Boeing Boeing 737 Max MCAS System

Astronuc

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A TV talking head (sorry no link) says this is a consequence of the 737 (all 737s) having so little ground clearance.
I read a discussion of this issue and various media organizations have published about it. The one I read highlighted the evolution of the 737 and the addition of the CFM International LEAP engine, which is larger than previous generations.
So while all 737s are certified as a single aeroplane type, the changes have been massive.
Most recently, the big push has been to reduce fuel consumption. For both economic and environmental reasons, this led to the latest 737 MAX 8 models – along with the competitor Airbus A320 Neo – being fitted with new, larger and more efficient CFM Leap engines. These must be mounted higher and further forward than previously, creating a handling problem that wasn’t unique to this aeroplane, but had to be addressed.
Ref: http://theconversation.com/boeing-737-max-after-two-fatal-crashes-an-expert-explains-the-issues-113833
Similarly:
https://www.businessinsider.com/boeing-737-max-design-pushed-to-limit-2019-3 (too much advertising)

The Wall Street Journal has published an article:
U.S. Federal Investigators Probe Boeing 737 MAX Development Choices
Probes to focus on stall-prevention system and examine whether any shortcuts compromised safety

 

CWatters

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Sorry if this has been mentioned already.


Selected quotes..

Regulators knew before crashes 737 MAX trim control could be confusing
The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said. It specifically noted that at speeds greater than 230 knots (425 km/h) with flaps retracted, pilots might have to use the wheel in the cockpit's centre console rather than an electric thumb switch on the control yoke.
The source said that training materials before the crash did not say the wheel could be required under those conditions but that Boeing advised the airline about it after the crash.
In the EASA document, the regulator said simulations showed the electric thumb switches could not keep the 737 MAX properly trimmed under certain conditions....
"It would be very unusual to use the trim wheel in flight. I have only used manual trim once in the simulator," said a 737 pilot. "It is not physically easy to make large trim changes to correct, say, an MCAS input. You - or more than likely the other pilot - have to flip out a little handle and wind, much like a boat winch."
So imagine you are both pulling hard trying to recover from a runaway nose down trim. Your colleague is yelling that the stick forces are too high for him to pull back on his own. One of you needs to stop pulling and hand crank the trim wheel instead. Ok so they can do both at once but I still have a problem with the "trim" being more powerful than "control".
 

russ_watters

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I "liked" most of your post, but...
...I still have a problem with the "trim" being more powerful than "control".
While I understand the sentiment with regard to MCAS or other runaway trim scenarios (with or without MCAS, it happens sometimes), as a general statement this is off base or even backwards.

For a plane with a fixed stabilizer and elevator, the purpose of trim is to reduce control input force by providing some of the force directly to the elevator so the pilot doesn't have to. Without trim adjustment, such airplanes would not be flyable for very long -- not even a Cessna 172. The required control force is just too much for too long.

So trim force has to be more powerful than control force so the pilot can use it to eliminate of all of the control force.
 
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FactChecker

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So trim force has to be more powerful than control force so the pilot can use it to eliminate of all of the control force.
I would assume that trim is needed to reduce stick force in relatively straight and level flight. I don't think that it would need enough control authority for extreme maneuvers. It may need more during a strong crosswind that lasts a long time, but I can't imagine it needing a large amount of pitch authority that this system apparently had.
 

CWatters

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Yes I understand that trim is used to eliminate stick forces. I flew gliders for several years.

I guess what I'm saying is why do you need to be able to eliminate control forces over such a large range of control throw? When under normal circumstances do they need to trim so nose down that the pilots can't recover due to high stick forces?
 

russ_watters

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I would assume that trim is needed to reduce stick force in relatively straight and level flight. I don't think that it would need enough control authority for extreme maneuvers.
I guess what I'm saying is why do you need to be able to eliminate control forces over such a large range of control throw? When under normal circumstances do they need to trim so nose down that the pilots can't recover due to high stick forces?
Cruise: The higher the speed, the more nose down trim is needed. Flaps: flaps down creates a pitch-up moment, requiring nose down trim.

Or looking at it from the other way, because there is no universal neutral point:

Trim for takeoff is significant nose up, otherwise it would take a huge amount of force to pull a plane up off the runway, and crashes on takeoff would be a much bigger risk.

The same amount of deflection requires more force at higher speed, so it might seem like a lot of force when it is meant to be used in a situation where the force is lower but the deflection is higher. The MCAS or any runaway nose down trim situation causes a deteriorating situation:

When you nose down, the plane speeds up, requiring more force to keep the nose down. In runaway down trim it's the opposite; the nose goes down, you pull it back up, but the plane is accelerating so the force you need to apply is steadily rising, making it harder and harder to keep the nose up. A setting that requires 30lb of force to stay level at 200kts requires 120lb at 400kts.

