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Can someone here explain a pretty basic question that I have had ever since this controversy came to light? I don't understand why any aircraft manufacturer, engineer, software developer, would make a device that forces the nose of the plane down. You can't land unless you're in control and landing safely is the only reason to go down that I know of. So I just don't get that as an initial thing, but the second part of that is, given the hazards of pointing the nose down, which are obvious even to me, why would you not design the system so that it's a warning, and allow the pilots who are actually sitting there to decide whether going down is a good idea or not? That just fundamentally, as a threshold issue, makes no sense to me at all. So I'm hoping you could help me understand. Thank you.

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    $\begingroup$ I've heard it's so that the MAX can have a common type rating with the older 737s, which airlines like in order to keep training costs down $\endgroup$ – Steve Kuo Jul 28 at 11:35

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The main thing to avoid in aeroplane stability & control, is an aerodynamic nose up moment that is not commanded by the pilot. The uncommanded nose-up moment would not auto-stabilise, but rapidly get progressively larger with increasing angle of attack, and run away to a stalled aeroplane.

During certification of a passenger aeroplane, many tests are carried out to check if the airframe does not start to have a mind of its own.

  • If the pilot does not provide a control input, the airframe must return to the trimmed position.
  • Forces and inputs to move the aeroplane away from trimmed position must be such that there is an ever increasing force required to achieve an ever increasing nose-up position. The nose-up position must always be commanded by the flight control surfaces, elevator and stabiliser, in a predictable way.

One of the tests to be performed during certification is stick-force-per-g. Bank the aeroplane and start turning while pulling the stick back in order to maintain altitude. Then bank more and pull back more, in ever tightening turns. It must be progressively harder to pull back on the stick to maintain altitude, never easier.

It was during this wind-up turn that due to the engine configuration of the MAX an aerodynamic nose-up moment appeared, which would cause the pitch stick force to suddenly become less than expected. Not as bad as a runaway pitch, but still an undesirable situation when the pilot is still straining to maintain the manoeuvre. This is the situation that MCAS was originally designed for, to auto-compensate for this situation only. The aeroplane should always stabilise itself and manner of control must be predictable and within the reaction times of humans.

More info in the links in this question, very interesting articles which illustrate how MCAS design bloated from the original scope, under time pressure.

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    $\begingroup$ The accelerated stall. They keep blaming the engine (could be part of it). Yet using smaller "more efficient" airfoils as stabiliators has one very troublesome aspect. With a flat plate, the coefficient of "lift" is linear to the angle of deflection all the way up to 45 degrees, along with loads of drag, making the pull harder and harder. Notice the lift curve on an airfoil is anything but linear post stall. Could this be part of the control issue? $\endgroup$ – Robert DiGiovanni Jul 29 at 3:15
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    $\begingroup$ @RobertDiGiovanni Not a stall, but a tight turn. I could not find any reference to a change in stabilator profile relative to the 737NG. $\endgroup$ – Koyovis Jul 29 at 5:11
  • $\begingroup$ Well, I am going to award the check mark to Koyovis, because his answer is the most detailed, albeit a little technical for me. But big thank yous also to Anthony X and Alexei for less technical answers that rounded out my understanding. Even the comments were helpful! $\endgroup$ – Malik A. Rumi Jul 29 at 15:46
  • $\begingroup$ The stabiliator may not have changed, but air flow around it possibly, with the engine exhaust and wing downwash interacting. Huge amount of energy being put into air aft of the wing, and flow pattern may have changed from NG. $\endgroup$ – Robert DiGiovanni Jul 29 at 21:06
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    $\begingroup$ The second sentence of the first paragraph appears to describe a situation in which an airplane becomes statically unstable in pitch (...would not auto-stabilize...run away to a stalled aeroplane.) As that issue has been raised in this answer, I think it would help to point out that the problem here was not that bad - the airplane does not become unstable (at least prior to the stall), but its handling characteristics, unmodified, were still unacceptable. $\endgroup$ – sdenham Jul 30 at 11:43
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Counter-intuitively, lowering the nose of an aircraft is not done for the purpose of "going down". Climb/descent is managed with throttle, and speed is managed with the control column/stick. The logic of this makes sense when you consider that going up or down involves the addition or removal of potential energy which is sourced by the engines and sunk into drag.

Pull back on the column/stick and you increase pitch angle, which increases angle of attack, which increases lift. That causes the aircraft to go up, yes, but going up requires extra energy which most immediately comes from the aircraft's forward motion - it slows down. As it slows, lift decreases and (ideally), equilibrium is restored at some lower airspeed and increased angle of attack.

Same thing in reverse - push forward and you go faster.

