Have we ever gotten an explanation for why the Boeing 737 MAX needed MCAS?

Has anyone ever seen an explanation of why Boeing couldn’t simply remove MCAS from the 737 MAX and tell pilots “you have to push forward on a go-around, just as you would in a Cessna 172 or Cirrus SR22”?

From FAR 25 (certification of airliners):

§25.145   Longitudinal control.

(a) It must be possible, at any point between the trim speed prescribed in §25.103(b)(6) and stall identification (as defined in §25.201(d)), to pitch the nose downward so that the acceleration to this selected trim speed is prompt with

(1) The airplane trimmed at the trim speed prescribed in §25.103(b)(6);

(2) The landing gear extended;

(3) The wing flaps (i) retracted and (ii) extended; and

(4) Power (i) off and (ii) at maximum continuous power on the engines.

(b) With the landing gear extended, no change in trim control, or exertion of more than 50 pounds control force (representative of the maximum short term force that can be applied readily by one hand) may be required for the following maneuvers:

(1) With power off, flaps retracted, and the airplane trimmed at 1.3 VSR1, extend the flaps as rapidly as possible while maintaining the airspeed at approximately 30 percent above the reference stall speed existing at each instant throughout the maneuver.

(2) Repeat paragraph (b)(1) except initially extend the flaps and then retract them as rapidly as possible.

(3) Repeat paragraph (b)(2), except at the go-around power or thrust setting.

(4) With power off, flaps retracted, and the airplane trimmed at 1.3 VSR1, rapidly set go-around power or thrust while maintaining the same airspeed.

(5) Repeat paragraph (b)(4) except with flaps extended.

(6) With power off, flaps extended, and the airplane trimmed at 1.3 VSR1, obtain and maintain airspeeds between VSW and either 1.6 VSR1 or VFE, whichever is lower.

In other words, an airliner meets certification standards unless it takes more than 50 pounds of push-forward on a go-around. I’m sure that there is some pitch-up moment on a 737 MAX, but it is tough to believe that a gradual trim-forward from MCAS would be sufficient if, in fact, more than 50 pounds of pushing on the yoke were required in the absence of any trimming.

The B737 already had a stick shaker for any time that it was getting near a stall (reminds pilots to push forward). So it should have been less likely to get into an elevator trim stall than a flight school Cessna 172.

Why couldn’t Boeing rip out MCAS, fire any of the coders and engineers involved in its design, tell airlines to give everyone an elevator trim stall demo in every recurrent sim session, and call the 737 MAX good?

(This is such an obvious and cheap fix that surely Boeing would have tried it if would work, so I assume that some rule would be violated, but which one?)

Related:

42 thoughts on “Have we ever gotten an explanation for why the Boeing 737 MAX needed MCAS?

  1. This is a billion dollar question. I haven’t heard one, but apparently they cannot do that with the CFM LEAP 1-B engines on the plane, or they would have, as you say. It may be that the plane is just uncertifiable without MCAS working in some form. Which rule(s) does it violate? I have absolutely no idea.

    R. John Hansman from MIT Aeroastro commented about the symptoms to Forbes, back in April of 2019.

    https://www.forbes.com/sites/petercohan/2019/04/02/mit-expert-highlights-divergent-condition-caused-by-737-max-engine-placement/#9037c4040aab

    https://aeroastro.mit.edu/r-john-hansman

    “As I understand it, at high angles of attack the Nacelles — which are the tube shaped structures around the fans — create aerodynamic lift. Because the engines are further forward, the lift tends to push the nose up — causing the angle of attack to increase further. This reinforces itself and results in a pitch-up tendency which if not corrected can result in a stall. This is called an unstable or divergent condition. It should be noted that many high performance aircraft have this tendency but it is not acceptable in transport category aircraft [emphasis mine] where there is a requirement that the aircraft is stable and returns to a steady condition if no forces are applied to the controls.”

