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Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car: if the computer commands acceleration, and the driver steps on the brake, cruise control disengages. The necessity of MCAS is to meet regulatory requirements for similarity in handling characteristics to achieve a common type rating. It's not strictly an engineering requirement, nor is it a safety-assisting technology. It's an attempt to mask the actual aerodynamic characteristics of the aircraft being flown. That is a conflicting mission if you will.

wp381640 8y ago
> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time
- oflannabhra 8y ago
  Yep, and on top of that the 737 MAX did not require retraining for pilots (cost savings for Boeing and airlines) and Boeing did not provide documentation to pilots that MCAS would re-engage after a short timeout. Pilots use the term "ahead of the plane" to describe being in a state where they understand exactly how a plane and its systems will respond to inputs (including theirs). MCAS (and lack of training) resulted in pilots being "behind the plane", which meant they could not regain control over the plane and ultimately crashed.
- briandear 8y ago
  They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for. Here’s some discussion about this from 2009: https://www.airliners.net/forum/viewtopic.php?t=761011 https://www.airliners.net/forum/viewtopic.php?t=761011 Even if the cause is an MCAS failure, the way that failure presents in the cockpit follows the same procedures as runaway trim: which means to me that the Ethiopian and Lion Air pilots were simply under-trained. The 200 total hours of the Ethiopian first officer supports that assertion. In the US, a first officer has a 1500 hour requirement: having a 200 hour pilot in the cockpit can’t be ignored as a contributing cause especially since following the standard checklists would have saved the plane— if the first officer would have known to consult those checklists. In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737.
  - amanaplanacanal 8y ago
    > A system malfunction should appear to a pilot the same way a runaway trim wheel appears. I thought the problem here was that it doesn't appear the same.
    - crooked-v 8y ago
      From what I understand, runaway trim looks like a continuous change, where runaway MCAS is intermittent changes every 5 seconds with longer delays if a pilot just touched the throttle trim controls.
  - cjbprime 8y ago
    > In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737. To be fair, it's not clear that the time someone would spend flying a Cessna between their 200-1500 hours in the US is actually helping them become a better 737 first officer..
  - anthonybsd 8y ago
    https://www.nytimes.com/2019/04/04/world/asia/ethiopia-crash-boeing.html https://www.nytimes.com/2019/04/04/world/asia/ethiopia-crash... Ah yes implying those weird "African" airlines cannot compare to the supreme training of the US ones. You closet racism is pretty self evident.
prolepunk 8y ago
From what I understand is that the comparison between cruise control and MCAS breaks down when it comes to how quickly the potential catastrophic situation may happen. When driver steps on the brakes -- the system disengages and car slows down. When pilot disables MCAS -- the system disengages and the plane is more likely to enter aerodynamic stall and crash. So with cruise control there is no potential for catastrophic positive feedback loop when the system disengages, whereas MCAS needs to be pretty aggressive in what it does.
losvedir 8y ago
> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first officers took over, who allowed it to get to full nose down trim. A constantly incorrectly trimming system is supposed to trigger the runaway stabilizer trim checklist, so Boeing thought a failure in this system was covered by existing training. Unfortunately, a periodic every-5-seconds trim wasn't recognized as such. I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant?
- lisper 8y ago
  > I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant? Yes, exactly. The problem (well, one of the problems) is that during normal operations the trim wheels are constantly moving intermittently, so additional intermittent motion isn't immediately seen as unusual. (Disclaimer: I am not a 737 pilot either, but I am a pilot, I fly a plane with an electric trim (Cirrus SR22) and the info above came from a very experienced 737 pilot named Juan Browne who hosts an excellent channel on YouTube called Blancolirio.)
- cameldrv 8y ago
  The problem is: 1. MCAS failure produces an intermittent stab trim movement. 2. Boeing claimed that the stab trim runaway checklist should have been run and would have let the crew recover. 3. Boeing's checklist only calls for action on "continuous" uncommanded stab trim movement https://www.satcom.guru/2019/03/taking-next-steps-while-awaiting-on.html https://www.satcom.guru/2019/03/taking-next-steps-while-awai... (second image) 4. Intermittent uncommanded stab trim movement is normal. The Speed Trim System and Elevator Feel Shift system regularly adjust trim, so merely seeing the wheels move by themselves is not cause for alarm. 5. MCAS is not documented at all in the flight manual, and the stab trim runaway checklist was not updated to address how MCAS might cause a failure necessitating its use, presumably to avoid the need for additional transition training. 6. MCAS is unreliable due to its reliance on a single AoA sensor.
mhandley 8y ago
I think that would open up a different safety issue. Suppose you're in a tight turn for some reason, pulling back hard on the yoke, as you have to do in a tight turn. The aircraft is at fairly high angle of attack, but not yet quite enough to enable MCAS. You've got quite a high column load, and you blip the trim upwards to relieve it a bit. If you blip the trim just as MCAS kicks in, and it disables, then you're now flying at high AoA without MCAS protection. You've not trained for that, and your chances of an accelerated stall just went up significantly as the plane no longer flies quite like you trained.
cmurf 8y ago
Pilot pulling back on the control column does not disengage the autopilot; pilot changing throttle lever settings doesn't disengage the autothrottle. And also, these things can have different behaviors in different modes (or laws, referring to airplane computer control not legal), which vary from make and model of aircraft hence part of the reason for type ratings. The problem with articles thus far: there's a lot of confusion whether MCAS exists to meet a FAR 25 requirement (aircraft airworthiness for transport category aircraft)[1], or FAR 61 requirement (pilot certification and ratings)[2]. I still don't know the answer to that, but either way it seems problematic to have an ostensibly required function that can be disabled and either have an instantly decertified (not airworthy) airplane, or an instantly decertified (not type rated) pilot, or both. [1] There's are many behavioral requirements in FAR 25, I'm cherry picking two: § 25.203 Stall characteristics. (my emphasis on paragraph a.) https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203 § 25.173 Static longitudinal stability. (my emphasis on paragraph c. and d.) https://www.law.cornell.edu/cfr/text/14/25.173 https://www.law.cornell.edu/cfr/text/14/25.173 [2] The pilot must have a type rating, 61.31 a(3) for aircraft the FAA says must have a type certificate https://www.law.cornell.edu/cfr/text/14/61.31 https://www.law.cornell.edu/cfr/text/14/61.31 which then points to this section which has all kinds of ins and outs https://www.law.cornell.edu/cfr/text/14/part-21/subpart-B https://www.law.cornell.edu/cfr/text/14/part-21/subpart-B
cameldrv 8y ago
This is incorrect. MCAS is required to meet the stability requirements of FAR 25 in addition to being required for the common type rating.

