operational safety of the fleet:

- Issued FAA emergency Airworthiness Directive (AD) 2018-23-51 on November 7, 2018
- Validated that airplane maintenance and functional check instructions on Angle of Attack (AOA) vane replacement were adequate
- Conducted simulator sessions to verify the Operational Procedures called out in FAA AD 2018-23-51
- Validated AOA vane bench check calibration procedures were adequate
- Reviewed Boeing’s production processes related to the AOA vane and Maneuvering Characteristics Augmentation System (MCAS)

Ongoing oversight activities by the FAA include:

- Boeing’s completion of the flight control system enhancements, which provide reduced reliance on procedures associated with required pilot memory items. The FAA anticipates mandating these design changes by AD no later than April 2019.

- Design changes include:

 MCAS Activation Enhancements
 MCAS AOA Signal Enhancements
 MCAS Maximum Command Limit

- Boeing’s plans to update training requirements and flight crew manuals to go with the MCAS design change include:

o Airplane Flight Manual (AFM) and Flight Crew Operations Manual (FCOM)
o Quick Reference Handbook (QRH) - notes in Speed Trim Fail checklist
o Airplane Maintenance Manual (AMM)
o Interactive Fault Isolation Manual (iFIM)
o Boeing has proposed Level A training impacts

Aircraft/engine make, model, and series: The Boeing Company Model 737-8 and 737-9 airplanes (737 MAX)

U.S.-registered fleet: 74 airplanes; Worldwide fleet: 387 airplanes

Operators: 59 operators worldwide: 9 Air, Aerolineas Argentinas, Aeromexico, Air Canada, Air China, Air Fiji, AIR ITALY S.P.A., American Airlines, Arkefly, Britannia Airways AB, Cayman Airways, China Eastern Airlines, China Southern Airlines, Comair, COPA Airlines, Corendon Airlines, Eastar Jet, Enter Air Sp. Z O.O., Ethiopian Airlines, Fertitta Enterprises, Inc., flydubai, Fuzhou Airlines Co., Ltd, Garuda Indonesia, Gol Linhas Aereas S.A., Hainan Airlines, Icelandair, Jet Airways, Jet Aviation Business Jets, JSC Aircompany SCAT, Kunming Airlines, Lion Air, Globus Airlines, LOT Polish Airlines, Lucky Air, Mauritania Airlines, Mongolian Airlines MIAT, Norwegian Air International Lt, Norwegian Air Norway, Norwegian Air Shuttle AS, Norwegian Air Sweden, Okay Airways Company Limited, Oman Air, Qatar Airways, Royal Air Maroc, Shandong Airlines, Shanghai Airlines, Shenzhen Airlines, SilkAir, Smartwings, Southwest Airlines, SpiceJet, Sunwing Airlines Inc., Thai Lion, TUI Airlines Belgium, TUI Airways, Turkish Airlines (THY), United Airlines, WestJet, Xiamen Airlines

Emergency AD

On 13 March 2019, the FAA issued an Emergency Order of Prohibition, prohibiting the operation of any MAX in the US.

Flight Standardisation Board (FSB) report

On 17 Apr 2019 the FAA issued a draft of the Flight Standardisation Board (FSB) report for the 737. The significant extracts are as follows:


The purpose of this revision is to add the B-737-7, B-737-8200, and Maneuvering Characteristics Augmentation System (MCAS). In Appendix 3, the Design Differences Table from the Boeing 737-800 to the Boeing 737-8 is revised to include ATA 27 Flight Controls addition of MCAS.


In March 2019, the FSB conducted an evaluation of the modified Maneuvering Characteristics Augmentation System (MCAS) for training and checking differences determination. The system enhancement is incorporated on all MAX series aircraft. The MCAS system was found to be operationally suitable.

9.2 Special Emphasis Areas.

B-737-MAX Maneuvering Characteristics Augmentation System (MCAS). The Speed Trim System (STS) provides speed and pitch augmentation. Speed stability augmentation is provided by the Speed Trim function of STS. Pitch stability augmentation is provided by the MCAS function of STS. MCAS ground training must address system description, functionality, associated failure conditions, and flight crew alerting. These items must be included in initial, upgrade, transition, differences, and recurrent training.