Due to the way the new engines were fitted to the 737 Max 8, it acquired a handling problem in which the nose could pitch up unexpectedly; if uncorrected, this pitch-up could induce a stall. MCAS was intended to compensate for this bit of bad behavior by detecting its occurrence and automatically pushing the nose down to maintain the expected attitude and prevent a stall. The crashes tragically revealed points of failure in the MCAS; in those cases, MCAS responded to a falsely indicated high angle of attack condition.

A fairly lengthy article was published at The Verge recently, which details the story of the 737's evolution into the Max 8. The article outlines the reason the MCAS exists and why it does what it does (both nominally and erroneously), as well as a short history of one of the accident aircraft leading up to its final moments. Not sure how authoritative the article is, but I believe all the stated facts are correct.

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To add a bit to the existing answers, the reason for the unexpected pitch-up moment on the 737 MAX, as far as I understand it, had to do with the flattened portion on the bottom of the engine cowling.

The root of the problem is that the 737 was designed back in the day of low-bypass turbofans (specifically, the Pratt & Whitney JT8D.) Due to the low bypass ratio around the core, these engines had much smaller diameters than today's high-bypass turbofans. The JT8D had a fan diameter of only 49 inches, while the LEAP-1B on the 737 MAX has a fan diameter of 69 inches (and even that is notably held smaller than the 78 inch diameter on the LEAP-1As for the A320neo series.)

Due to the massive increase of engine diameter with the switch to high-bypass turbofan engines, ground clearance became a problem, since the landing gear height was designed with much smaller engines in mind. So, you will see even on the 737 NG series that there is a flattened portion on the bottom of the engine cowling in order to improve the ground clearance a bit while still allowing the landing gear to fit into the wheel wells. Since the LEAP engines on the MAX are even larger than the CFM56 engines on the NG, the flattened part became larger in order to fit the new engines under the wing.

Boeing 737
737NG with flattened engine cowlings (Source)

It turns out that this flattened portion can create a significant amount of lift (and, due to its positioning, nose-up moment) at high angles of attack. This causes the aircraft to want to pitch up even more at high AoA, which is bad for the reasons that Koyovis has already explained well. Thus, MCAS was designed to prevent this tendency for AoA to continue uncommanded increases at high AoA by intentionally pushing the nose down if the AoA got too high.

The idea itself is not necessarily a bad one and systems with similar purposes (pushing the nose down to prevent AoA from getting too high) exist on nearly all Airbus planes still flying and also on other newer Boeing designs. The problem was with the implementation of the MCAS, which apparently did not cross-check the AoA vane inputs against each other or otherwise do sufficient sanity-checking on the inputs from from the AoA vanes before acting on one of the AoA vane inputs to push the nose down without command from the pilots. In the case of the Ethiopian crash, preliminary information from the investigators indicates that the AoA vane that the MCAS was using for input was likely completely sheared off of the aircraft, possibly during a bird strike or similar FOD event during takeoff. Due to the counterweight of the vane still being attached, this caused it to indicate an extremely high AoA, which in turn triggered the MCAS.

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  • $\begingroup$ Source for the profile of the engines creating lift, please? The origin of the pitching moment is the moving of the thrust-line of the engines in the MAX 8 away from the centre of gravity. This was necessitated by the use of new larger, more efficient, high-bypass turbofan engines. In a throttle-up scenario (eg taking-off) the aircraft now pitches up more than its predecessor version. To reduce pilot training costs for the new type, MCAS attempted to silently reduce the pitch moment perceived by the pilot, by using the horizontal stabilizer to apply a corresponding pitch-down moment. $\endgroup$ – Ben Jul 30 at 10:59
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    $\begingroup$ @Ben High power settings do increase the pitch-up moment further, but that's not what MCAS was designed to counter and MCAS isn't active during takeoff anyway (it's locked out when the flaps are selected.) See here for a more detailed description. It's the nacelle shape that causes the lift generation. If it were just the thrust, it would apply to virtually all airliners these days, as the engines are almost always mounted well forward of the CoG and create pitch-up moment at higher power settings. $\endgroup$ – reirab Jul 30 at 11:19
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    $\begingroup$ @Ben It's both the nacelle shape and the position of it (and the combination of the two.) Normally-shaped nacelles wouldn't generate as much lift at high AoA as far as I know. Of course, the farther forward you move the engine, the larger the pitching moment will be for a given lift. In this case, both the lift increased due to larger and reshaped nacelle and it was moved a bit farther forward. $\endgroup$ – reirab Jul 30 at 11:29
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    $\begingroup$ @Ben Indeed, as noted in the answer. It was expanded from the NG to the MAX, though, in order to compensate for the LEAP engines, which are larger than the CFM56s on the NG. The lift at high AoA existed in the NG, but it's larger and a bit farther forward in the MAX, resulting in more pitch-up moment than the NG nacelles caused. $\endgroup$ – reirab Jul 30 at 11:35
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    $\begingroup$ @Ben Yes, pitch up moment from thrust was not the reason MCAS was created. $\endgroup$ – reirab Jul 30 at 11:55
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MCAS forces the nose of the 737 MAX down because, under some circumstances, the nose of the aircraft can pitch up and this may result in a stall.

