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As far as is publicly known, the failure has occurred three times, and has led to two crashes. It is possible to recover from if you know what you're looking f

As far as is publicly known, the failure has occurred three times, and has led to two crashes. It is possible to recover from if you know what you're looking for and you act promptly. However, Boeing did not tell pilots what to look for. The pilots did not diagnose it as stab trim runaway because it's not stab trim runaway. When MCAS is malfunctioning in this way, it engages nose down trim for 9.3 seconds, and then pauses for 5 seconds. A runaway trim is where you have a stuck trim microswitch or relay or some sort of short circuit. You're expecting a continuous trim input. The confusing thing about MCAS is that it appears to resolve itself, but then waits 5 seconds and comes right back. By the way, the wheels going clack clack is a normal thing at that phase of flight. The 737 has something called Speed Trim and another thing called Mach Trim that automatically adjust the trim as the speed of the aircraft changes. In the initial climbout, this will be running constantly. Both the speed trim and mach trim systems are documented in the 737 manual and pilots are required to know how they work to get a type rating. The workings of MCAS were deleted from the manual before it was released because Boeing was concerned that the FAA would require more training on the system, which would cause Boeing to have to pay a contractual penalty to Southwest Airlines. edit: Also, Boeing's own test pilots failed to respond promptly to this failure in the simulator: https://aviationweek.com/air-transport/aircraft-propulsion/boeing-aware-pilot-response-issues-during-max-certification https://aviationweek.com/air-transport/aircraft-propulsion/b... It's very easy as a pilot to say that you would do a better job and something like this would never happen to you. The earth is pockmarked with the smoking holes of these people's airplanes.

WalterBright 3y ago
> The pilots did not diagnose it as stab trim runaway because it's not stab trim runaway Runaway stab trim is uncommanded movement of the trim. MCAS failure exhibits as exactly this. The most likely cause of uncommanded movement is an electrical fault of some sort. Electrical faults are often intermittent. When you don't know why an intermittent fault is happening, you turn it off. This is not rocket science. In the first LA crash, the crew restored normal trim 25 times. The uncommanded trim happened twenty five times. In what universe is this not runaway trim? From what I've seen the definition you're using is one invented after the fact by parsing word meanings like a lawyer. If it's your butt in the pilot's seat, I recommend thinking like a pilot, not like a lawyer. From your link: "in at least one simulator session, Boeing pilots took more than 10 sec. to react to a runaway stabilizer" First off, they don't seem confused about what a "runaway" is. Secondly, 10 seconds was plenty of time, as in both incidences the crew battled it for several minutes. "Aerodynamic loads on the mis-trimmed aircraft made the trim wheel hard to turn" The loads were too high because the pilots were at full throttle and ignored the overspeed warning horn. > It's very easy as a pilot to say that you would do a better job and something like this would never happen to you. And in the first incident, that's what the crew did. They just turned it off and continued to their destination. > the wheels going clack clack is a normal thing at that phase of flight When the wheels are clack clacking and the nose suddenly pitches sharply down, there's the clue that the stab trim is running away. > The earth is pockmarked with the smoking holes of these people's airplanes It sure is, and that's why airline pilots are supposed to know things like what the switches in the cockpit are for and what stabilizer trim runaway is and read and understand all Emergency Airworthiness Directives. Flying an airplane is not a joke and you'd better pay attention in training. My dad flew 23 years in the AF, including combat) and anything less than 100% proficiency was unacceptable.
- cameldrv 3y ago
  Look, take it up with Boeing, not me. Here's the runaway stab trim checklist from 2018, before the accident [1]. Right at the very top, it says: Condition: Uncommanded stabilizer trim movement occurs continuously. MCAS produces an intermittent fault, not a continuous one, therefore you are not supposed to run that checklist. There should be a checklist for MCAS failure, and the system and its failure modes should be explained, like every other system on a 737, but Boeing deliberately decided not to. To your point, yes, you'd better pay attention in training. But, in order for that training to be of any use, they can't keep aircraft systems that can fail and kill you a secret from the pilots! [1] http://www.b737.org.uk/images/runawaystab2013.jpg http://www.b737.org.uk/images/runawaystab2013.jpg edit: Further to your point about the overspeed, the overspeed was not caused by the thrust levers being at full. The overspeed was caused by MCAS pushing the nose down and causing the plane to pick up speed. The pilots were trying to get the nose back up. The difficulty in moving the wheel comes from a combination of the airspeed and how far out of trim the plane is. Both of these problems were caused by MCAS putting in nose down trim. Yes, the pilots could have throttled back and bought themselves a little more time, but that is not the primary reason that they were unable to turn the trim wheel.
  - WalterBright 3y ago
    > MCAS produces an intermittent fault, not a continuous one, therefore you are not supposed to run that checklist. Again, if you're a pilot and want to live, stop parsing sentences like Bill CLinton and parse them like a pilot. Remember, I worked on the 757 trim system. I guarantee you that everyone there would ridicule your interpretation. Do you really imagine it has to run all the way to the stops to satisfy you that it is runaway? > they can't keep aircraft systems that can fail and kill you a secret from the pilots! The pilot had everything he needed to know to save the airplane. He did not need to know the cause of the runaway trim, just how to stop it. > the overspeed was not caused by the thrust levers being at full When you're in a dive, it behooves you to pull the thrust levers back (and they were at full). Overspeed can tear the airplane apart. It just takes a second to pull the levers back, and reducing the airspeed would have made their attempts at manual trim workable. Frankly, if I was teaching ground school and encountered a student who made arguments like yours, I'd wash him out. I'm reminded of that episode of The Office where the characters were driving using navigator software. The software directed them to drive into a swamp, and the characters were arguing over whether they should follow the directions into the swamp or not. There's a reason that airliners still have pilots in them and not have computers run it all.