The level of training specified for MCAS is "B" which corresponds to "Oral or written exam or Tutorial computer-based instruction self-test (TCBI)"

The draft report is available here:

On 23 May 2019 the FAA met with 30 international air regulators including China, the European Union, Brazil and Canada, in Texas to discuss progress on the MAX recertification including design changes, MCAS training and simulator requirements. When asked by reporters about a date for a return to service, the acting head of the FAA, Dan Elwell, said that he does not have a specific timetable to approve the 737 MAX for flight. “It’s a constant give and take until it is exactly right, It’s taking as long as it takes to be right,” he said, adding: “I’m not tied to a timetable.” “If you said October I wouldn’t even say that, only because we haven’t finished determining exactly what the training requirements will be, If it takes a year to find everything we need to give us the confidence to lift the [grounding] order so be it.”

By late August 2019, the FAA said its staff has spent over 110,000 man hours dealing with MAX issues since the grounding.

Joint Authorities Technical Review panel

The JATR panel was convened by the FAA and first met on 29 April 2019. The FAA said about the JATR: “The JATR team will conduct a comprehensive review of the certification of the aircraft’s automated flight control system, The team will evaluate aspects of the 737 Max automated flight control system, including its design and pilots’ interaction with the system, to determine its compliance with all applicable regulations and to identify future enhancements that might be needed.”

The JATR is led by veteran NTSB investigator, Chris Hart and has is comprised of representatives from the civil aviation authorities of Australia, Brazil, Canada, China, the European Union, Japan, Indonesia, Singapore and United Arab Emirates.

The group is expected to take up to 90 days to conclude, which suggests that the 737 Max will be grounded until at least August.

Technical Advisory Board (TAB)

On 7 May 2019 the FAA said it had convened a multi-agency Technical Advisory Board (TAB) to review Boeing’s proposed software fix. The board consists of experts from the FAA, U.S. Air Force, NASA and the DOT's Volpe National Transportation Systems Center that were not involved in any aspect of the Boeing 737 Max certification. The board’s recommendations will “directly inform the FAA’s decision concerning the 737 Max fleet’s safe return to service.”. The new panel is separate from two other existing reviews created by FAA.

“The TAB is charged with evaluating Boeing and FAA efforts related to Boeing’s software update and its integration into the 737 MAX flight control system. The TAB will identify issues where further investigation is required prior to FAA approval of the design change,”


Government investigations and reviews

US DoJ Fraud Section Review of the FAA and Boeing

On 18 Mar 2019, the Department of Justice’s Fraud Section opened a criminal investigation into the development and certification of the Boeing 737 MAX by the FAA and Boeing. The Department of Transportation’s Inspector General and the FBI are participating in the investigation. Federal attorneys are gathering evidence through a federal grand jury seated in Washington, D.C. Grand jury proceedings are conducted in secret and the Justice Department has declined to comment on the investigation. The FAA and Boeing have also declined to comment.

The former 737 MAX chief technical pilot, Mark Forkner, now a first officer for Southwest Airlines, has invoked his Fifth Amendment rights and refused to turn over documents subpoenaed by the Justice Department as part of its broad investigation into the crashes in Indonesia and Ethiopia. He referred questions to his attorney when reached by phone. His attorney, David Gerger, of Houston, did not respond to inquiries.

Forkner played a key role in the development of the aircraft and worked for Boeing from 2011 to 2018. He’s now a first officer for Southwest. As chief technical pilot, Forkner’s job would have been to “provide flight operations, safety and technical support to Boeing internal and external customers at multiple levels” according to a job posting for a chief technical pilot on another airframe.

While “taking the Fifth” is often perceived as an admission of guilt, it’s use to avoid supplying documents is relatively rare and may just imply some legal wrangling between Forkner and the Justice Department according to experts consulted by the Seattle Times. The Times has previously reported that it was Forkner who suggested to the FAA that MCAS was not included in the FCOM. He, his lawyer and officials with the Justice Department all declined to comment on the legal move.

