This has become a trope and is flatly incorrect.
MCAS was cooked up to maintain the handling characteristics required by the certification specifications.
The standard requires stick resistance to increase as the critical (stall) angle of attack is approached. The MAX violated that requirement due to the aerodynamics of the new engine cowls, so MCAS was Boeing's (?crap) implementation of "artificial feel" to meet the spec.
Yes, while the spec exists presumably to prevent the pilots from themselves taking the ship into a stall, MCAS doesn't change wing or aircraft performance in a material way that would be meant by "prevent a stall."
(2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than the minimum speed of the applicable speed range prescribed in paragraph (b)(1), nor speeds that require a stick force of more than 50 pounds)[...]
The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak.
As far as 'authoritative sources' go, I'll just mention the Gell-Mann amnesia effect.²
(me: non-active aircraft dispatcher)
¹—https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1... ²—https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov...
How do you know this? Haven't seen that reported anywhere but in user comments here.
When speaking to the uninitiated, anti-stall system gets the point across.
When speaking reguspeak, and actual aerodynamics, MCAS is a compensatory system that is meant to allow the plane to be certified airworthy. It is the same class of system as a mechanical stick-pusher (though even less effective since all it does is changes the yoke pressure required to bring the plane to a stall).
It's not a meme sadly, which makes this all the more tragic in hindsight. Sibling poster has quoted the CFR.
Where did you get this information? My understanding is the airframe has the potential to nose up due to extra lift provided by engine placement (eg, lift not commanded by stick pull). MCAS is used to prevent this stall condition during high AoA.
One of the most important things to get right with a control system is to maintain some level of consistency throughout the entire range of the system you're actuating.
Get on your computer, and move your mouse around. Now go into the settings and muck with your mouse sensitivity/acceleration curves, and try to drop your cursor on a particular file.
Odds are, you'll have some degree of difficulty until your brain adjusts to the new settings.
Same thing is relevant with airplane yokes. The force required to deflect a control surface (and therefore the plane) in an airstream is a result of the forces imparted on the surface + plane while by the impinged airstream.
Now, a force response curve should be smooth and predictable. Some distance of deflection on the yoke at one position should yield approximately the same level of deflection from the yoke at another position. If there is a sudden change in the number of degrees of AoA you get per degree of deflection, this is a highly undesirable flight characteristic.
For a pilot's perspective, I recommend the D.P. Davies Interview Podcast from the Royal Aeronautics Society, specifically the Boeing episode where he describes his certification flights of the Boeing 727-300.
That plane had a similar interaction with high lift devices at high AoA, making the plane more eager to pitch into a stall when you got to a certain point AoA-wise. This was in direct violation of this CFR. As Davies puts it, "You can't certify a plane that wants to stall itself!"
The 727 was eventually certified though when they added a mechanical stick pusher which would shove the stick forward when the pilot was getting close to that flight regime to make sure they didn't stall.
MCAS, by all indications, serves a similar purpose, it detects current AoA and trims the plane to counter the extra lift so that the deflection of the elevator at the top of the AoA curve has to fight the nose-down trim, which to a pilot would feel like the plane takes the same amount of force to deflect for those last few degrees. Basically, to use the mouse analogy, it's bumping down the sensitivity of the yoke to compensate for the extra sensitivity created by the lift at high angles of attack.
All of this is fine and dandy at 17 or so degrees AoA. Not so much though in attempted level flight with a busted AoA sensor.
Much of the art behind control systems comes from translating technological activity into humanly processable control schemes. That CFR is a common sense guideline basically specifying that a transport plane must have a predictable response curve.
It reminds me of the way nuclear reactors work, which kinda made it click. There they make criticality dependent on neutrons donated by fission byproduct decay, allowing for control rod situations, and thereby criticality level changes to take place over minutes instead of seconds.
Plus, look at the name
Maneuvering Characteristics Augmentation System.
The important takeaway is it doesn't really do squat to make stalls not happen beyond making it increasingly impossible for the pilot to get enough oomph out of the control surfaces to get the plane in stall and stay there. I don't know if there is also a stick pusher for MAX aircraft to help recover from a stall.
If the MCAS was constantly variable, like a mechanical stick pusher, then what you're saying would make sense. IMO, as reported, it seems like an 'anti-stall device' that automatically adjusts the plane's pitch regardless of stick input. Depending on conditions, this presumably could be little to no stick input.
But AoA sensors freezing is apparently common enough where this system is frequently kicking in when the plane isn't in the situation it was designed for.
From the article: “A properly trained pilot should be able to solve an MCAS anomaly or any uncommanded flight-control input through procedures that are taught to all 737 pilots,” said Menza, noting that the emergency information Boeing distributed in December reiterated those procedures.
