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Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

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Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#31

Post by PhantomGremlin »

toss1 wrote: post #22
blattimwind wrote: post #18

Earlier quoted context omitted.

The good thing with cars is that the time to avert a crash is generally very low (<1 second - <10 seconds), so while you're bending the supports of your seat pushing down on the brake pedal in panic, that state is not going to last very long.

Well also with a car, you can jut put it in Neutral and let the engine rev and throw a piston while you coast off to the side

I don't know if it works that way any more.

Nowadays with CVT or with PDK it seems to me that you're merely making a "suggestion" to put the transmission into Neutral. If the transmission software is borked, you're screwed?

But I'm not sure, maybe there is some fail safe override that occurs when you physically move the transmission control stalk into the neutral position?

I'd say that you could always turn the engine off. But Toyota made a mockery of that. Q: Who knew that you needed to press and hold the OFF button for a full five seconds before the car decided to comply? A: Not everyone knew --- people died!

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#32

Post by EdwardDiego »

sokoloff wrote: post #29
kart23 wrote: post #13

Earlier quoted context omitted.

So scary. It's like driving a car, trying to slow down, slamming on the brake, but it just keeps accelerating. Cant imagine everyone's feelings on those flights.

Though no doubt terrifying, the crew of ETH302 failed to reduce thrust; doing so may have allowed them to save the airplane.

Do you have something I can read on that "may"? I don't disagree that the throttles weren't moved, but the part about it maybe having saved them is new information.

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#33

Post by mannykannot »

mlyle wrote: post #16
cjbprime wrote: post #9

Earlier quoted context omitted.

I don't agree. I think the lift from the nacelles results in decreasing stick backpressure while the nose is being raised in some circumstances, which is both a handling issue and an airworthiness violation. It's an airworthiness violation because it exacerbates rather than discourages stalls. If the pilot continues pulling back on the stick with reduced backpressure the plane would stall, therefore MCAS is (indirect…

I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable". The stick force inversions you describe are just a symptom of divergent longitudinal stability. If you are trimmed stick-neutral for climb, deceleration doesn't cause sufficient nose drop, which in turn causes further deceleration and the aircraft does not re…

> I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable."

One should look at the actual requirement, not just the title, and for convenience, I have copied it below. As you can see, the issue in this requirement is not one of fundamental aerodynamic stability; this is a handling characterisics issue (and also, 14 CFR 25.175, which does require static lonitudinal stability, also requires more than that (a "stable slope" of the stick force curve in several different cases), and so can be failed by an airplane that is nevertheless statically longitudinally stable.)

14 CFR 25.173:

Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight.

(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.

(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#34

Post by toss1 »

PhantomGremlin wrote: post #31
toss1 wrote: post #22

Earlier quoted context omitted.

Well also with a car, you can jut put it in Neutral and let the engine rev and throw a piston while you coast off to the side

I don't know if it works that way any more. Nowadays with CVT or with PDK it seems to me that you're merely making a "suggestion" to put the transmission into Neutral. If the transmission software is borked, you're screwed? But I'm not sure, maybe there is some fail safe override that occurs when you physically move the transmission control stalk into the neutral position? I'd say that you could always turn the engin…

Agree, I'm not happy with the electronics just taking it as a suggestion to go to N or shut off engine while rolling.

Sure, it's got risks, but it is a key element of driver control. At least with my Mazda, I couldn't get a manual transmission, but the thing will go to N while rolling (haven't yet tried Off).

And yes, even with lots of experience, I'm not sure I'd figure out to hold the OFF button for 5 full seconds in a unintended acceleration scenario -- that's damn deadly!!

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#35

Post by chawco »

tus88 wrote: post #8

But presumably the pilots of both downed MAXs were pulling back as hard as possible.

Even more than that, you pick up speed due to the nose down configuration. This means the forces on the control surfaces are so great that you can no longer manually trim via the trim wheels. This was, apparently, a well known handling characteristic of the 737, with original versions describing a "roller coaster" maneuver where you dive briefly to relieve the forces on the control surfaces and incrementally trim. Unfortunately, because of how MCAS was set up, it was inactive while flaps were set, but kicked in immediately upon flaps up, meaning that they essentially had no altitude to work with.

See: https://www.youtube.com/watch?v=aoNOVlxJmow&t=1008s for a flight sim recreation of this situation on an NG.

Everything here seems to be that exceptional pilots can (and did) save the situation, but that it's not straightforward even with training.

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#36

Post by mlyle »

mannykannot wrote: post #33
mlyle wrote: post #16

Earlier quoted context omitted.

I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable". The stick force inversions you describe are just a symptom of divergent longitudinal stability. If you are trimmed stick-neutral for climb, deceleration doesn't cause sufficient nose drop, which in turn causes further deceleration and the aircraft does not re…

> I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable." One should look at the actual requirement, not just the title, and for convenience, I have copied it below. As you can see, the issue in this requirement is not one of fundamental aerodynamic stability; this is a handling characterisics issue (and also, 14…

I think it's hilarious that I discuss the test methods, cite the CFR, and mention the actual test results, and in turn you paste back the CFR to me that I cited. Thanks!

737's with smaller engines have very close to neutral pitch stability when at high AOA and high thrust. It takes 5ever to get back to trim speed. 737 MAX is even worse; depending on CG it can be divergent.