U.S Senate Aviation and Space subcommittee Public Hearing

On 27 March 2019, the U.S Senate’s Aviation and Space subcommittee held a public hearing, where Daniel Elwell, the FAA’s acting administrator, defended the agency’s oversight of the jet. A second hearing, where Boeing officials might testify, is pending without a date


US DoT Review of the FAA

On 7 May 2019, the US Department of Transportation appointed a committee to review the FAA’s process for certifying the Boeing 737 Max 8. The special committee is specifically tasked to review the 737 Max 8 certification process from 2012 to 2017. The findings from that review will provide the basis for recommendations for future improvements. The committee’s goal is to make proposals to improve “the FAA's aircraft certification process, including recommendations on delegations of authority and training, and improvements to other certification processes. Topics for investigation include the FAA’s certification process and timelines, and the process under which the FAA delegates some certification and oversight work to aircraft manufacturers and their employees.

The committee is led by co-chairs Lee Moak, the vocal former president of the Air Line Pilots Association (ALPA), and Darren McDew, former commander of the Air Force’s transportation command, a unit that oversees US military transportation. four other members hail from government, academia and industry. They include Amy Pritchett, head of The Pennsylvania State University’s aerospace engineering department and former director of NASA’s aviation safety programme, and Gretchen Haskins, chief executive of HeliOffshore, an association focused on safe offshore operation of helicopters. Other members include Amtrak’s chief safety officer Kenneth Hylander, who formerly was chair of the Flight Safety Foundation and has worked at Delta Air Lines and Northwest Airlines, and David Grizzle, a Republic Airways board member who formerly was the FAA’s chief counsel and head of its air traffic organisation.


US House Aviation Subcommittee

On 15 May 2019, the U.S. House Aviation Subcommittee of the Transportation and Infrastructure Committee held a 3 hour hearing on the status of the 737 MAX. Daniel Elwell, acting administrator of the FAA and NTSB chair Robert Sumwalt testified. The hearing was held to start producing answers on how the FAA certified the safety of the 737 MAX. So far, requested information from Boeing has not been forthcoming, noted Transportation and Infrastructure Committee Chairman Representative Peter DeFazio, a Democrat from Oregon who opened his statement by saying that Boeing has yet to turn over a single document as requested by the committee.

In this meeting, Elwell came under sharp questioning over how the FAA and Boeing had certified the plane as safe. The FAA has also been criticized for not requiring a clear description of the automated MCAS feature in documentation for pilots. House Transportation Committee Chairman Peter DeFazio, D-Ore., cited a Boeing engineer who was taped during a November meeting with a pilots' union saying MCAS might be seen just once in a million miles, and that "we try not to overload the crews with information that's unnecessary." "Do we really think that was unnecessary? It wasn't in the manual, and they didn't even know about it," DeFazio told Elwell.

On 19 June, airline pilots, including Chesley “Sully” Sullenberger, told the committee that the US FAA should require more frequent flight simulator training for pilots and ensure adequate aircraft information is provided during training.

Sullenberger and Dan Carey, who represented the union of pilots at American Airlines, said during their testimonies that Boeing did not provide pilots with any information about this manoeuvring characteristics augmentation system (MCAS) prior to the Lion Air Crash in October. Acting FAA chief Dan Elwell has also said pilots were not given adequate information about the flight control software. "This is a global aviation crisis of trust and will require global solutions to restore and bolster aviation’s global safety culture and reputation," Carey said.

On 24 July 2019, The NTSB Chairman Robert Sumwalt said that "We will let the facts drive us, but I am told that our staff is working on a recommendation package that I would suspect we would have out in the next 60 days regarding design certification issues."


US Securities and Exchange Commission (SEC)

The US Securities and Exchange Commission (SEC) is investigating whether Boeing properly disclosed issues tied to the grounded 737 Max jetliner, according to people familiar with the matter, as regulators intensify their scrutiny of the company following two deadly crashes. Officials in the SEC’s enforcement division are examining whether Boeing was adequately forthcoming to shareholders about material problems with the plane, said the people who asked not to be named because the probe isn't public. The agency is also reviewing the aircraft manufacturer's accounting to make sure its financial statements have appropriately reflected potential impacts from the problems, the people said.


The Proposed Fix

Boeing have been working on a software modification to MCAS since the Lion Air accident. Unfortunately although originally due for release in January it was not released due to both engineering challenges and differences of opinion among some federal and company safety experts over how extensive the changes should be.

