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Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

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Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#151

Earlier quoted context omitted.

> The MCAS is designed to prevent an aerodynamic stall [...] 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 Boe…

There seem to be a handful of people spreading this talking point here. In every authoritative source, MCAS is described as anti-stall system.

Define authoritative source.

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#152

Earlier quoted context omitted.

There seem to be a handful of people spreading this talking point here. In every authoritative source, MCAS is described as anti-stall system.

I believe the critical passage is here in CFR 14 §25.175¹ (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…

> The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak.

How do you know this? Haven't seen that reported anywhere but in user comments here.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#153

Earlier quoted context omitted.

There seem to be a handful of people spreading this talking point here. In every authoritative source, MCAS is described as anti-stall system.

Define authoritative source. 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).…

> MCAS is a compensatory system that is meant to allow the plane to be certified airworthy

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#154

Earlier quoted context omitted.

This is my understanding. 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…

Yeah. I'm also not clear on the practicality of retrimming the aircraft at low altitude (Ethiopian was never >1000ft above ground) before impacting terrain after you disable the trim motor. You have to use a hand crank, because you just killed the trim motor altogether, and I think it takes several minutes (!) of cranking to get the jackscrew from one end to the other. I wonder if they killed the trim motor and then…

Fatigue as well. Looking at the flight track after the last and final nose down, it's they were overcome. Nose down -> more speed -> more stabilizer nose down force -> more nose down attitude even without a change in stabilizer -> more speed -> more stabilizer force -> more nose down attitude.

And then the ground got in the way.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#155

Earlier quoted context omitted.

Define authoritative source. 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).…

> MCAS is a compensatory system that is meant to allow the plane to be certified airworthy 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.

From an understanding of control systems.

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#156
post #96

Earlier quoted context omitted.

Even if it has enough sensprs, the flight software has no situational awareness. 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 p…

Not sure about the "able to do so since the 80s" part, but generally, I agree. 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…

The USSR were flying their Buran space shuttle in the 80s with full autonomous capabilities.

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#157
post #134

> "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" It blows my mind that anyone still talks like this. My first job was writing (non-life-critical) scientific software for detail-obsessed university researchers working in a lab with notebooks and procedures. If "just solve it with training" could work for an…

Not only that, but the pilots received no training, to uphold the fiction that the 737 MAX flies the same as the old 737.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#158

Earlier quoted context omitted.

> MCAS is a compensatory system that is meant to allow the plane to be certified airworthy 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.

From an understanding of control systems. 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…

Given that the MCAS operates in intervals and does not provide constant input, it doesn't really stand that it's similar to a stick pusher as far as maintaining constant response. It seems as though it will make an evaluation and push the nose down regardless of how much stick input there is.

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#159

Earlier quoted context omitted.

From an understanding of control systems. 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…

Given that the MCAS operates in intervals and does not provide constant input, it doesn't really stand that it's similar to a stick pusher as far as maintaining constant response. It seems as though it will make an evaluation and push the nose down regardless of how much stick input there is. If the MCAS was constantly variable, like a mechanical stick pusher, then what you're saying would make sense. IMO, as reporte…

That's what is so insane about this system. It was put in to make the thing act like a 737 within a very small part of the flight envelope. It never should have kicked in in normal flight.

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.

Re: Lack of redundancies on Boeing 737 MAX baffles some involved in developing it

#160
post #116
post #108

Earlier quoted context omitted.

Automation is definitely not the enemy. 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.

Have fun using any product. If your company gets big enough the probability that one of its products was faulty because of negligence slowly approaches 100%. 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…

AFAIK the automation had no good reason to be there first place, unless you consider racing Airbus for contracts a good reason. I mean, the simpler the better especially when lives are at stake, right?
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