    In the end, though, he expressed confidence that MCAS+improved software and training would carry the day:

    “There is not a fundamental problem in the design of the aircraft.”

    I don’t doubt his sincerity or expertise, I’d just like to know if he still thinks the same way.

    • Hansman is a great guy and almost certainly correct, but his bottom-line statement is also true of a lot of low-performance certified airplanes, such as the Cessna 172 and the Cirrus SR22: “a pitch-up tendency which if not corrected can result in a stall” (at least at go-around power)

      Maybe the B737 MAX is worse in that it would be unstable in pitch, at high angles of attack, even without high power.

  2. Mentour Guy said the different stall characteristics were in a category which required a new type rating, so they would have to make it feel like an old 737 through software. Of course, babysitting the MCAS ended up requiring retraining anyway.

    • lion2: Yes, that makes sense as an initial goal. But once the planes were grounded, why not abandon it? Boeing kicking in for a couple of extra hours of sim time at the next recurrent training so that pilots at customers could have an extra type rating would be a lot cheaper than 800 airplanes grounded.

    • I am a retired military and airline pilot. If an aircraft is aerodynamically unstable in a critical stage of flight and needs a electronic bandaid to fix the problem it damn sure needs an ejection seat!

    • Bob: With a robot voice for the PA saying “Ladies, gentlemen, and those who identify as being at various intermediate stages between ladies and gentlemen… if you look out the sides of the aircraft you’ll see your pilots hanging from parachutes and waving to wish you a pleasant rest of your journey.”

    • Toucan: As noted above, the single type rating is a nice goal, but once 800 airplanes are grounded, why not abandon it? Airline pilots need to go into the sim every 6-12 months anyway. Why not have them come out with an additional type rating? The ATP standard for a type rating is not very different from the recurrent training standard.

    • My guess would be a public perception problem. This is simply a guess, but it seems as if the public in general has blamed the airplane not the pilots for the crashes so the public demand a fix to the airplane. However I do not share that belief, I am appalled that a 200 hour pilot was in the front seat of an airliner.

    • My hunch is it’s a high-stake gamble for Boeing (perhaps nothing but a sunk cost fallacy). Maybe–just maybe–they will be able to find a magic fix. And if not, the government is going to let Boeing down, right? right? RIGHT?
      Admitting that the long term business strategy was wrong is hard.
      Looks like a replay of 2008 to me.

    • In other words, this is not about technology at all, it’s about long term business strategy.
      If things stop making sense, it’s only prudent review the “most solid” assumptions behind the model.
      Just like in 2008, when Goldman quants reported events 11 standard deviations away from the mean instead of admitting that the consensus models fell apart.

  3. I wrote my Capstone Project on this issue. I’m no aircraft expert but to compete with the new fuel efficient engines that Airbus had decided to go with, Boeing needed to follow suit. Their main goal with the redesign was to avoid the need/requirement of fresh simulation training.

    They knew that the new engines would change the planes aerodynamics which caused the change in the angle of attack under certain conditions.

    In the instances of the crashes, it wasn’t so much the MCAS that failed. The MCAS was fed invalid data from the angle of attack sensor that told the MCAS that it was pitching up. The real problem arose when pilots that didn’t know that the MCAS had taken over (or that it existed) so they had no clue as to how to shut it down.

    Had they simply redesigned the plane to fix the issue with the aerodynamics of the new engines, the MCAS would not have been needed though pilots would have had to go through sim training which was what Boeing was trying to avoid all together to save their customers from going to Airbus.

  4. I am not a pilot nor do I have any pilot’s training,but it appears to me that somewhere along the line,the pilots with Lion Air and the Ethiopian Airlines were not given sufficient training in learning to fly these new 737’s and then being able to utilize all of the newfangled
    software that came with it.It appears to me that this type of plane obviously needed all sorts of software to correct the problems of aerodynamic feasability with the design of this plane and the pilots were supposedly never properly trained to handle this,or if they were,the training obviously needs to be revised properly.At the Air Show these planes performed beautifully,but they really are a unique type of plane that needs to properly address their unique and revolutionary capabilities.

