Here’s How Boeing is Fixing the 737 MAX MCAS Software

The Lion Air Boeing 737 MAX crash in October revealed that the Maneuvering Characteristics Augmentation System (MCAS) software installed on the aircraft could push the plane’s nose down as a result of faulty Angle of Attack readings.

While pilots can disable this system, many weren’t aware of the way that it worked, having only received limited training as they transitioned from earlier generation 737s to flying the MAX.

Boeing has unveiled software changes that should make erroneously engaging the system less likely, and less challenging for pilots if it ever does happen. They are awaiting certification for the changes after “hundreds of hours of analysis, laboratory testing, verification in a simulator and two test flights, including an in-flight certification test with Federal Aviation Administration representatives on board as observers.”

  • The system will compare data from both sensors to avoid inaccurate readings instead of relying on one sensor at a time. If inputs are off by more than five and a half degrees, MCAS and the speed trip system will be inhibited for the rest of the flight.

  • MCAS will drop the plane’s nose only once each time it detects a high angle of attack instead of doing so persistently.

  • Nose down movement will be easier to overcome by pilots in order “to override MCAS input with sufficient manoeuvring ability that the aircraft can still climb.”

  • Better training on the differences between earlier generation 737s and the MAX.

Given that the result of failure of an angle of attack sensor can be hazardous relying on a single input seems like it was a questionable decision. This software update is badly needed, even though every 737 MAX pilot is certainly well aware at this point how to disengage the MCAS system — as well as under what circumstances they’d want to do so.

About Gary Leff

Gary Leff is one of the foremost experts in the field of miles, points, and frequent business travel - a topic he has covered since 2002. Co-founder of frequent flyer community InsideFlyer.com, emcee of the Freddie Awards, and named one of the "World's Top Travel Experts" by Conde' Nast Traveler (2010-Present) Gary has been a guest on most major news media, profiled in several top print publications, and published broadly on the topic of consumer loyalty. More About Gary »

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Comments

  1. Sure seems like these are things that probably should have been part of the system before 300+ people died. Especially #1. Planes have two(or more (AOA) sensors for a reason.

  2. Gary, I understand that Boeing will also retrofit a digital AOA readout and warning light to alert pilots that AOA sensors disagree. These two components we’re not standard equipment in MCAS but provided as an $80,000.00 option. Since when is safety sold as optional equipment?

  3. First things first:

    If I fail to feed a parking meter in order to save $1, I have to pay something like $100 in fine: that’s when nobody got hurt. So Boeing as well as the airlines should be fined a zillion times whatever money they made / saved by not providing the proper safety equipment and training to the pilots.

    Then, the victims’ families should be allowed to collect whatever damages the courts see fit.

    Finally, all individuals who were responsible at any company, certainly the key decision makers, should go to jail for a very long time.

  4. @AKTCHI – I could not disagree more. Aircraft are incredibly complex machines, and even accounting for these crashes, are safer than they have ever been. If Boeing had been aware of these problems, and covered them up or refused to deal with them, I would agree that fines or potential imprisonment should be on the table, but nobody is making such claims. The advancement of technology inevitably means that mistakes will be made and unfortunately when it comes to aircraft that may mean people die. We should do everything possible to avoid that, but it still may happen. Destroying a company because honest mistakes were made helps nobody. Many 737 MAX pilots will protest that in spite of the errors in the AOC sensor and MCAS software, the pilots in these crashes likely could/should have been able to disengage the system and safely fly the aircraft. The most important thing at this point is to learn from the crashes and make changes in equipment/software/pilot training that will make such events less likely in the future.

  5. Sorry – you’re letting Boeing off the hook here. You say, “While pilots can disable [the MCAS], many weren’t aware of the way that it worked….” That’s just frankly dishonest. The truth is that pilots weren’t notified that the system is there. My question (and I think it’s a fair one) is, “How were pilots of the Max series aircraft expected to know how to adjust for or disable the MCAS if they didn’t even know it existed?”

    Boeing hid this information and endangered all of the passengers on these planes (as well as people on the ground). I’m livid about it – it’s unconscionable.

  6. @Doug: Boeing was certainly aware of the problems, which is why it produced a half-fix, but then decided to peddle it for $80,000. Their greed for taking this “why-not-grab-some-money” route. Airlines’s greed for saying “No thanks, we will take our chances with this Russian roulette”.

  7. @Steve – too many examples to list but lets start with AAA 191 where AA was too cheap to pay for two stick shakers.

  8. In linked article: “…the updates do not represent any concession by Boeing that the 737 Max was unsafe to begin with.”
    A half decent powerup self diagnostic that indicated anomaly in the angle of attack sensors would have saved everyone’s life on Lion Air. A few unusual beeps would have alerted the pilots. The lack of such basic safety feature is beyond belief.