wp381640 8y ago

> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time

oflannabhra 8y ago
Yep, and on top of that the 737 MAX did not require retraining for pilots (cost savings for Boeing and airlines) and Boeing did not provide documentation to pilots that MCAS would re-engage after a short timeout. Pilots use the term "ahead of the plane" to describe being in a state where they understand exactly how a plane and its systems will respond to inputs (including theirs). MCAS (and lack of training) resulted in pilots being "behind the plane", which meant they could not regain control over the plane and ultimately crashed.
briandear 8y ago
They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for. Here’s some discussion about this from 2009: https://www.airliners.net/forum/viewtopic.php?t=761011 https://www.airliners.net/forum/viewtopic.php?t=761011 Even if the cause is an MCAS failure, the way that failure presents in the cockpit follows the same procedures as runaway trim: which means to me that the Ethiopian and Lion Air pilots were simply under-trained. The 200 total hours of the Ethiopian first officer supports that assertion. In the US, a first officer has a 1500 hour requirement: having a 200 hour pilot in the cockpit can’t be ignored as a contributing cause especially since following the standard checklists would have saved the plane— if the first officer would have known to consult those checklists. In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737.
- amanaplanacanal 8y ago
  > A system malfunction should appear to a pilot the same way a runaway trim wheel appears. I thought the problem here was that it doesn't appear the same.
  - crooked-v 8y ago
    From what I understand, runaway trim looks like a continuous change, where runaway MCAS is intermittent changes every 5 seconds with longer delays if a pilot just touched the throttle trim controls.
- cjbprime 8y ago
  > In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737. To be fair, it's not clear that the time someone would spend flying a Cessna between their 200-1500 hours in the US is actually helping them become a better 737 first officer..
- anthonybsd 8y ago
  https://www.nytimes.com/2019/04/04/world/asia/ethiopia-crash-boeing.html https://www.nytimes.com/2019/04/04/world/asia/ethiopia-crash... Ah yes implying those weird "African" airlines cannot compare to the supreme training of the US ones. You closet racism is pretty self evident.