  5. I would bet that removing MCAS and creating a new type was a course of action identified in the tradespace. Boeing probably wasn’t very good at estimating the cost and time required to fix MCAS, resulting in a flawed decision. Maybe we’ll see a new type soon.

  6. (For reference, the cost of a B737 type rating at retail from a place such as https://aerostartyperatings.com/ is about $10,000. That’s for someone who has zero experience with jets, potentially, and none with any other version of the 737. If you figure that an airline would have 12 pilots for every airplane (https://www.statista.com/statistics/740450/pilots-airline-industry-per-aircraft-type/ says 11) and that the cost of getting a B737 pilot through a B737 MAX type course is no more than getting a Cessna 172 pilot through, that’s $120,000 per airplane to get it back in the air minus the MCAS.)

  7. I am not a pilot. What I do know is that this week the Airbus consortium admitted bribery of Government buyers, and Dennis Muilenberg’s contribution to Boeing was less than competent. I have friends who have engineered for Boeing in the 90’s who said the culture went from safety to cost cuts. Simple as that.

    The problem here is both companies need to be broken up, and criminal penalties and civil damages made available for provable negligence, and torts through records.

    Also, how did Boeing get so comfy with the FAA, that despite our military complaints of metal safety hazards near electrical harnesses on delivery, they could still put pressure on the FAA to sign off MCAS, and make redundant safety pitot tubes only available as an upgrade?

    I have a friend who has been a United Captain for decades now. He lands in crosswinds that the FO and SO are not confident with. Why? Because he can, but too much automation is his main concern as it robs pilots of gaining necessary skills.

    We need to pay all pilots a living wage, and will someday likely find trucks and planes will fall under control of our military if ever weaponized again.

    Do others not see what a disaster the American transportation system has become, and what will change it?

    I am already afraid to fly on commercial jets, and I flew a hang glider as a kid. How do you think the general public feels about this Boeing – Wall Street partnership?

    If pilots don’t find their voice to criticize, it will cost society much more in lives. Pilots
    care about my safety. Boeing millionaires don’t. Pilots need to take charge of this.

    M Gale

  8. Answer is – no, we never have gotten explanation

    Reason is because disabling MCAS results in aircraft decertification under:
    §25.173 Static longitudinal stability.
    §25.175 Demonstration of static longitudinal stability.
    and maybe
    §25.181 Dynamic stability.
    and not §25.145 Longitudinal control, as you propose

    • Anonymale: These FARs are a promising starting point. Thank you!

      But are you sure that they wouldn’t be met without MCAS? If MCAS is required to meet the basic FARs then wouldn’t it have had to be significantly redundant? Airplanes that actually need computer augmentation for stability typically have three computers voting. I never heard about anything like that for MCAS. It seemed like a “nice to have” not “need to have”.

    • MCAS is a stability augmentation system. (note the -AS suffix)

      See
      §25.672 Stability augmentation and automatic and power-operated systems.

      Redundancy, triple or otherwise, not required under statute.

    • But FAR 25.672 says “If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part … [it needs] A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system”

      But MCAS by default didn’t have any warnings, right? They had an optional “AOA Disagree” warning, I think, but didn’t we hear that MCAS wasn’t included as part of pilot training? If it could speak up with a warning of some kind then pilots would have needed to learn about it (to know what to do after receiving the warning, for example).

      https://www.boeing.com/commercial/737max/737-max-software-updates.page

      says “The Maneuvering Characteristics Augmentation System (MCAS) flight control law was designed and certified for the 737 MAX to enhance the pitch stability of the airplane – so that it feels and flies like other 737s.”

      They make it sound like a pride-of-ownership thing, not a necessary-for-certification thing.