Note that as MCAS is an FCC function, the modifications to MCAS are made in the FCC software. The revision will be known as FCC P12.1

There are three significant changes to MCAS software being worked on by Boeing:

  1. To give the system input from both angle-of-attack sensors, Currently MCAS only uses data from the angle of attack sensor on the side of the active FCC, (see AoA source). The system will have split vane monitor and Mid Value Select (MVS) input. This will both enhance detection of erroneous AoA vane behaviour and the MVS signal selection will pick the average of ADIRU L & R and the previous MVS output. If the output of the two AoA vanes differ by more than 5.5 degrees MCAS will be disabled.
  2. To limit how much MCAS can move the horizontal stab to guarantee sufficient handling capability using elevator alone. In its original report, Boeing said that MCAS could move the horizontal stabilizer a maximum of 0.6 degrees. However, after the Lion Air crash, it told airlines that MCAS could actually move it 2.5 degrees, or half the physical maximum. Boeing reportedly increased the limit because flight tests showed that a more powerful movement was needed at high AoA rather than at high Mach.
  3. A modification to the activation and resynchronisation schedule. MCAS will be limited to operate only for one cycle per high AoA event, rather than multiple. At present it will operate for 10s, pause for 5s and repeat for as often as it senses the high AoA condition is present. Furthermore the logic for MCAS to command a nose up stab trim to return to trim following pilot eletric trim intervention or exceeding the forward column cutout switch, will also now be improved.

Furthermore there is expected be:

  • Changes to the on board displays. This includes making the AoA Disagree Alert standard on all aircraft and offering a free option to have the AoA display on the PFD;
  • FCOM explanation of MCAS and an updated Speed Trim Fail Non-Normal Checklist and revised QRH.
  • Type conversion training to cover speed trim and MCAS, existing crew procedures and related software changes.
  • Enhancements to the maintenance documentation, including specifically to the Airplane Maintenance Manual (AMM) and Interactive Fault Isolation Manual (iFIM)

On 16 Apr 2019 the MCAS flight test program concluded after 120 flights / 203 hours flight test time. The Boeing CEO, Dennis Muilenburg flew on a 737 Max demonstration flight, where he “saw first-hand this software in its final form, operating as designed across a range of flight conditions”.

On 29 May 2019 the Boeing CEO, Dennis Muilenburg said in an interview with CBS Evening News "The implementation of that software — we did not do it correctly. Our engineers discovered that. We are fixing it now. And our communication on that was not what it should have been. We clearly fell short. And the implementation of this [alert] was a mistake," he added. "We did not implement it properly."

On 5 Aug 2019 the Boeing CEO, Dennis Muilenburg said that the MAX has conducted almost 500 test flights with the new FCC software. Muilenburg said that he has personally flown on two of the test flights, and that Boeing employees are “eager to do the same.”

A MAX MCAS flight test. Note the airspeed and offshore location.

Details from Boeing here:


The Return to Service of the MAX

Update 25 Aug 2019

The certification test flight for the changes to Boeing’s 737 MAX flight architecture is now expected to occur in early October. If succussful, a return to service should occur sometime in the fourth quarter of 2019.


Update 5 Aug 2019

Boeing now plan to submit a certification package to the FAA in September, and then expect the MAX to return service early in the fourth quarter.


Update 31 May 2019

Boeing is planning the return to service of the MAX. CEO Dennis Muilenburg has described a disciplined schedule that would start with Boeing teams helping to take about 500 parked 737 jets out of storage. That total includes about 100 newly built aircraft that can't be delivered until the grounding is lifted. They are stored around the Seattle area and on a large Boeing maintenance base in San Antonio. Boeing has two other sites where it could park planes if needed.

Boeing doesn't plan to increase the production rate, dropped from 52 to 42 aircraft per month in April, nor reinstate its financial forecast, until it's clear that its supply-chain is healthy and moving in sync.

CFM International, in a break from its earlier struggles with delays, is starting to deliver turbofans on schedule, Muilenburg said. They are also feeding a pool of spare engines.

Boeing is still responding to questions from the FAA and international regulators. EASA Director Patrick Ky wrote in a letter dated May 27 that EASA will do a separate, in-depth review to examine the 737 Max's entire flight-control system, including the plane's displays, alerts and air-data systems, as well as the aircraft's autopilot function,

"The regulators aren't on the same page," said De Juniac, the IATA chief. "Otherwise they'd have a similar time line, a similar set of measures."


The EASA Position

The European Aviation Safety Agency (EASA) has said it will want to satisfy itself about the MAX's suitability for return to service with no delegation to the FAA.