  9. Essentially, MCAS is an Easter Egg. It’s an undocumented bug, er feature, that turned out not to be quite as fun as planned. And Boeing should pay big time for the tragic consequences.

    Question: what other undocumented features are there in the 737 MAX or other Boeing products?

  10. The MCAS is there to address a risk of stall resulting from the change of engine position on the max. Wonder if all the ‘fixes’ makeing the MCAS less persistent and easier to disable will result in more stalls. Will the plane now be dangerous for that reason?

  11. Although, personally, I would fly in a 737max, if I was Boeing, i would recall every single 737Max and provide 737 Regular in its place. Ethiopian airlines was sold a bill of goods. All they needed was another 737, not 14% (???) fuel savings and a convenient learning curve for the pilots. Boeing should sacrifice their profits to protect their customers safety records. Or rename the plane 737 Bumblebee painted in black and yellow so passengers can make an informed decision. My family may travel on one in two or three years, after a 1,000,000 or so proven flights. Otherwise, I would rather be driving.

  12. No matter what Boeing does to make this plane safe, I will not fly this version(737-800 MAX) of their planes. It is like a lemon…Even if it means more work me to decide which plane flies which route or changing carriers.

  13. There is so much Monday Morning Quarterbacking going on here by haters who don’t want to wait on facts and analysis before they haul out the accusations of greed and laziness.

  14. According to CNBC “Fixes made to Boeing 737 Max

    Among the notable changes to the MAX flight controls:

    – The plane’s Maneuvering Characteristics Augmentation System, or MCAS automated flight control system, will now receive data from both angle of attack sensors, instead of just one.
    – If those disagree by more than 5.5 degrees, the MCAS system will be disabled and will not push the nose of the plane lower.
    – Boeing will be adding an indicator to the flight control display so pilots are aware when the angle of attack sensors disagree.”

    So now, if one sensor malfunctions, the MCAS would not be activated when the critical angle of attack is exceeded. This could cause instability of the plane, a departure stall, and eventually loss of control at low altitude.

    MCAS was installed in the Max for a reason. The new engines shifted the CG aft, making the model inherently unstable during a steep climb. The aft CG would not be a problem for a seasoned, well trained test pilot. However, it could cause a major issue to an airline pilot who did not go through very specific training.

    The only safe solution would be a complete redesign of the Max, especially the wings. Moving the CG forward would increase stability and make the plane safer. It could also result in increased fuel consumption, making the Max loose its competitive edge. The redesign would cost billions,and take years. I doubt that any such radical design could be retrofitted on the existing Maxs.

    Two angle of attack sensors do not add the same margin of safety that two engines do. In fact, they would make the plane even more dangerous since having two sensors makes it twice as likely that one of them would fail.

    The FAA is not likely to pull its certification of the existing Maxs. It will, again, take Boeing’s word that the plane is airworthy at face value. The Max will remain a dangerous plane.

  15. So now, if one sensor malfunctions, the MCAS would not be activated when the critical angle of attack is exceeded. This could cause instability of the plane, a departure stall, and eventually loss of control at low altitude.

    MCAS was installed in the Max for a reason. The new engines shifted the CG aft, making the model inherently unstable during a steep climb. The aft CG would not be a problem for a seasoned, well trained test pilot. However, it could cause a major issue to an airline pilot who did not go through very specific training.

    The only safe solution would be a complete redesign of the Max, especially the wings. Moving the CG forward would increase stability and make the plane safer. It could also result in increased fuel consumption, making the Max loose its competitive edge. The redesign would cost billions,and take years. I doubt that any such radical design could be retrofitted on the existing Maxs.

    Two angle of attack sensors do not add the same margin of safety that two engines do. In fact, they would make the plane even more dangerous since having two sensors makes it twice as likely that one of them would fail.

    The FAA is not likely to pull its certification of the existing Maxs. It will, again, take Boeing’s word that the plane is airworthy at face value. The Max will remain a dangerous plane.

  16. Like what I heard someone put it succinctly, if it is software and not aerodynamics that puts us in a plane in the air, God bless us.

  17. This is NOT a software error, a project error. I agree I will not fly a 737-8Max. I do not know how many Easter Eggs the plane will have.