    • https://news.ycombinator.com/item?id=19512094 discusses this issue as well. Towards the bottom there is a quote that mostly makes sense:

      This new location and size of the nacelle cause the vortex flow off the nacelle body to produce lift at high AoA; as the nacelle is ahead of the CofG this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to further increase the back pressure on the yoke and send the aircraft closer towards the stall. This non-linear/reducing stick force is not allowable under FAR §25.173 “Static longitudinal stability”. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.

    • But “this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to further increase the back pressure on the yoke” doesn’t make sense. If the airplane is pitching up, why would the pilot increase back pressure on the yoke? Maybe unintentionally?

    • @philg

      The system could (does?) have aspects of both a

      (i) Stability Augmentation System for high AOA, and probably aft CG.
      (ii) Legacy Flight Characteristics emulator for everywhere else in flight envelope

      But only the first behavior (SAS) is regulated under §25.672 as necessary to show compliance with the flight characteristics requirements of this part

      Then you elided over an important clause in §25.672(a)

      warning…for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure.

      Per statue, if MCAS failure would not result in an unsafe condition, then warning light not required.

      Failure of MCAS to engage may not have been deemed unsafe to flight, when acting as a stability augmentation system, because presence of stick shaker provided warning of unsafe conditions (stall, upset) in exactly the places stability augmentation required (aft CG, high AOA). ‘

      You make that point above in your post.

      Now back to physical reality vs. statutory analysis.

      I think failure analysis failed to extend to improper engagement of MCAS. I don’t think any analysis was done on the altitude/airspeed envelope from which improper engagement would be unrecoverable.

      I also don’t think any analysis was done on how the strong pitch doublets resulting from repeated trim cycles during activation on a non-idealized flight crew, and how that would futher reduce the envelope for recovery.

      I think that work was missing.

      On the hacker news piece, I don’t think it’s necessary to propose the mechanism by which §25.173 wasn’t met. There are a couple possible contributors. I don’t have a view on which one (or combinations) were causal.

    • You wrote
      ====================================================
      https://www.boeing.com/commercial/737max/737-max-software-updates.page

      says “The Maneuvering Characteristics Augmentation System (MCAS) flight control law was designed and certified for the 737 MAX to enhance the pitch stability of the airplane – so that it feels and flies like other 737s.”

      They make it sound like a pride-of-ownership thing, not a necessary-for-certification thing.
      ====================================================

      Try this:
      enhance pitch stability…so that it feels and flies like other 737s. [which, unlike this one, meet the pitch stability requirements listed above without augmentation]

      Seems true enough. Not a false or misleading statement. Maybe a material omission, maybe not.

    • Anonymale: I think your explanation is at least plausible. The plane becomes too unstable to meet certification requirements if MCAS goes offline, but on the other hand is not “unsafe” to fly to a landing (but is there a notification to the crew that maintenance is due and the aircraft is grounded until repaired?). It is interesting that our media has run thousands of stories on the B737, including “investigating” supposedly secret emails and complaints and so forth, but no journalist has ever tried to present a regulation-based analysis of why the plane actually needs MCAS! They keep beating the same “Boeing wanted to avoid the requirement for extra pilot training” drum when it should be obvious that the training ship sailed years ago.

    • Yes. Exactly.

      As drafted, predicate of “safe” is different from predicate of “certifiable”.

      That’s the whole point, and wholly consistent with all of Boeing’s disclosures to date.

      https://boeing.mediaroom.com/news-releases-statements?item=130431

      Stick shaker and the pitch limit indicator are the primary features used for the operation of the airplane at elevated angles of attack….Neither the angle of attack indicator nor the AOA Disagree alert are necessary for the safe operation of the airplane

      MCAS is the prerequisite for “certifiable”

      Stick shaker and PLI are the prerequisites for “safe”.

      I can’t speak to maintenance – per your earlier point, the AOA indicator purported to have failed high and the data was treated as a valid input.