Perhaps we've gotten to a point with technology (or did 60 years ago?) that elevators shouldn't be used anymore. I'm not totally clear on what their upside is vs an all moving stabilizer.
 
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CWatters

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As I said much earlier in the thread I believe most jets have a combination of all moving tailplane and elevator (with trim moving the AMT and the pilot moving the elevator).
 

FactChecker

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Trim for takeoff is significant nose up, otherwise it would take a huge amount of force to pull a plane up off the runway, and crashes on takeoff would be a much bigger risk.
I have never worked on a commercial airplane, but I believe that the actual mechanism on a modern large plane would have hydraulic or electric power
 

russ_watters

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As I said much earlier in the thread I believe most jets have a combination of all moving tailplane and elevator (with trim moving the AMT and the pilot moving the elevator).
Hmm, the wiki on stabilators confirms that. I guess I misread that the difference between Airbusses and Boeings was that Airbusses used all moving stabilators for control and tabs (elevators) for trim.
 

russ_watters

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I have never worked on a commercial airplane, but I believe that the actual mechanism on a modern large plane would have hydraulic or electric power
Sure, but while that can - if the designers choose - completely eliminate the need for pilot input force it doesn't eliminate the control surface force and trim is still needed. The pros and cons of the design choices are interesting but I'm not sure if they are relevant: a plane like the 737 that isn't fly-by-wire requires force to move the control wheel. The wiki article says that progressive force - inherent stability - is required by law. I'm not sure the extent of that, since obviously on a full fly by wire the only progressive force is a spring attached to the joystick.

Like I said, maybe we're at the point where this issue should be eliminated.

[edit] Let me rephrase: I don't know why hydraulic assist control systems still require significant control force. Perhaps because the control wheel is still mechanically linked to the control surfaces, control wheel force can only be proportional to control surface force. Or perhaps it is a purposely made choice to keep the control wheel force high to keep the direct feedback feel "normal".

Airliners crash very infrequently these days. It can certainly be said that the Boeing design philosophy contributed to these crashes, but it can also be said about the Airbus design philosophy. Neither is perfect.
 
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[edit] Let me rephrase: I don't know why hydraulic assist control systems still require significant control force. ... Or perhaps it is a purposely made choice to keep the control wheel force high to keep the direct feedback feel "normal".
On the F-16 (fly by wire), that is exactly correct. The feel on the stick maps to the roll and pitch rates (or g's) desired. In general, the mapping remains unchanged regardless of flight condition. That makes flying the plane much easier.
 
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It can certainly be said that the Boeing design philosophy contributed to these crashes,
I have worked with some Boeing flight control "worker bees" and found them to be very smart, conscientious and reliable. I trust and respect them. I would guess that the problems (if any) may be with higher-level decision makers. But that is pointless guesswork on my part.
 

russ_watters

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I have worked with some Boeing flight control "worker bees" and found them to be very smart, conscientious and reliable. I trust and respect them. I would guess that the problems (if any) may be with higher-level decision makers. But that is pointless guesswork on my part.
I don't mean for that to sound like blame; I'm talking broader than just the MCAS issue itself. In essence, my perception is that the Airbus philosophy is that computers fly airplanes and pilots are there to tell the computer where the plane should go. The Boeing philosophy is that pilots fly airplanes and computers help. Both of these philosophies have their pros and cons and both contribute to crashes. These days crashes are so infrequent that it seems to me that most are caused by poor relationships/communication between the pilot, the computer and the plane -- because everything else has become so close to perfect.
 

atyy

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It seems the Ethiopian Airlines pilots followed the instructions Boeing reiterated after the Lion Air Crash and were still unable to control the plane. How could that be? Could Boeing's instructions have been wrong or inadequate? There's interesting commentary in the April 6 update at http://www.askthepilot.com/ethiopian-737max-crash/.
 

nsaspook

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atyy

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Is that the current manual or an old one? The Ask the Pilot article says "However, as an obscure phenomenon that no pilot was likely to ever encounter, it was eventually forgotten as the 737 line evolved, to the point where no mention of it appears in the manuals of later variants."
 

nsaspook

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Is that the current manual or an old one? The Ask the Pilot article says "However, as an obscure phenomenon that no pilot was likely to ever encounter, it was eventually forgotten as the 737 line evolved, to the point where no mention of it appears in the manuals of later variants."
It's from 1982. I've no idea if it's in the current manual but I suspect not.
 