Here's a video of a competent 737 pilot showing those exact procedures: https://www.youtube.com/watch?v=xixM_cwSLcQ
Obviously the MCAS system is badly designed, but any automation system can fail and pilots need to know how to deal with it.
https://www.wsj.com/articles/ethiopian-airlines-pilots-initi...
Did the pilots fail to notice the MCAS problem before it put the plane too far out of trim?
Could they have saved the plane if they stuck to the procedure and kept adjusting the trim by hand?
Why does MCAS override even direct pilot trim up commands?
Is the runway trim procedure even viable on older 737s once the trim goes past a certain point? Or are we just lucky that it never happens?
In a typical run-away trim, the trim wheel will move a great distance. It's very noticeable, both audibly (the wheel makes a clack-clack-clack noise) and visibly (there are white paint flashes on the wheel). And, obviously, the plane nose goes up or down by more than expected. The correct remedy is to disable the auto trim control via switches on the panel (located near the trim wheels).
The MCAS will adjust the trim in small increments every 10 (or is it 20?) seconds. Yes, a pilot should notice this, but because it's intermittent, it's more likely they don't "see" it as run-away trim, and just a slightly abnormal trim (EDIT - problem made worse because pilots were not informed MCAS existed - it's not a failure more they have trained on). They may attempt to remedy this with the manual trim control (a rocker switch on the control yoke). This does NOT disable MCAS, it only re-trim the plane. MCAS will re-engage due to faulty AoA sensors repeatedly until either the pilot disables all auto-trim with the switch on the panel OR the plane runs into the ground.
I wonder if they killed the trim motor and then failed to reach level flight before the ground got in the way.
And then the ground got in the way.
Yes, mistakes were made, yes, we should find ways to avoid this happening in the future, no, automation isn't the enemy. Without automation in flying at the current number of airplanes in the sky we'd probably have one or two crashes a day. The fact that 2 crashes in half a year make the news is thanks to the amazing safety of modern airplanes.
However, automation that is designed in a criminally negligent manner is. People who create such designs should be put in prison, and we should not be using products from them ever again.
Any big company has a scandal, learns from it, the people that learned the lesson leave at some point, a new generation of employees makes a similar mistake again, wash rinse, repeat.
You probably can't fly any airplanes if you will not use any airplanes from a company that had a scandal like this.
One can't teach others without having first learned the lesson oneself. I'm not convinced Boeing has. I'm not convinced the company had adequately consider all possible scenarios and how pilots would react in them based on their intuition.
This is by design. The flight software is deliberately kept simple enough that the pilots can be trained to understand the full workings of the flight software and what it will do in any situation.
The entire flight control algorithm is probably only a few hundred lines of psudocode.
It's a catch 22:
We are fully capable of designing a plane that safely flys itself from airport to airport without any pilots, to handle most emergency situations. We have been able to do so since the 80s.
But we are not able to put a pilot in the loop on such a plane without massive safety implications. To have a pilot in the loop, the software has to be kept super simple so the pilot can diagnose it in an emergency and take the correct action.
Either we have the flight software that is situational aware and authorised to take any action, or we have a super simple software and a pilot in the loop. There is no safe middle ground.
It's the same problem we see with Tesla, Uber, etc and autonomous cars. If it's to be truly autonomous, it needs to do so without ANY human intervention. As soon as a human is added to the loop, the system needs to be understandable by any human operator.
In the case of the MAX, even with Redundancy sensors, there is the possibility enough of them fail that the plane doesn't "know" how to resolve the problem. So, control must be returned to the pilot with enough time and enough information to avert disaster.
It could launch into space, orbit, de-orbit, fly though re-entry and land at a runway without any external control (it was designed to complete it's mission under signal jamming conditions).
It had full situational awareness and could autonomously make decisions like redirecting to a different runway or airport depending on weather and other factors.
Maybe it's a bit of a stretch to say Buran was safe enough to fly passengers autonomously, but it was in the right direction.
Compared to a self driving car where you have to detect respond to other cars and solid objects, the amount of processing power to maintain a dynamic aerodynamic flight model and fly under instrument conditions is actually pretty low.
No, the sensors existed before the MAX they are used by the autopilot system. With the MAX they are now also used by the MCAS system.
These sensors exist to solve a problem with the MAX design. Changing and moving the engines increased the likelihood of a stall when the engines could push the nose up (as I understand it). Fine.
But here's the kicker: this should be something that pilots should be trained on. They should be aware of how the MAX is different to the previous 737s and know what to do to disable this system if it causes problems.
But that might be the end of a common type rating, which is something the airlines (and apparently Boeing) didn't want.
The whole 737 MAX situation just looks like a giant clusterf--- that was a kneejerk reaction to the unexpected success of the A320neo to which Boeing had no answer and to avoid years-long development delays and losing more customers to Airbus, it really looks like they made shortcuts. And that's so damaging to their brand it really defies belief.