In a presentation by Patrick Ky, Executive Director of EASA, on 3 Sep 2019 to the European Parliament’s transport committee, the EASA chief showed extracts from a letter he wrote to the FAA on 1 Apr 2019 giving a list of four conditions which would have to be met before EASA would allow the MAX to return to service:

Ky noted that the FAA now finds itself in a “very difficult situation”, indicating that the hierarchy between certification authorities and the US agency may be forever changed, saying, “It is very likely that international authorities will want a second opinion, or a further opinion … It was not like this a year ago.”

The presentation can be viewed here: (Slides 6-16 apply)

Slides 1-5

The presentation was not exclusively about the MAX and these slides deal with other non-MAX issues such as EASA Strategic Priorities etc

Title Page (Slide 6)

Lion Air accident (Slide 7)

Ethiopian accident (Slide 8)

Decision to ground / ban (Slide 9)

Preliminary information On the Contributing factors (Slide 10)

Return to Service - EASA conditions (Slide 11)

  • 1. Design changes proposed by Boeing are EASA approved (no delegation to FAA)
  • 2. Additional and broader independent design review has been satisfactorily completed by EASA
  • 3. Accidents of JT610 and ET302 are deemed sufficiently understood
  • 4. B737 MAX flight crews have been adequately trained

EASA Design Review (Slide 12)

Objective: to ensure that no similar weaknesses in the design are present in the other (safety critical) areas of the 737 MAX design.
Activities are being carried out on the Flight Control System and all associated functions/systems including but not limited to the following: displays, alerting system, autopilot and air data system.

  • - Review of Functional Hazard Assessments and Safety Analysis
  • - Review of Development Assurance process - Review of Flight Controls and Autopilot architecture and logics
  • - Review of Human Factors aspects
  • - Review of the methodology applied for the assessment of the Crew Training needs

EASA activities (Slide 13)

EASA Findings (Slide 14)

Significant technical issues:

  • - Lack of exhaustive monitoring of the system failures resulting in a stabiliser runaway
  • - Too high forces needed to move the manual trim wheel in case of a stabiliser runaway
  • - Too late disconnection of autopilot near stall speed (in specific conditions)
  • - Too high crew workload and risk of crew confusion in some failure cases, especially Angle of Attack single failure at take-off

Findings communicated to Boeing and FAA in July 2019

Latest Status (Slide 15)

Of note here is that EASA states that there is "still no appropriate response to Angle of Attack integrity issues".

Next major milestones (Slide 16)

  •  Safety assessment of the new design changes proposed by Boeing, including operational procedures
  •  Human factor evaluation and functional tests of the new software
  •  Flight tests on a modified B737 max [one full week - at Boeing Flight Test Center]
    •  MCAS operations (nominal behavior)
    •  Flight without MCAS (including high speed turns and stall)
    •  Scenario of stabiliser runaway (uncommanded MCAS activation, manual trim wheel forces)
    •  Approach to stall with autopilot engaged
  •  Crew Training requirements, in particular using Computer Based Training or Simulator
  •  Coordination with EASA Member States on Return to Service actions


MCAS Media Videos

The following are a selection of videos and explanations of MCAS issues from the media:

BBC News, 4 Apr 2019

In this video I explain and demonstrate a stall and then later the effect of MCAS trimming the stabiliser down and how it would be unexpected by crew.

60 Minutes Australia, Published on 5 May 2019.

Liz Hayes investigates the disaster of Boeing’s 737 MAX jetliner. Why two supposedly state-of-the-art and safe planes crashed killing 346 people; why pilots now fear flying the 737 MAX; & whether Boeing could have averted the catastrophes.

In this video I discuss and try to simulate the effect of an unanunciated AoA disagree (ie airspeed unreliable) and MCAS activation shortly after take-off. Note that due to the considerable editing down process this video does not show the recall actions and is in no way intended to be a demonstration to flight crew of how to handle such a scenario. It has been edited by non-aviation professionals for the consumption of the general public.

BBC Panorama, Published on 29 Jul 2019.

The Boeing 737 Max was set to dominate the skies, but two deadly crashes have left the aircraft grounded and the company in crisis. Both flights were forced down by software specially designed for the new aircraft. Reporter Richard Bilton investigates the plane’s fatal flaws and asks whether Boeing should have done more to protect passengers.

Footer block

This site has had visitors to date.