      Entirely valid to assume 2nd mishap aircraft presented as a ready jet sitting on the ramp at zero airspeed.

      Training/common type rating not a plausible inference anymore. Absolutely.

      That leaves Part 25 and certification.

      This was a very helpful conversation and I sincerely thank you. I hadn’t had the opportunity to discuss this with someone who is an ATP and expert witness and an EECS prof. – which curiously, seems to represent the minimal spanning tree of skills/experience to reason about what is going on here. It was nice to unburden.

      Media was always going to miss causality.

    • Last item – just noticed

      Neither the angle of attack indicator nor the AOA Disagree alert are necessary for the safe operation of the airplane

      In referring to AOA indicator, if they are talking about only a cockpit AOA display, can’t read anything into sentence.

      If they are talking about AOA sensor – and if AOA sensor is a primary input for MCAS activation, inference is MCAS also not necessary for safe operation of airplane.

      MCAS also not enumerated as necessary for safe operation (That’s the PLI and stick shaker).

      Dunno. There’s some wiggle room between an indicator, display, and sensor. and what components those nouns represent.

    • > This was a very helpful conversation and I sincerely thank you. I hadn’t had the opportunity to discuss this with someone who is an ATP and expert witness and an EECS prof. – which curiously, seems to represent the minimal spanning tree of skills/experience to reason about what is going on here.

      I’ll say. I’m blown away by the analysis and I learned a lot. There is a big difference between Easy Answers and Correct Answers, or more properly, the Best Answers the Analysis Allows. Thank you both. Now pretty please, with sugar on top – would you guys go and solve laser fusion? Philg is going to depress me and tell me it can’t be done. I know, I know…

  9. My guess would be: leadership defined the success/failure of the project as not requiring any additional simulator time. Management kept reporting up “it’s ok, we’ll issue a software patch, we got this.” The project was eventually managed into the ground.

    Here’s hoping Boeing fares better fixing software issues on the Starliner. At least they are not trying to fix hardware with software on that project (that we’ve heard).

  10. Anonymale, great discussion. One question: I have heard Boeing used only one sensor for MCAS because comparing two would have invoked a stricter requirement (higher implied safety effect if redundancy is required). If true, this seems almost criminal.

    • Thanks.

      See §25.1309 Equipment, systems, and installations,

      and related pdf providing advisory guidance

      https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/50bfe03b65af9ea3862569d100733174/$FILE/AC%2025.1309-1A.pdf

      I don’t know Boeing’s reasons for using one sensor, but that decision doesn’t seem to meet §25.1309 , at least as explained in para 5(a) of the Advisory Circular

      a. The following basic objectives pertaining to failures apply:

      (1) In any system or subsystem, the failure of any single element, component, or connection during any one flight (brake release through ground deceleration to stop) should be assumed, regardless of its probability. Such single failures should not prevent continued safe flight and landing, or significantly reduce the capability of the airplane or the ability of the crew to cope with the resulting failure conditions

      Doesn’t prevent use of single-failure components – just limits their use to non-safety-of-flight.

      I don’t know – Maybe Boeing just didn’t look at the MCAS-on-when-should-be-off failure mode. Lack of imagination? Convinced themselves that runaway-trim procedure covered the problem of uncommanded trim input and were assumed away altitude/airspeed issues?

      That might have been a decent assumption if the initial pull back on stick disabled further MCAS inputs – but that would have defeated its design function as a Stability Augmentation System.

      I agree with @philg that MCAS-off-when-should-be-on doesn’t create an unsafe condition.

  11. Simple Answer: Capitalism
    Profits trump Safety
    1. Airbus was ahead of Boeing on new plane
    2. Boeing tried to just modify existing plane to compete
    3. To fly safely, a new plane was needed=wrong answer.
    4. To fly the modification safely, pilot training was necessary = wrong answer
    5. Give control of the plane to a computer=right answer
    6. Computer crashes the plane

    • You gotta point there. Trump may be right, “to many gadgets”. Flying military aircraft we did just fine with the HSI’s and ADI’s no problems ever. Course this was mid sixties.