From Wikipedia:

The Maneuvering Characteristics Augmentation System (MCAS) is a flight control software system developed for the Boeing 737 MAX to provide handling qualities similar to the Boeing 737 NG, especially in low-speed and high angle of attack (AoA) flight. It lowers the nose without pilot action when it determines the aircraft is too nose-high, based on input from airspeed, altitude and angle of attack sensors. However, it is susceptible to erroneous activation, as evidenced in the deadly crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302. The 737 MAX is indefinitely grounded until regulators decide the aircraft is airworthy, pending software and instrumentation updates and revisions to information for flight crews. They may also be required to undergo MCAS training sessions in flight simulators.

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    $\begingroup$ Reasonable answer, but yet again you are using wording that appears to have led to the original question: MCAS is NOT supposed to push the nose down, it IS supposed to push the nose lower than it is - the assumption being that the nose is too high; MCAS should make it slightly less high. $\endgroup$ – Mike Brockington Jul 29 at 10:26
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    $\begingroup$ @MikeBrockington: What's the difference between lower and down? A 737 isn't going to be flying inverted, so the two appear functionally equivalent. $\endgroup$ – MSalters Jul 29 at 13:01
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    $\begingroup$ @MSalters - Both words refer to the direction of change, but 'down' can be misinterpreted as meaning 'below horizontal' $\endgroup$ – Robin Bennett Jul 29 at 13:12
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    $\begingroup$ @RobinBennett: So can 'lower'. Perhaps the better description would be that MCAS is supposed to level off. $\endgroup$ – MSalters Jul 29 at 13:14
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    $\begingroup$ @MSalters To a layman, 'Down' means below the horizontal, i.e. diving. 'Lower' means less far up, less of a climb; this may possibly mean a transition from climb to level, from level to dive, but not necessarily. $\endgroup$ – Mike Brockington Jul 29 at 15:51
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IIRC, the modifications to the 737 (including the new engines that powered it) meant the engines had to be positioned further forward and higher. (This was a similar case when they swapped from the original "cigars" to the CFM-56). And under some flight conditions, this resulted in automatic pitching up of the aircraft.

MCAS was designed to counter this tendency, to avoid pilots constantly having to make corrections. Since it was inherent behaviour of the new design, it was implemented in such a way that the pilots were not intended to even know such a system existed on their aircraft, for all intents and purposes it was flying "just as the other ones did".

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  • $\begingroup$ I've seen this explanation elsewhere, but I would have thought that raising the thrust line of the engines would cause a nose-down moment, not nose-up? Maybe it was just that these engines were more powerful than the old ones? $\endgroup$ – Robin Bennett Jul 29 at 9:37
  • $\begingroup$ @RobinBennett that's an interesting topic, maybe you should ask this as a new question. $\endgroup$ – leftaroundabout Jul 29 at 12:14
  • $\begingroup$ The new engines are so big that they couldn't raise them far enough to maintain the original thrust line. They were raised such that part of the intake nacelle was already above the wing! $\endgroup$ – Ames Jul 29 at 15:17
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    $\begingroup$ The issue with the 737 MAX is not pitch caused by the thrust of the engines (though that does exist, on the 737 and other airliners). The issue is that the engines with their large fairings generate a decent amount of lift at higher angles of attack; combined with the forward position of the engines that lift generates the nose-up tendency that MCAS was meant to counter. $\endgroup$ – Roel Schroeven Jul 29 at 15:28
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For stable flight all forces and moments must be at equilibrium. For a given speed and thrust there is a window of acceptable angles of attack (AoA) when the wing produces enough lift. If the angle of attack is too high, the wing stalls and plane loses lift abruptly. The critical AoA can be reached by either excessive pitch up for a given speed or slow-down for a given pitch.

The Boeing 737 design dates back to 1964 and the latest 737s are evolutions of that design. The most apparent differences in design of the now-grounded planes are:

  • Hull lenght
  • Wingspan
  • Engine thrust

If you compare engine positions and the fact that the thrust doubled over time you can clearly see where the unintended pitch up is coming from.

This pitch up can lead to stalling and pitch down countermove must be performed to avoid such situation because power-up will lead to further pitch up move.

Also note that the MCAS intervention led to a disater only twice; in all other cases not worth to be documented it worked as intended. The issue, that made all 737-MAX grounded, is NOT about that such system was implementd but how it was implemented and how it was documented - which is a comletely different story. Also note that it is not the first case of anti-stall not-well-documented safety system malfunction leading to disastrous consequences.