CWatters

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There's interesting commentary in the April 6 update at http://www.askthepilot.com/ethiopian-737max-crash/.
Quote from that link...

The reason, many now believe, is a design quirk of the 737 — an idiosyncrasy that reveals itself in only the rarest of circumstances, and that few 737 pilots are aware of. When the plane’s stabilizers are acting to push the nose down, and the control column is simultaneously pulled aft, a sort of aerodynamic lockout forms: airflow forces on the stabilizers effectively paralyze them, making them impossible to move manually.
OMG
 

CWatters

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I was trying to think of a analogy. Best I could come up with is...

You're driving on the freeway and re-engage the cruise control. The car accelerates towards the preset speed. Suddenly a car pulls into your lane forcing you to brake. You slow down a bit but the brake pedal forces required seem very high, you're virtually standing on the pedal but not much is happening. In fact you are unable to depress the brake pedal any further because the controls have locked-out.

This is deemed acceptable by the regulators because..

a) it only happens rarely (eg when the car is accelerating you are trying to brake)
b) drivers are warned about it by an entry in the user manual. The solution is to release the brake pressure and start pumping the hand brake.

If you were a regulator would you find that acceptable?
 

FactChecker

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Quote from that link
When the plane’s stabilizers are acting to push the nose down, and the control column is simultaneously pulled aft, a sort of aerodynamic lockout forms: airflow forces on the stabilizers effectively paralyze them, making them impossible to move manually.


OMG
That sounds like the aerodynamic hinge moment is so large one way that the actuators do not have enough power to move stabilizers the other way. I thought that only happened at very great speeds, but the low altitude and dense air might cause that in a steep, fast, dive.

In those airplanes, does "manually" mean that there is no hydraulic assistance?
 
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Hello, just catching up on the latest replies. It's interesting to see the latest information coming out of the crashes. I think what we hypothesized early was fairly accurate. The memory items were not adequate to regain control of the aircraft. And as suggested, there is a software issue.

I can't help but think back to the beginning of this discussion where we discuss the need for a master cut out that essentially lets the pilot command absolute manual control of the airplane. Or direct law. This is exactly how we see the cruise control systems work in cars. It's interesting because with the tesla and the growing automation in these cars, we're seeing these same paradigms shift, so that the automated systems get more and more authority. Perhaps in 90% of use cases this isn't problematic and in fact beneficial statistically, but it's the other 10% when Murphy's law comes into play. Alas, often the impetus in engineering is to run before we can walk consistently and accurately. I think we're in this place with automation and AI.

It's always useful to step way way back when you're deep into a complex problem.

When we look at how MCAS operates on the trim, it seems to me it is over-stepping the bounds of what an automated system should control.

With autopilot, we have a computer working with essentially the yoke and throttle and commanding them to maintain altitude and heading etc... The limits to that system and how to circumvent it are very well understood by pilots. It's a tool for them that they control.

But with MCAS we're employing an automated system that is essentially, working "behind the scenes" to try to create a scenario or desired flight characteristics that is expected by the pilot. This is a big paradigm shift the way I see it. This is not like autopilot or the speed trim system. The pilot is not aware in the general situational sense, that any autonomous system is engaged and the ways to disengage this system are fundamentally different than autopilot. This override scenario is metaphorically akin to doing a sort of "airplane system emergency surgery" to disengage a fundamental system integral to how the plane flies as expected.

I think when they sketched up MCAS they didn't really have the objective, stepped back, forest for the trees approach to understand how this system is very different than anything they've implemented (taking the Boeing model of flight controls in account). It's really not like the speed trim system.

I think they might of thought of MCAS as augmenting the speed trim system. And so from that perspective, the MCAS is just the speed trim system going farther and farther down the direction of increased automation authority.

Anyway, wanted to add my thoughts. Really engrossing discussion here on aerodynamics, HCI and system design.
 
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So trim force has to be more powerful than control force so the pilot can use it to eliminate of all of the control force.
You're right. So the issue is that we have an automated system controlling that trim, and thus has tremendous pitch authority. And the communication and feedback to the pilot of that system, as well as how easily that system can be circumvented is problematic.
 
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In those airplanes, does "manually" mean that there is no hydraulic assistance?
No I don't think that's the case for Boeing aircraft in these scenarios. These planes are so big that hydraulic augmentation is fundamental to controlling the aircraft. Such that there is a redundant hydraulic system. A and B. If both systems fail the pilot is still linked in mechanically, such that the pilot would still be connected to the control surfaces. But that is not a very good scenario at all and the control forces would be unmanageable for anything but minor attitude adjustments.