  12. According to the Boeing 737 Max Technical Site the Flight Characteristics of the Max did not meet the requirements for the FAA regulation regarding “Stall Characteristics” in 14CFR 25.203(a), thereby requiring MCAS or repositioning the larger CFM Leap 1B engines PROPERLY under the wing, close to the Center of Gravity to obtain near Next Generation 737 levels of flight stability. This would require Taller Main Landing Gear, and new engine hangers (Pylon’s) that would utilize more than 96% of the existing Aircraft. The 737NG aircraft do not have, and do not need MCAS.

    My (Dennis E Sullens) understanding is that the 737Max without MCAS has Negative Static Stability, where if a gust of wind pushed the nose up, if left uncorrected by the Pilot or Autopilot, the Max nose would continue pitching up, steeper and steeper until STALL. See below from the Boeing 737 Max Technical Site. There is a lot more information on the website which helped me to understand what the problem was, and the best way to correct it. Dennis E Sullens, 29 year’s in Aviation Quality Assurance, 19 year’s with Boeing. Retired.

    FLAGNOTE 05. Boeing 737 Max Technical Site.¶

    “The LEAP engine nacelles are larger and had to be mounted [if main landing gear is not taller as on the Boeing 737 Max 10] slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary [17 inch] ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA [Angle of Attack]. As the nacelle is ahead of the C of G [Center of Gravity], 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 [Angle of Attack] when flaps are up. [Dennis E Sullens: According to “Aviation Best Practices” and many Aviation Engineers (FN01), at this point of failing the Wind Tunnel and Flight Testing, Boeing should have made BOTH main and front Landing Gear taller and then place Lager Max Engines PROPERLY under the wing, thereby bringing the Max to near 737NG levels of flight stability, passing the FAA Flight Stability requirements, and thereby eliminating the need for MCAS. No MCAS, no Problems. Everybody is happy.]”¶

    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.”¶

    http://www.b737.org.uk/mcas.htm¶

  13. As a Boeing retired airline pilot I am very critically upset with the FAA and Boeing for certificating the Max.
    Having flown the 757, 767 and 777 I am well trained on the flight management systems of Boeing aircraft.
    All of those aircraft have double or triple backup inputs to the three flight management computers. The MAX only takes its input from the captain’s angel of attack vain.
    The angle of attack vain.

    The 737 was issued its certification in the mid 60’s and the Boeing’s I flew were were all certified in the 80’and 90’s. The certification standards had changed greatly during that period with the advent of advanced computers being introduced introduced into jets in the late 70’s. Which required additional inputs.
    The 777 FMC can be engaged after take off at 200 feet and it can remain on until the airplane decelerates thew 60 knots after landing and disengaged by the captain to taxi to the gate.
    I believe the original 737-100 did not have auto landing capabilities.

    I also believe Boeing engineering and production standards have gone down considerably in the last ten or fifteen years.

  14. Lengthen the landing gear in order to better accommodate post 737 200 powerplants.

    Lower and move aft the 737 MAX powerplant’s mount positions.

    The pre 1966 design of the 737 that lingers today in 2020 has long been overdue for retirement.

    The first indication of future issues became apparent when the 737-300’s CFM-56 accessory gearbox had to be relocated due to much needed ground clearance.

    The 737 has had it’s the day. Today it is somewhat like a Sopwith Camel with the latest Avionics available…

    Some things you just can’t push into the future especially when things around it keep advancing.

  15. The obvious reason boeing does not simply abandon MCAS is that it would be a tacit adminsion that it was not really needed in the first place (other than to avoid a different type rating). That leads to the conclusion that the real motive was to maximise sales and profits. This in turn would likely lead to more lawsuits and possible (well deserved in my opinion) criminal charges.

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