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    $\begingroup$ Yes, not the first case indeed. Airbus has also had its fair share of problems with its alpha protection systems, too, including one incident that nearly caused a mid-air collision between an A330 and an A340 over the Atlantic from a dramatic uncommanded climb and another in which an A330 made two extreme uncommanded dives (one of which actually went into negative g territory) that injured several passengers and crew members before the pilots were able to get the airplane back under control. $\endgroup$ – reirab Jul 29 at 18:41
  • $\begingroup$ “...led to disaster only twice.” Once too many. $\endgroup$ – Koyovis Jul 30 at 0:52
  • $\begingroup$ @Koyovis Twice too many, I'd argue. Sound software engineering and a culture of airlines trying to make sure their pilots learn as much as possible about the immensely complicated machines they're flying rather than as little as they can get away with would have prevented both disasters. $\endgroup$ – reirab Jul 30 at 15:07
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I don't understand why any aircraft manufacturer, engineer, software developer, would make a device that forces the nose of the plane down.

Because history has proven that pilots themselves don't always do it.
I recon the engineers thought it would improve safety.
Looking at the Air France flight 447, if (a working) MCAS was in place it probably would have saved the plane.

In short the first officer stalled the plane in to the water.

First officer Robert said to himself, "climb" four times. Bonin heard this and replied, "But I've been at maximum nose-up for a while!" Captain Dubois realized Bonin was causing the stall, causing him to shout, "No no no, don't climb!"

This is just one accident where lowering the nose would have saved the plane.
I'm not blaming the air crew, it's "counter-intuitive" to lower the nose and once your reptile brain is in charge it is hard to get back in control of your mind.

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  • $\begingroup$ Piloting 101: when in a stall, lower the nose. The pilot may do that. $\endgroup$ – Koyovis Jul 31 at 14:35
  • $\begingroup$ @koyovis yes. They should do that but I believe AF447 is a clear example that it's not always done. Many other accidents has been close to ground and but AF447 had lots of time to solve the problem yet they didn't. Is a MCAS or similar needed, perhaps not needed but it could help. $\endgroup$ – Andreas Jul 31 at 14:42
  • $\begingroup$ It's worth consideration I guess, T-tail aircraft have stick pushers as well. $\endgroup$ – Koyovis Aug 1 at 12:56
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Planes fly with speed. If they don't have enough speed, they fall down like bricks.

Nose down trades altitude for speed. You're losing altitude, but you're "flying more".

Nose up tries to trade speed for altitude. At best, you're gaining altitude, but you're "flying less". At worst, you're not gaining anything, you're just slipping down while looking up. This is how Air France Flight 447 crashed: copilot kept pulling up and they've lost all altitude just by that.

737 MAX, due to it's unplanned engine size, has natural tendency of pulling the nose up, which is super-dangerous because of the reasons above. Pushing it down is a way to keep this danger at bay. The problem is not in pushing the nose down itself, but pushing it down too often and too much. They've replaced big danger with as smaller one - which unfortunately proven to be designed even worse than oversized engines.

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The real problem is not that the lift from the nacelles provide a upward pitching moment. The issue is why. Stability demands that the center of lift of the total airplane be aft of the center of gravity. Total lift comes from several sources, the wing, some contribution from the fuselage, contribution due to the engine thrust angle relative to the angle of attack, and engine nacelle contribution. At high angle of attack the engine nacelle contribution becomes significant. Add all of these sources and the result is that center of lift of the total airplane moves forward at high angle of attack. As angle of attack is increased the center of lift moves forward resulting in a pitch up. As angle of attack is progressively increased the center of lift moves forward of the center of gravity and potentially uncontrolled pitch up occurs.

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I think the issue for Boeing was the risk of falling behind Airbus in sales. So they modifed the 737 using far larger engines which because of limited ground clearance had to be mounted in such a way that increased the risk of stall during take off. So with a plane that is not as aerodynamic as it should be Boeing fitted a system to overcome the aerodynamic flaws, MCAS. It seems strange to knowingly produce a plane which is not as aerodynamic as its predecessor, but in their desperation to keep up with Airbus, Boeing did exactly that.

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    $\begingroup$ Also the regulator allowed it happen $\endgroup$ – Len Cannon Jul 30 at 8:01
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    $\begingroup$ There was indeed pressure on Boeing to come up with a competitor for the Neo, and the limited ground clearance made things difficult. It's important to state that there seemed to be nothing wrong with the original design of MCAS: automatic correction in a very specific circumstance that pilots would almost never find themselves in. The design scope had to be expanded in a hurry though, and that is where the problems seem to stem from. $\endgroup$ – Koyovis Jul 30 at 9:53
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    $\begingroup$ This had nothing to do with takeoff. The MCAS is not even active during takeoff. It's locked out when flaps are selected. $\endgroup$ – reirab Jul 30 at 15:09

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