prolepunk 8y ago

From what I understand is that the comparison between cruise control and MCAS breaks down when it comes to how quickly the potential catastrophic situation may happen. When driver steps on the brakes -- the system disengages and car slows down. When pilot disables MCAS -- the system disengages and the plane is more likely to enter aerodynamic stall and crash. So with cruise control there is no potential for catastrophic positive feedback loop when the system disengages, whereas MCAS needs to be pretty aggressive in what it does.

losvedir 8y ago

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first officers took over, who allowed it to get to full nose down trim. A constantly incorrectly trimming system is supposed to trigger the runaway stabilizer trim checklist, so Boeing thought a failure in this system was covered by existing training. Unfortunately, a periodic every-5-seconds trim wasn't recognized as such. I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant?

lisper 8y ago
> I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant? Yes, exactly. The problem (well, one of the problems) is that during normal operations the trim wheels are constantly moving intermittently, so additional intermittent motion isn't immediately seen as unusual. (Disclaimer: I am not a 737 pilot either, but I am a pilot, I fly a plane with an electric trim (Cirrus SR22) and the info above came from a very experienced 737 pilot named Juan Browne who hosts an excellent channel on YouTube called Blancolirio.)
cameldrv 8y ago
The problem is: 1. MCAS failure produces an intermittent stab trim movement. 2. Boeing claimed that the stab trim runaway checklist should have been run and would have let the crew recover. 3. Boeing's checklist only calls for action on "continuous" uncommanded stab trim movement https://www.satcom.guru/2019/03/taking-next-steps-while-awaiting-on.html https://www.satcom.guru/2019/03/taking-next-steps-while-awai... (second image) 4. Intermittent uncommanded stab trim movement is normal. The Speed Trim System and Elevator Feel Shift system regularly adjust trim, so merely seeing the wheels move by themselves is not cause for alarm. 5. MCAS is not documented at all in the flight manual, and the stab trim runaway checklist was not updated to address how MCAS might cause a failure necessitating its use, presumably to avoid the need for additional transition training. 6. MCAS is unreliable due to its reliance on a single AoA sensor.

mhandley 8y ago

I think that would open up a different safety issue. Suppose you're in a tight turn for some reason, pulling back hard on the yoke, as you have to do in a tight turn. The aircraft is at fairly high angle of attack, but not yet quite enough to enable MCAS. You've got quite a high column load, and you blip the trim upwards to relieve it a bit. If you blip the trim just as MCAS kicks in, and it disables, then you're now flying at high AoA without MCAS protection. You've not trained for that, and your chances of an accelerated stall just went up significantly as the plane no longer flies quite like you trained.

cmurf 8y ago

Pilot pulling back on the control column does not disengage the autopilot; pilot changing throttle lever settings doesn't disengage the autothrottle. And also, these things can have different behaviors in different modes (or laws, referring to airplane computer control not legal), which vary from make and model of aircraft hence part of the reason for type ratings. The problem with articles thus far: there's a lot of confusion whether MCAS exists to meet a FAR 25 requirement (aircraft airworthiness for transport category aircraft)[1], or FAR 61 requirement (pilot certification and ratings)[2]. I still don't know the answer to that, but either way it seems problematic to have an ostensibly required function that can be disabled and either have an instantly decertified (not airworthy) airplane, or an instantly decertified (not type rated) pilot, or both. [1] There's are many behavioral requirements in FAR 25, I'm cherry picking two: § 25.203 Stall characteristics. (my emphasis on paragraph a.) https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203 § 25.173 Static longitudinal stability. (my emphasis on paragraph c. and d.) https://www.law.cornell.edu/cfr/text/14/25.173 https://www.law.cornell.edu/cfr/text/14/25.173 [2] The pilot must have a type rating, 61.31 a(3) for aircraft the FAA says must have a type certificate https://www.law.cornell.edu/cfr/text/14/61.31 https://www.law.cornell.edu/cfr/text/14/61.31 which then points to this section which has all kinds of ins and outs https://www.law.cornell.edu/cfr/text/14/part-21/subpart-B https://www.law.cornell.edu/cfr/text/14/part-21/subpart-B

cameldrv 8y ago

This is incorrect. MCAS is required to meet the stability requirements of FAR 25 in addition to being required for the common type rating.