737: The Max Mess | Hacker News Reader

737: The Max Mess

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35 pointsbreadbox44 comments

A comparison to the A320 is in order.

The A320 (as well as modern Boeing Airliners like the 777 and 787) has a comprehensive system of "flight envelope protection" which automatically acts to keep things like a pitch runaway from happening.

In a modern aircraft, flight envelope protection is closely integrated with the fly-by-wire system in normal operation. If sensors are degraded, the system reverts to a "control rule" which has less protection.

In the case of the 737 they retrofitted a partial flight envelope protection system onto an old aircraft. Thus it wasn't properly designed, tested, and pilots not trained how to use it.

The aircraft is also uncontrollable when this system is disabled unless you're a crack Boeing test pilot.

Citation? This is a very significant claim if true, it would be a clear violation of FAR 25.173. I don't see how a plane is certified to be airworthy by software work around, in particular one that can be disabled instantly rendering the plane not airworthy.

Not exactly as strong as "crack test pilot", but from the article:

"after a few minutes of wrestling with the control yoke, the pilots on Flight 302 did invoke the checklist procedure, and moved the STAB TRIM switches to CUTOUT. The stabilizer then stopped responding to MCAS nose-down commands, but the pilots were unable to regain control of the airplane."

That is due to the mistrim situation that a perturbed MCAS had already put the plane into. Proposing that without MCAS (from the outset) the aircraft is inherently unstable is what I'm questioning.

You can see the previous flight made by the same lion air plane that crashed here[1]. They not only did not declare an emergency but they contained the flight as normal and flew for over an hour with no MCAS and no electric trim.

[1]: https://leehamnews.com/wp-content/uploads/2018/11/Previous-f...

I should point out they found the stab cut out procedure by trial and error and were able to maintain control of the plane simply by counteracting the errant downward trim. The reason the Ethiopian plane crashed is that they didn't execute the airspeed disagree procedure and left the throttles on basically maximum.

Yes MCAS is an issue, But since no one on this site has gotten this right I might as well point that out

On what basis do they follow the IAS disagree procedure, and not the procedure in emergency AD 2018-23-51?

KNKT's preliminary report says the prior flight, Lion Air 34, was not airworthy due to continuous stick shaker activation. Further, that flight expressly requested a much lower altitude, citing control problems for that request. Neither are normal.

You're saying that IAS disagree and unreliable airspeed ought to have caused them to throttle down? That seems counter-intuitive. If you don't know how fast you're going, how are you supposed to avoid stalling?

Are you looking at a specific QRH entry? What does it say?

Egypt air pilots performed the recovery procedure and still crashed. Pretty damning evidence that it can't fly safely with MCAS off.

Post hoc ergo propter hoc is a logical fallacy. Very basic.

MCAS caused the extreme mistrim that resulted in the crash. Turning it off does not solve the mistrim.

The procedure includes manually correcting the trim. It didn't work.

I think it's generous to consider MCAS a variety of "flight envelope protection" considering in the working case it causes a MAX to behave unlike an NG in the same situation. In the perturbed case it's a likely saboteur.

Air France was criticized for not having high altitude training for the “vol avec IAS douteuse” procedure, in the Air France 447 final report. Pilots need to know all behaviors, natural and artificial, of an airplane.

I am very curious to what degree airlines knew of MCAS, and whether they instigated or were part of the conspiracy to withhold necessary training from pilots in order to avoid different a type certification for the 737 MAX, and thereby avoid pilots needing a type rating for it. The decision to deny training for MCAS is 180 degrees from the lesson that should have been learned from AF 447.

> By the time of the Ethiopian crash, 737 pilots everywhere knew all about MCAS and the procedure for disabling it. A preliminary report issued last week by Ethiopian Airlines indicates that after a few minutes of wrestling with the control yoke, the pilots on Flight 302 did invoke the checklist procedure, and moved the STAB TRIM switches to CUTOUT. The stabilizer then stopped responding to MCAS nose-down commands, but the pilots were unable to regain control of the airplane.

> It’s not entirely clear why they failed or what was going on in the cockpit in those last minutes. One factor may be that the cutout switch disables not only automatic pitch trim movements but also manual ones requested through the buttons on the control yoke. The switch cuts all power to the electric motor that moves the stabilizer. In this situation the only way to adjust the trim is to turn the hand crank wheels near the pilots’ knees.

Wow, so disabling MCAS also disables all stabilizer commands?

