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The MCAS failure didn't appear as a runaway stabilizer problem to the pilots. Here's what Chesley Sullenberger writes about the experience (http://www.sullysull

by fsh 7y ago

The MCAS failure didn't appear as a runaway stabilizer problem to the pilots. Here's what Chesley Sullenberger writes about the experience (http://www.sullysullenberger.com/my-letter-to-the-editor-of-new-york-times-magazine/ http://www.sullysullenberger.com/my-letter-to-the-editor-of-...): I know firsthand the challenges the pilots on the doomed accident flights faced, and how wrong it is to blame them for not being able to compensate for such a pernicious and deadly design. These emergencies did not present as a classic runaway stabilizer problem, but initially as ambiguous unreliable airspeed and altitude situations, masking MCAS.

WalterBright 7y ago
> The MCAS failure didn't appear as a runaway stabilizer problem to the pilots. The LA and EA pilots countered the stab trim movement repeatedly with the electric trim switches. They knew the stab was running out of control. There is no other definition of runaway trim and there is No Such Thing as "classic" runaway trim. There is no mistaking the 737 stab trim running. Two large wheels, painted black and white so spinning can be easily seen, bracket the center console. They are directly connected with a cable to the stab trim jackscrew. They also make a loud clacking sound when they spin. There is no possible way the pilots could not know it was running with the direct effect of pushing the nose down. Indeed they did know, because they countered it. Source: I worked on the design of the 757 stab trim system for 3 years.
- fsh 7y ago
  I'm inclined to take the word of a famous pilot and safety expert who replicated the accidents in a simulator over that of an engineer who probably didn't. My guess is that the failure of one of the AOA sensors triggered all kinds of alarms and conflicting instrument readings, while MCAS behaved a lot like other systems that occasionally rotate the trim wheels. The Boeing checklist gives as a condition for runaway stabilizer trim: Uncommanded stabilizer trim movement occurs continuously. It probably didn't help that at this point the pilots didn't even know about MCAS' existence.
  - WalterBright 7y ago
    The facts are that the LA and EA pilots both acted multiple times to counter the runaway trim. > Uncommanded stabilizer trim movement occurs continuously It occurred continuously enough that both crews actively tried to counter it each time. They obviously knew that was the problem. Running for several seconds each time is plenty "continuous". You don't need to wait until it goes all the way to the stops, and you'd better counter it before it does. Consider this: suppose an electrical short caused the trim to run erratically. Are you going to decide that isn't really runaway trim because it's erratic? You're going to ride that into the ground? I don't think so. > engineer who probably didn't. Except that I worked for 3 years on the 757 stab trim system, and Sully is an Airbus (very different controls from Boeing) pilot and hadn't flown a 737 in many years and is not certified to fly one.
    - salawat 7y ago
      I'll vouch for Walter, even if his insight I've received less than coolly in the past. He is technically correct that the pilot's did have information in hand through Airworthyness Directives at the time. The only place I tend to depart from him point of view-wise is how the human factors come into it. He believes, (and if you mull on it from the technical point of view, he's right) that it should have been covered with the same procedure. I may not be an aero engineer, but I've done stints in both Quality, and Safety industries. My stint in safety brought me into contact frequently with the aviation guys, and unintuitively enough, they were very particular about how information was presented to pilots. When I asked why, th He answer was that they needed to fly the plane, not be burdened with how everything went together. Their job was seen as needing to know everything that affected the operation and aeronautic behavior of the craft first and foremost. That's where I believe these Airworthyness Directives, though accurate, failed. The pilot's mental model was built up during their training period which included no mention of MCAS. The Airworthyness Directive in this case would translate to informing a pilot of a Major feature release through an off hand set of patch notes. Which is why I still hold Boeing at fault, even if a faulty misunderstand may have been arrived at by the pilot's reading alone of a notification outlining the exact solution to the feature. You don't build the kind of instinctual multi-level reflexive behavioral binding through a singular notification. You must explain the why, and provide repeated examples to get good retention.

by fsh 7y ago

WalterBright 7y ago

> The MCAS failure didn't appear as a runaway stabilizer problem to the pilots. The LA and EA pilots countered the stab trim movement repeatedly with the electric trim switches. They knew the stab was running out of control. There is no other definition of runaway trim and there is No Such Thing as "classic" runaway trim. There is no mistaking the 737 stab trim running. Two large wheels, painted black and white so spinning can be easily seen, bracket the center console. They are directly connected with a cable to the stab trim jackscrew. They also make a loud clacking sound when they spin. There is no possible way the pilots could not know it was running with the direct effect of pushing the nose down. Indeed they did know, because they countered it. Source: I worked on the design of the 757 stab trim system for 3 years.

fsh 7y ago
I'm inclined to take the word of a famous pilot and safety expert who replicated the accidents in a simulator over that of an engineer who probably didn't. My guess is that the failure of one of the AOA sensors triggered all kinds of alarms and conflicting instrument readings, while MCAS behaved a lot like other systems that occasionally rotate the trim wheels. The Boeing checklist gives as a condition for runaway stabilizer trim: Uncommanded stabilizer trim movement occurs continuously. It probably didn't help that at this point the pilots didn't even know about MCAS' existence.
- WalterBright 7y ago
  The facts are that the LA and EA pilots both acted multiple times to counter the runaway trim. > Uncommanded stabilizer trim movement occurs continuously It occurred continuously enough that both crews actively tried to counter it each time. They obviously knew that was the problem. Running for several seconds each time is plenty "continuous". You don't need to wait until it goes all the way to the stops, and you'd better counter it before it does. Consider this: suppose an electrical short caused the trim to run erratically. Are you going to decide that isn't really runaway trim because it's erratic? You're going to ride that into the ground? I don't think so. > engineer who probably didn't. Except that I worked for 3 years on the 757 stab trim system, and Sully is an Airbus (very different controls from Boeing) pilot and hadn't flown a 737 in many years and is not certified to fly one.
  - salawat 7y ago
    I'll vouch for Walter, even if his insight I've received less than coolly in the past. He is technically correct that the pilot's did have information in hand through Airworthyness Directives at the time. The only place I tend to depart from him point of view-wise is how the human factors come into it. He believes, (and if you mull on it from the technical point of view, he's right) that it should have been covered with the same procedure. I may not be an aero engineer, but I've done stints in both Quality, and Safety industries. My stint in safety brought me into contact frequently with the aviation guys, and unintuitively enough, they were very particular about how information was presented to pilots. When I asked why, th He answer was that they needed to fly the plane, not be burdened with how everything went together. Their job was seen as needing to know everything that affected the operation and aeronautic behavior of the craft first and foremost. That's where I believe these Airworthyness Directives, though accurate, failed. The pilot's mental model was built up during their training period which included no mention of MCAS. The Airworthyness Directive in this case would translate to informing a pilot of a Major feature release through an off hand set of patch notes. Which is why I still hold Boeing at fault, even if a faulty misunderstand may have been arrived at by the pilot's reading alone of a notification outlining the exact solution to the feature. You don't build the kind of instinctual multi-level reflexive behavioral binding through a singular notification. You must explain the why, and provide repeated examples to get good retention.