Further, when you say "manual control", that would be a scenario for the pilot where they are in control of the airplane without the intervention of any autonomous systems and yes, it would be hydraulically assisted.
 
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Hello, just catching up on the latest replies. It's interesting to see the latest information coming out of the crashes. I think what we hypothesized early was fairly accurate. The memory items were not adequate to regain control of the aircraft. And as suggested, there is a software issue.

I can't help but think back to the beginning of this discussion where we discuss the need for a master cut out that essentially lets the pilot command absolute manual control of the airplane. Or direct law. This is exactly how we see the cruise control systems work in cars. It's interesting because with the tesla and the growing automation in these cars, we're seeing these same paradigms shift, so that the automated systems get more and more authority. Perhaps in 90% of use cases this isn't problematic and in fact beneficial statistically, but it's the other 10% when Murphy's law comes into play. Alas, often the impetus in engineering is to run before we can walk consistently and accurately. I think we're in this place with automation and AI.

It's always useful to step way way back when you're deep into a complex problem.

When we look at how MCAS operates on the trim, it seems to me it is over-stepping the bounds of what an automated system should control.

With autopilot, we have a computer working with essentially the yoke and throttle and commanding them to maintain altitude and heading etc... The limits to that system and how to circumvent it are very well understood by pilots. It's a tool for them that they control.

But with MCAS we're employing an automated system that is essentially, working "behind the scenes" to try to create a scenario or desired flight characteristics that is expected by the pilot. This is a big paradigm shift the way I see it. This is not like autopilot or the speed trim system. The pilot is not aware in the general situational sense, that any autonomous system is engaged and the ways to disengage this system are fundamentally different than autopilot. This override scenario is metaphorically akin to doing a sort of "airplane system emergency surgery" to disengage a fundamental system integral to how the plane flies as expected.

I think when they sketched up MCAS they didn't really have the objective, stepped back, forest for the trees approach to understand how this system is very different than anything they've implemented (taking the Boeing model of flight controls in account). It's really not like the speed trim system.

I think they might of thought of MCAS as augmenting the speed trim system. And so from that perspective, the MCAS is just the speed trim system going farther and farther down the direction of increased automation authority.

Anyway, wanted to add my thoughts. Really engrossing discussion here on aerodynamics, HCI and system design.

The first report on the crash reported the co pilot requested to trim stab cut out, was permitted.

That was well into the issue, inputting nose up trim via yoke, then as per mcas, nose down stab trim.

With stab trim off, co pilot tried manual trim. He said it wasn't working. Pilot concurs (did he try? why didn't both try at same time for more power?) Note Boeing's procedural fix is to get trim where you want it first (this is specifically for runaway mcas trim) using yoke inputs; then stab trim cut out. That's just to make it so there no need for a huge manual (with the trim wheels) trim adjustment.

Apparently pilots are (well) aware of the effects of airspeed, altitude ect on aerodynamics. Including that manual trim in such conditions can be very difficult and a valid option is a "relief trim" input and then manually adjust trim; if altitude permits of course.

Odd thing looks like stab trim was re-engaged after that. Likely to try a final nose up trim; followed by a Boeing "correction" designed to avoid the costs of a new type certificate hard nose down; kaput.

Grr....

all the info in here, except for my figure pointing, is from Juan Brown's "debriefing" of the accident from a pilot to the general public lol (his yt channel is called blancolirio)

Juan's debrief of the initial report
Note in that vid, he mentions a system that may operate the stabilizer trim even with the stab trim cut-off in the off position. Regarding a "mach trim". It was recorded that while the stab trim cut off was in off position that the stab trim move a bit (with no explanation on what moved it), Juan posits it could be a mach trim, given the speed of the craft at the time.

So such "behind the scenes" automation can work, it just needs to have an acceptable logic to it.
 
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Yes, it is relevant. Apparently, the engine position of the MAX changed and moved the CG so that the MCAS system was required for preventing a stall. There should be a mandated stability margin that would make a commercial airplane safe. The location of stored luggage is also a concern.
The relevancy may not be so obvious. Strong note; mcas is not there to prevent a stall; the plane does have pilots on board that are there to fly the plane. The pilots prevent stalls, implicitly.

The mcas was to maintain the flight characteristics of the plane as not to require a new type certificate. Simple as that.

Apparently the plane flies "just fine" without mcas; however would require all new pilot training....try selling that to the airlines.

So they half ass this to circumvent the actual flight characteristics. Using automation to make this different beast fly like the previous version the pilot is certified to fly.

The initial flight report doesn't sound like a "well behaved" airliner.
 

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