No, it says that moving the pitch trim switch to cutout disabled the trim buttons on the yoke. They should have still had manual pitch control the good old fashioned way (with the primary control inputs through the yoke), as well as the manual pitch trim wheels on either side of the throttle quadrant. However, it appears that due to an existing issue with the 737 design, they could not use the manual trim wheels for some reason while they were holding the yoke all the way back. That's a very bad situation to be in, as controlling an airplane that's severely out of trim can require huge amounts of force on the control inputs and could have just exhausted the pilots.

They don't have manual pitch control through the yoke (elevator) when the stabilizer is severely mistrimmed because the stabilizer is "stronger" than the elevator -- it's a much larger surface, the entire horizonal stabilizer moves. Yoke movement doesn't move the stabilizer.

The stabilizer trim switches enable/disable the electric stabilizer trim. There's a wheel with a foldout hand crank to manually control the stabilizer trim. Based on the preliminary report, the copilot was unable to turn the wheel which is why they turned the electric stabilizer trim back on.

One hypothesis is that because the pilot was fighting the extreme nose down stabilizer with nose up elevator the jackscrew that moves the stabilizer was so heavily loaded that the co-pilot didn't have enough strength to manually turn the trim wheel which is connected to the jackscrew by cables.

Another theory is that it was possible, but requires both pilots to turn their cranks together -- not possible if one is pulling back on their yoke and unable to let go without that being deadly.

My understanding is that the horizontal stabilizer position is determined by the sum of the yolk ("steering wheel") position and the trim wheel position. MCAS might actually spin the trim wheel. In any case, MCAS trimmed the plane way out of whack, such that even with the yolk all the way back, it wasn't flyable. The only way to fix the trim was to turn back on the electric motors on the trim system (including its MCAS subsystem) or else crank the trim wheel like mad by hand and hope their hands are fast enough to get the plane sanely trimmed before the plane lost too much altitude.

Edit: I see inamberclad mentioned an additional detail that the trim wheels get stuck when the yolk is pulled all the way back. I wasn't aware of this, but it makes the pilots' actions make much more sense.

NB: "yoke", not "yolk".

That doesn't sound right. It sounds like it might be correct on an Airbus, but not a Boeing. On a 737, yoke movement mechanically controls the elevator and the trim system mechanically (and with a motor to help) controls the stabilizer. There is no connection between the yoke and the stabilizer on a Boeing. Does that make sense?

It's not just that the trim wheels don't work when the yoke (not yolk) is pulled, but that it can require too much torque to move them -- it's a purely mechanical connection with the motor off -- when the elevator is deflected in opposition to the stabilizer.

Yes, all electric ones. The pilots have a purely mechanical method for moving the stabilizer but it seems like it wasn't possible in this particular situation, perhaps for aerodynamic reasons.

Great article! The only thing missing is a discussion of whether manual retrim after disabling the stabilizer trim motor was actually possible given the aerodynamic constraints. This theory's been discussed quite a bit here on HN and pilot forums.

The theory is that, on the Ethiopian flight, after disabling the stabilizer trim motor while there is mistrim, the aerodynamic load on the jackscrew was too great for it be movable by the copilot using their crank. The pilot was perhaps using their strength to pull the yoke back, which meant both that he was unavailable to help crank, and that the aerodynamic load was increased by the elevator directing airflow in opposition to the stabilizer.

There is an old “yo-yo maneuver” that stopped being mentioned in Boeing manuals decades ago that describes having to relieve load on the stabilizer before manually trimming, in this case by releasing the elevator and pushing the nose down even further, which I’m sure would have been very unattractive given their high airspeed and low altitude if the pilots even understood the procedure, which is no longer part of simulator training.

This may explain why the trim motor appears to have been re-enabled towards the end of the Ethiopian flight: because purely manual trim was impossible.

This is dangerous conjecture at best. I'm a little sick of reading that MCAS shouldn't exist.

MCAS must exist.

The flight regime where MCAS would need to act should be exceedingly rare. The bug isn't that MCAS exists. The bug is that there is a failure in the code/inputs to the system which allow it to trigger in regimes where it shouldn't.

Are you asserting that MCAS must exist as a condition of its certification? And that without it the airplane is not airworthy?

Can you reference any checklist item that when followed causes the airplane to become unairworthy?

If MCAS is required for the airplane to be airworthy, following the FAA and Boeing's directive to set stab trim to cutoff in the specific case of "MCAS upset" under discussion, directs pilots to make the airplane unairworthy as a work around for a perturbed system. I don't see how this is a defendable regime.