WalterBright 3y ago

> The pilots did not diagnose it as stab trim runaway because it's not stab trim runaway Runaway stab trim is uncommanded movement of the trim. MCAS failure exhibits as exactly this. The most likely cause of uncommanded movement is an electrical fault of some sort. Electrical faults are often intermittent. When you don't know why an intermittent fault is happening, you turn it off. This is not rocket science. In the first LA crash, the crew restored normal trim 25 times. The uncommanded trim happened twenty five times. In what universe is this not runaway trim? From what I've seen the definition you're using is one invented after the fact by parsing word meanings like a lawyer. If it's your butt in the pilot's seat, I recommend thinking like a pilot, not like a lawyer. From your link: "in at least one simulator session, Boeing pilots took more than 10 sec. to react to a runaway stabilizer" First off, they don't seem confused about what a "runaway" is. Secondly, 10 seconds was plenty of time, as in both incidences the crew battled it for several minutes. "Aerodynamic loads on the mis-trimmed aircraft made the trim wheel hard to turn" The loads were too high because the pilots were at full throttle and ignored the overspeed warning horn. > It's very easy as a pilot to say that you would do a better job and something like this would never happen to you. And in the first incident, that's what the crew did. They just turned it off and continued to their destination. > the wheels going clack clack is a normal thing at that phase of flight When the wheels are clack clacking and the nose suddenly pitches sharply down, there's the clue that the stab trim is running away. > The earth is pockmarked with the smoking holes of these people's airplanes It sure is, and that's why airline pilots are supposed to know things like what the switches in the cockpit are for and what stabilizer trim runaway is and read and understand all Emergency Airworthiness Directives. Flying an airplane is not a joke and you'd better pay attention in training. My dad flew 23 years in the AF, including combat) and anything less than 100% proficiency was unacceptable.

cameldrv 3y ago
Look, take it up with Boeing, not me. Here's the runaway stab trim checklist from 2018, before the accident [1]. Right at the very top, it says: Condition: Uncommanded stabilizer trim movement occurs continuously. MCAS produces an intermittent fault, not a continuous one, therefore you are not supposed to run that checklist. There should be a checklist for MCAS failure, and the system and its failure modes should be explained, like every other system on a 737, but Boeing deliberately decided not to. To your point, yes, you'd better pay attention in training. But, in order for that training to be of any use, they can't keep aircraft systems that can fail and kill you a secret from the pilots! [1] http://www.b737.org.uk/images/runawaystab2013.jpg http://www.b737.org.uk/images/runawaystab2013.jpg edit: Further to your point about the overspeed, the overspeed was not caused by the thrust levers being at full. The overspeed was caused by MCAS pushing the nose down and causing the plane to pick up speed. The pilots were trying to get the nose back up. The difficulty in moving the wheel comes from a combination of the airspeed and how far out of trim the plane is. Both of these problems were caused by MCAS putting in nose down trim. Yes, the pilots could have throttled back and bought themselves a little more time, but that is not the primary reason that they were unable to turn the trim wheel.
- WalterBright 3y ago
  > MCAS produces an intermittent fault, not a continuous one, therefore you are not supposed to run that checklist. Again, if you're a pilot and want to live, stop parsing sentences like Bill CLinton and parse them like a pilot. Remember, I worked on the 757 trim system. I guarantee you that everyone there would ridicule your interpretation. Do you really imagine it has to run all the way to the stops to satisfy you that it is runaway? > they can't keep aircraft systems that can fail and kill you a secret from the pilots! The pilot had everything he needed to know to save the airplane. He did not need to know the cause of the runaway trim, just how to stop it. > the overspeed was not caused by the thrust levers being at full When you're in a dive, it behooves you to pull the thrust levers back (and they were at full). Overspeed can tear the airplane apart. It just takes a second to pull the levers back, and reducing the airspeed would have made their attempts at manual trim workable. Frankly, if I was teaching ground school and encountered a student who made arguments like yours, I'd wash him out. I'm reminded of that episode of The Office where the characters were driving using navigator software. The software directed them to drive into a swamp, and the characters were arguing over whether they should follow the directions into the swamp or not. There's a reason that airliners still have pilots in them and not have computers run it all.