Before 737 Max, Boeing’s Flight-Control System Included Key Safeguards
wsj.com
wsj.com
The key quote from there:
Notably, pilots of the KC-46 are able to override the MCAS—or Maneuvering Characteristics Augmentation System—simply by pulling on the controls, the Journal reports. It quotes an Air Force procurement chief as saying, “We have better sensor data. But most importantly, when the pilot grabs the stick, the pilot is completely in control.”
A better way to write the article would be "Boeing and the airlines have given up on trying to do MCAS without telling pilots anything about it."
MCAS is there mostly because of certification requirements for hands-off stability. It is not a mechanism to prevent the pilot from stalling the aircraft.
I don’t believe this is correct. During testing the MAX was discovered to have an inverted force curve on the stick approaching a stall — as the aircraft approaches stall angle, forces flip around and it becomes easier to pull the stick back (into the stall) than forward (out of the stall angle).
This violates a fundamental airworthiness requirement — commercial aircraft cannot be certified as airworthy if the stick forces invert.
MCAS “solves” this by commanding the stabilizers down as the airplane approaches stall — effectively using the stabilizer to put the forces on the stick that are “missing” due to the MAX’s aerodynamics, ensuring the force curve never inverts.
MCAS is entirely about generating force on the stick to meet airworthiness regs, not hands-off stability. It actually can’t disengage when the pilot pulls back on the stick, as that would defeat its entire purpose re: the airworthiness force-on-stick requirement.
The WSJ kind of muddled things here. The KC-46A does mid-air refueling, and its semi-similar MCAS system is about hands-off stability — compensating automatically as the weight and balance of the plane dynamically changes as fuel is offloaded. The pilot can override it with the stick because its purpose isn’t to provide stick force.
It's hard to know what the real issue is, because Boeing isn't forthcoming about it. But your claim that the airframe is fundamentally aerodynamically unstable ("needed for hands-off stability") is a very extreme claim that I don't think is supported.
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 return to the trim speed.
Both test methods are described in 25.175 and were problematic for the 737 MAX.
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.
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
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.
>As the speed shot through 450 knots, the pilots hauled back on their control columns to no effect. Six minutes after takeoff, the airplane hit the ground doing approximately 600 miles an hour. It buried itself into a 30-foot-deep crater in farmland about 32 miles southeast of the airport. Within a week, the Boeing 737 Max was grounded worldwide. (Ethiopian Airlines Flight 302) From: https://www.nytimes.com/2019/09/18/magazine/boeing-737-max-c...
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!
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.
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!!
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...
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.
https://news.ycombinator.com/item?id=10178989
https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...
> But please don't post complaints about paywalls. Those are off topic.
> It's ok to post stories from sites with paywalls that have workarounds.
Where's the workaround? I don't find workarounds for WSJ. WSJ is locked out. Please post the workaround, or kindly stop badgering people as it is not productive.
If there is no workaround that allows us to view WSJ articles, then WSJ article are NOT allowed here, per the rules you yourself selectively cited.
The rule about paywalls are that paywalled articles must come with a workaround, and that people should not post complaints about paywalls. His post did not complain about paywalls themselves nor does mine. He requests that paywalled articles be tagged as such, a much more relaxed rule than the official rules which are that paywalled articles without workarounds, such as WSJ articles (unless someone would like to post a legal workaround allowing us to view WSJ articles), are not allowed.
That is asking.
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