It took China for once having an independent policy instead of "impartial" agencies pushing their agenda onto a defenseless world.

May I remember that most HN jumped to defend Boeing and the FAA, without a shred of evidence?

I welcome our Chinese overlords.

Competition is good.

I don’t remember that, can you show some examples?

This is the story [1].

Without a truly independent power standing strong, we would still be flying the Max.

[1] https://news.ycombinator.com/item?id=19356138

The first three top-level comments all laud the decision. That would appear to be the opposite of your central claim that May I remember that most HN jumped to defend Boeing and the FAA, without a shred of evidence?.

Maybe you would be able to make a better point if you dropped the jingoism and histrionics. It can also be helpful to not lump the tens of thousands of people here into one bin of “HN did x” when trying to make that point.

My central claim is this:

> It took China for once having an independent policy instead of "impartial" agencies pushing their agenda onto a defenseless world.

And I stand by my comment about HN: I clearly remember the bias on the discussion. The top comments are top comments once the story settles. And the replies to those comments show a different picture.

> Maybe you would be able to make a better point if you dropped the jingoism and histrionics.

There is something worse than being slightly offensive: being biased and pushing damaging narratives onto the rest of us.

Let's notice that the truth has come out because a Max crash would also affect americans. Whenever interests are not so nicely alligned, we in the rest of the world end up paying the price.

You keep raising the issue of bias, but the one piece of supporting “evidence” for that bias doesn’t support your claim. You brush that off by saying it changed “after the story settled” as if that meant something.

I’m thinking that a mirror might help you with your bias concerns.

You keep adressing my secondary point. The one that seems to personally hurt you.

Stories evolve. Top voted comments change. The replies to those are also part of the discussion, and some are highly voted too.

My top comment in this discussion has started being downvoted. This might change, but it will still be the case that a big part of the HN audience disagrees with me.

You keep adressing my secondary point. The one that seems to personally hurt you.

Here’s an alternative theory... I didn’t take it personally because I didn’t take the stance you’re criticizing. Now you’re left in the untenable position of assuming I was offended on the behalf of some strangers, or what bothers me is unrelated to “hurtfulness” and has to do with something else.

My top comment in this discussion has started being downvoted. This might change, but it will still be the case that a big part of the HN audience disagrees with me.

Again, perspective is your friend. Maybe people disagree with you, maybe they disagree with how you’re saying what you’re saying, or maybe they agree with parts, but not the whole. This is yet another illustration of the perils in treating s diverse community as a monolith. For example my problem with your original comment was the signal:noise ratio, best illustrated by the complete innacuracy of how you characterized the reaction of so many people. Your latter day hand-waving about “it changed over time” is more of the same.

I looked at every top-level comment on that story and didn’t see a single one defending Boeing or the FAA.

See my reply to your sibling

I don’t see how that addresses what I said. I didn’t just look at the most upvoted comments. I looked at all of the top-level comments. Not one of them was as you said.

See

https://en.wikipedia.org/wiki/Comac_C919

which is another boring narrowbody which is not technically ambitious at all and could only compete with the long-in-the-tooth 737 and the slightly newer A320.

I'd like to see somebody shake up the market with a plane that is small on the outside and big on the inside like

https://en.wikipedia.org/wiki/Embraer_E-Jet_family

but you won't see it used heavily in the U.S. because of pilot union scope rules.

I meant competition against the FAA, not against boeing.

What do you think of https://en.wikipedia.org/wiki/Irkut_MC-21 ?

It's not about China being any "smarter* than anyone else.

Here's the reality, the consequences, both financial and otherwise, for being wrong in China are a good deal more deleterious. (Ever lose that kind of a job for incompetence in China? You won't be getting another one.) So, not surprisingly, Chinese airlines, experts and regulators will more readily err on the side of caution. Simple as that. It's just basic human psychology.

By comparison, there are no consequences at all if you or I are incorrect in some assertion we make on Hacker News. Certainly no lasting repercussions of any great concern.

So making that comparison as you did is a bit non-sensical. Of course experts do things differently than some randoms on HN.

I am not saying they are smarter. I am saying they are getting big enough that they can face the bully.

I misinterpreted your post. I apologize.

That could have been more clearly and nonpejoratively articulated.

I fail to see where I was pejorative. I was confrontational, yes.

You are right on the clarity department.

Characterisation of "most HN" and "Chinese overlords".

Your comment would read better absent both comments.

The notion of China being able to respond independently, with agency, is valid.

Reply on news.ycombinator.com