7 ms·

737: The Max Mess

35 points by breadbox 8y ago

PaulHoule 8y ago
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.
- kevin_thibedeau 8y ago
  The aircraft is also uncontrollable when this system is disabled unless you're a crack Boeing test pilot.
  - cmurf 8y ago
    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.
    - tyingq 8y ago
      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."
      - cmurf 8y ago
        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.
        mike_john23 8y ago
        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-flight.png 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
        cmurf 8y ago
        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.
        cjbprime 7y ago
        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?
    - kevin_thibedeau 8y ago
      Egypt air pilots performed the recovery procedure and still crashed. Pretty damning evidence that it can't fly safely with MCAS off.
      - cmurf 7y ago
        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.
        kevin_thibedeau 7y ago
        The procedure includes manually correcting the trim. It didn't work.
- cmurf 8y ago
  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.
gonvaled 8y ago
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.
- mikeash 8y ago
  I don’t remember that, can you show some examples?
  - gonvaled 8y ago
    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 https://news.ycombinator.com/item?id=19356138
    - toufiqbarhamov 8y ago
      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.
      - gonvaled 8y ago
        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.
jayess 8y ago
> 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?
- inamberclad 8y ago
  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.
  - cjbprime 8y ago
    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.
- cornellwright 8y ago
  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.
huslage 8y ago
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.
- cmurf 8y ago
  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.
cjbprime 8y ago
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.

35 points by breadbox 8y ago

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.

kevin_thibedeau 8y ago
The aircraft is also uncontrollable when this system is disabled unless you're a crack Boeing test pilot.
- cmurf 8y ago
  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.
  - tyingq 8y ago
    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."
    - cmurf 8y ago
      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.
      - mike_john23 8y ago
        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-flight.png 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
        cmurf 8y ago
        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.
        cjbprime 7y ago
        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?
  - kevin_thibedeau 8y ago
    Egypt air pilots performed the recovery procedure and still crashed. Pretty damning evidence that it can't fly safely with MCAS off.
    - cmurf 7y ago
      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.
      - kevin_thibedeau 7y ago
        The procedure includes manually correcting the trim. It didn't work.
cmurf 8y ago
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.

gonvaled 8y ago

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.

mikeash 8y ago
I don’t remember that, can you show some examples?
- gonvaled 8y ago
  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 https://news.ycombinator.com/item?id=19356138
  - toufiqbarhamov 8y ago
    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.
    - gonvaled 8y ago
      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.

jayess 8y ago

> 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?

inamberclad 8y ago
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.
- cjbprime 8y ago
  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.
cornellwright 8y ago
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.

huslage 8y ago

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.

cmurf 8y ago
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.

cjbprime 8y ago

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.