  18. From a more general perspective, please could someone inform the general public as to who is actually flying the aircraft? Is it the pilot, or is it software? If it is the pilot, the aircraft manufacturer needs to provide an aircraft that can be flown and controlled by competent pilots…. If it is the software, why do we need the pilots?
    The most significant concern seems to be that aircraft manufacturers now need to add all kinds of “safety” features to warn incompetent pilots from making fatal mistakes. And pilots, once they have done the basics, (with the few exception of enthusiastic fliers) no longer seem to have any inclination to maintain the basic skills of flying. They are operators…. without the basic awareness of general flight characteristics – the ability to fly by the seat of their pants when necessary. Thus the need for more and more features to tell them when they are reaching the edges of the envelope! A tragic example of this was Air France flight 447 where “final report stated that the probable cause of the crash was the aircraft’s pitot tubes icing over leading the autopilot to disconnect and handing full control of the aircraft to the pilots. The pilots were confused by various warnings and messages from the aircraft’s on board systems and pulled the nose of the plane up to the point where the aircraft stalled. The pilots failed to recognise that the aircraft had stalled until it was too late to prevent an uncontrolled and rapid descent into the Atlantic Ocean”
    So the real question is: “Are the manufacturers putting in too many “safety” systems when what we really need is just decent competent pilots to fly the aircraft?

  19. I have a friend who works at Boeing as an engineer (not on the 737 MAX) for decades. He told me that the company has been having a lot of quality issues. He said it’s mainly the upper management. When people inspect someone else’s work and find a problem, the management usually tends to ask if it will remain on schedule if they fix it and how much it will cost. When different issues either cause big delays or cost a lot extra, they tend to deny the issue from being worked on. With that being said, he told me that Boeing’s quality has gone downhill. He said that managers who are focused on quality and safety have left the company due to upper management creating issues. He said that when an issue, like this 737 MAX issue comes up, they usually blame the lower-level employees and have them go through these bogus safety classes and training. It’s sad that a company compromises quality and safety over profits, which may eventually be lost in the long-term if this trend continues.

  20. Very questionable to require a software fix to what is essentially bad aerodynamic design issue of mounting the engines too far forward instead of mounting them lower, further back and using longer undercarriage struts.

  21. Should we ask why Delta a past large user of Boeing has moved more and more to Air Bus – more than just $ ?- how many 37’s Max did they order if any? Can we remember the term DC 10 ??????
    What else don’t we know????

  22. I suspect any aircraft with large powerful engines mounted below and forward of the center of gravity would have a tendency to increase its angle of attack in climb configuration. Ironically, the probable exception to this is earlier model 737’s which have relatively small engines mounted close to the wing. This could allow 737 pilots to become complacent about monitoring the angle of attack during initial climb. Boeing created software to prevent the 737 Max which has larger engines mounted further forward from stalling due to pilot inattention. Sensor malfunctions may have occurred, but both crashes seem to be a result of the pilots reacting improperly – i.e. fighting the system rather than disabling it. My guess is that both planes crashed as a result of stalling when the pilots finally succeeded in in putting the aircraft in an unrecoverable attitude,. This would suggest that they were not properly trained, which is not the manufacturer’s responsibility.

    If you think such lack of training is impossible, refer to the Kegworth 737 crash which resulted from the pilots shutting down the functioning engine rather than the damaged one. And before you try to blame Boeing for that one, explain why they didn’t advance one throttle at a time to see which engine caused the vibration.

  23. April 4, 2019… today it is reported that Boeing has discovered the MCAS system had malfunctioned and has apologized. “New details emerge that the pilots of two planes could not counteract a malfunction of the system using the company’s recommended procedures.” For the sake of salvaging Boeing, the 737 Max needs to recalled in its entirety.
    The pilots properly followed Boeings procedure toe deactivate MCAS, and also tried to manually adjust Trim, and the plane was going to fast for the system to respond.
    These real world flight characteristics either cannot or have not been recreated in a flight simulator. The plane is like a bumblebee and fly’s despite the laws of physics. If it must rely on engines to fly and cannot predictably glide due to an unnatural and unusual weight distribution, then permanently ground it. For its replacement, the Board of Directors gets to go on the test flights, then maybe they will think twice before letting Management make a life or death decision on behalf of Boeing, Inc.

  24. 737 MAX is unsafe and will remain unsafe. Inherently unsafe. The plane cannot fly safely without MCAS and there is no proposed way to disable MCAS in flight.
    After the Lion Air crash, Boeing’s advisory to pilots was to turn off the power trim system. That turns off the powered system that drives the horizontal stabilizer (tail). It does NOT deactivate MCAS. In Ethiopian, the pilots followed Boeing’s advisory and switched off the power trim. MCAS kept sending signals to push down the nose (dive), but there was no power system to carry out the signal. Unfortunately, without a powered system, the pilots were unable to manually trim the stabilizer. Trimming involves cranking a small wheel (about 5-7″ diameter) in an awkward position with enough force to pull a cable the length of the plane that in turn moves the stabilizer. Next to impossible to do in normal flight, impossible in a high speed dive close to the ground. It seems that in desperation, the Ethiopian pilots switched the power trim back on. Within seconds, MCAS send a “nose down” signal and drove the plane into the ground.
    Even now, Boeing has not proposed any fix to turn off MCAS. That is probably because the plane cannot fly safely without MCAS because the plane is inherently unstable.