Gesticulating about stick forces and "handling" is obfuscation and misses the point. Yes, that's fucked up, too. The descriptions of allowing you to pull "harder" assume you're not trimmed for the climb and are holding a constant pull, which uh, some crews do, but autopilot won't (it'll trim off those forces), and lazy crews with weak arms like me won't either. :P

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#37

Post by gonzo »

toss1 wrote: post #22
blattimwind wrote: post #18

Earlier quoted context omitted.

The good thing with cars is that the time to avert a crash is generally very low (<1 second - <10 seconds), so while you're bending the supports of your seat pushing down on the brake pedal in panic, that state is not going to last very long.

Well also with a car, you can jut put it in Neutral and let the engine rev and throw a piston while you coast off to the side

You can turn off the key, too.

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#38

Post by sokoloff »

EdwardDiego wrote: post #32
sokoloff wrote: post #29

Earlier quoted context omitted.

Though no doubt terrifying, the crew of ETH302 failed to reduce thrust; doing so may have allowed them to save the airplane.

Do you have something I can read on that "may"? I don't disagree that the throttles weren't moved, but the part about it maybe having saved them is new information.

I'll give you the short explanation and then an article reference supporting. That might give you other terms to further research.

The trim is driven by a jackscrew. That jackscrew is driven by a mechanism in the cockpit (a large double wheel near each flight crew station). At low and moderate airspeeds, the jackscrew turns relatively easily. At high airspeeds and low elevator deflections (meaning "when the aircraft is approximately in trim"), the jackscrew turns relatively easily. At high airspeeds and high elevator deflections, the forces on the tail make the jackscrew difficult to turn due heavy forces. This would be the case where the aircraft is trimmed heavily nose down and the crew is holding large amounts of up elevator to compensate. (You're basically cranking the tail into a different position against the air loads on it.)

Slowing the airplane down would almost surely have helped the crew by reducing these loads. If you're going too fast (information the crew had available to them both in cockpit and confirmed by radio after they inquired while fighting), 94% N1 is not where you want the engines set.

It's not a popular or comfortable position to look into the actions of the crew and find areas for improvement in light of the fact that they were dealt a very difficult situation that morning.

Even with that, it appears that they correctly diagnosed the root of the issue, correctly disabled the electric trim [memory item on the checklist], for some reason left the engines at takeoff power [possibly just overlooked them, though overlooking engine power setting is like overlooking breathing to pilots], and when even knowing it was part of the problem, they correctly re-enabled the electric trim system in order to help them manage the situation, they then failed to command continuous nose up electric trim [which would have cut out MCAS and given them assistance in trimming the aircraft nose up]. The only reason to have the malfunctioning electric trim back on is to command nose up trim. Why turn it on, immediately command nose up, then stop and leave the faulty system engaged?

That crew was so close to saving the aircraft that it's frustrating as a pilot and a human. Slight adjustments to the above sequence and they're rightly heroes alongside Captains Sullenberger and Haynes.

https://www.reuters.com/article/us-ethiopia-airplane-procedu...

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#39

Post by mannykannot »

mlyle wrote: post #36
mannykannot wrote: post #33

Earlier quoted context omitted.

> I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable." One should look at the actual requirement, not just the title, and for convenience, I have copied it below. As you can see, the issue in this requirement is not one of fundamental aerodynamic stability; this is a handling characterisics issue (and also, 14…

I think it's hilarious that I discuss the test methods, cite the CFR, and mention the actual test results, and in turn you paste back the CFR to me that I cited. Thanks! 737's with smaller engines have very close to neutral pitch stability when at high AOA and high thrust. It takes 5ever to get back to trim speed. 737 MAX is even worse; depending on CG it can be divergent. Gesticulating about stick forces and "handli…

I am simply pointing out what can and cannot be deduced just from these CFRs. Evidence showing directly that the 737 MAX (without MCAS) was found to be divergent within the operational scope of the CFRs (which excludes, for example, CofG well behind the aft limit), would be more persuasive than these specific CFRs themselves.

Your comment about earlier 737s is informative, as the time taken to return to trim speed is not a parameter in the CFRs in question. Given that response time, could there have been any plausible doubt that the 737 MAX would require some additional form of stability augmentation? The story, as commonly told, is that MCAS was a response to an unanticipated problem, but now I am wondering if Boeing knew from the start that it would likely have to withold information in order to keep its 'no new training' pledge.

While we are on the topic of what the CFRs say, they also do not support the often-repeated claim that the purpose of MCAS is as a stall-prevention device, which is where this thread started.

Re: Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards

#40

Post by blattimwind »

PhantomGremlin wrote: post #31
toss1 wrote: post #22

Earlier quoted context omitted.

Well also with a car, you can jut put it in Neutral and let the engine rev and throw a piston while you coast off to the side

I don't know if it works that way any more. Nowadays with CVT or with PDK it seems to me that you're merely making a "suggestion" to put the transmission into Neutral. If the transmission software is borked, you're screwed? But I'm not sure, maybe there is some fail safe override that occurs when you physically move the transmission control stalk into the neutral position? I'd say that you could always turn the engin…

> Nowadays with CVT or with PDK it seems to me that you're merely making a "suggestion" to put the transmission into Neutral. If the transmission software is borked, you're screwed?

Nice thing about a standard transmission is that unless the linkage breaks you can always pull it out of gear, even under load.

> I'd say that you could always turn the engine off.

You can't necessarily turn a Diesel off [1], and I would not be surprised if modern very-high compression gasoline engines are able to pull the same runaway off.

[1] Big Diesel generators have countermeasures for runaway, but cars don't.

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