  25. er /? now from model aircraft days cg a little forward meant a slight trim to level . not a biggie; cg aft is a biggie at take off . (biggie or more of a problem.)
    twin jets under wing causes pitch upward at high thrust. take off. rollout .
    on every twin jet ever made. include models. because power rolls the cg aft.
    \
    Can somebody explain why moving cg further forward (max 8) has effect to exacerbate pitch up.
    doesnt moving cg back have that effect ?
    in models we try to center gravity on center of lift
    my guess is . moving engine closer to wing causes pendulum to rotate faster to vertical. despite cg further forward,,, at high power settings the distance engine to wing ( there is no under wing pod holding engine max 8) overwhelms any cg fwd.
    and despite my guess above I would have thought that about 600mm distance has little effect in flight character . and some pilots could determine little if any…. in flight testing.
    so why mcas is needed ? just switch that sh1t off.

    anyway what happens to max 8 in a glide/? u have to trim stab so much up to prevent nose down /?like a nose down flying brick? or is it nose up /?
    in something that transports for money wouldnt people want inherently stable/???

    as said why make aircraft unstable
    look management you get on the plane … no no get your children and your whole family on when testing. now you believe in your calcs get on and lets see

  26. It is not just a software fix. It is about fundamental design of aerodynamic stability. In conclusion, they need to reengineer the whole plane. Otherwise, it is garbage. I would not fly on this plane even if it was for free.

  27. why not installed only a warning lights or buzzer so to get the attention of pilot that MCAS wants the plane to move little down or more depending how the pilot observe the plane the way it is flying.So the MCAS will not directly maneuver the aileron.Or why not find a suitable engine so the location is appropriate in relation to moment of force so the thrust will balance as it fly.Moving the source of thrust away from center surely affect flying performance.I think this is a simple Algebra and Physics.

  28. From the article:

    “While pilots can disable this system, many weren’t aware of the way that it worked, having only received limited training as they transitioned from earlier generation 737s to flying the MAX.”

    This statement is misleading and incorrect.

    The way to “disable” MCAS on the MAX was exactly the same as the way to disable any automatic setting of the trim. Namely, to operate the cutout switches. Those cutout switches are present on all versions of the 737, and that procedure has always been the same on all versions of the 737.

    Bottom line – if a pilot was not aware of how to use the cutoff switches, it was not because he or she had transitioned to the MAX. It was because he or she was already improperly trained on the basics of the 737.

  29. Thank you Joe B. Your response is the only one that it factually correct. In future the 737 MAX will probably be one of the safest planes to fly in since all and sundry now know how to deactivate MCAS. Do not think for one moment that Airbus is any better at certifying new aircraft. They certified the A380 knowing that there was a problem with the Rolls Royce Trent engines. Quantas nearly lost an A380 when a disc in the Rolls Royce engine disintegrated, slicing through hydraulic lines, a wing spar and puncturing the fuselage. If it were not for the fortunate coincidence that they had a second crew on board they would most certainly have crashed. Rather look at the First officer with barely more than 200 hours TOTAL FLYING TIME experience. He should not have been in that cockpit. As you rightfully said, ANY EXPERIENCED 737 pilot would have known how to deactivate the system based on what the automation was doing. Even without knowing about the existence of MCAS an experienced 737 pilot would have known how to flick the 2 switches on the console to deactivate the auto trim. This would have disengaged the MCAS. As for those who want to claim the 737 Max is”ïnherently unstable” I suggest you reserve you opinion for yourself as it merely shows you up for being totally ignorant of airplane design. If anything rather examine the credentials of the 200 hour first officer….

  30. Why is MCAS needed in the Boeing 737 Max? All previous 737 was fine without it. Is it to inflate the plane’s selling price or there is a need for it now?

    The 737 Max 10 was reported by Virgin Airlines to be replacing most their Max 8’s in a change of their order status with Boeing. Also saying that the Max 10 does NOT have MCAS. This is probably true because the Max 10 has 10 inches Taller Main and Front landing gear in order to accommodate the longer fuselage to keep from dragging the tail on the airport runway during take off. The added benefit is with the Taller gear the larger Max Engines can be placed properly on the wing, thereby avoiding the inherent bad Aerodynamics of the Max 7, 8, and 9’s that have the Engines up and in front of the wing which requires MCAS.

    Raising the landing gear, and placing the larger Max Engines properly on the wing, as with the Max 10, is a good solution that has been suggested by the European Union Aviation Safety Agency (EASA), and everybody would be happy with if we could talk Boeing into spending the money. No MCAS, no problem!! It is only perhaps a dream, but it would be wonderful.

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