6 ms·

> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is

> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wheel. This is extremely (EDIT: No longer) well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. (EDITED TO ADD: Apparently this information & procedure was removed from Boeing's manuals after the 1980s, and pilots no longer train on it, as the issues that led to its necessity were thought solved. The MCAS apparently has reintroduced the need for them.). This is compounded by a new change in the 737-MAX: in prior 737 series aircraft, there are two stab trim cutout switches: one to cut the autopilot's ability to command the stabilizer trim, the other to cut electronic power to the stabilizer trim adjustment entirely, so you could stop the autopilot from changing the trim but still retain electronic control and not need to worry about the manual crank forces. The two switches are MAIN ELEC and AUTO PILOT. In the MAX, this changed, and it's now an all or nothing setup: you have to kill electronic power to kill autopilot commands to the stabilizer. The two switches are now PRI and B/U. Both must be cut out to stop any stabilizer runaway, including MCAS runaway. This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control.

Rooster61 8y ago
> They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to another flight that lost all hands. I'm trying very hard to look at this objectively, but this is an extremely bad look for Boeing.
- daddylonglegs 8y ago
  > > They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. > All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to another flight that lost all hands. Additionally an angle of attack (AOA) sensor failure also triggers an unreliable airspeed warning so the pilots no longer trust their airspeed sensors or their AOA sensors while trying to judge pitch and speed. If they go too fast the control forces overpower them, if they go too slow they are more likely to stall. I don't think Boeing gamed out or tested in a simulator the human factors involved in an AOA failure at takeoff, particularly for pilots who - like the Lion Air pilots - are entirely ignorant of MCASes existence.
  - Rooster61 8y ago
    I don't remember where I read it, but my understanding is that the MAX simulator does NOT simulate MCAS behavior. EDIT: Found it: https://www.reuters.com/article/us-ethiopia-airplane-simulator-exclusive/ethiopia-crash-captain-did-not-train-on-airlines-max-simulator-source-idUSKCN1R20WD https://www.reuters.com/article/us-ethiopia-airplane-simulat...
    - daddylonglegs 8y ago
      Thank you for that. Putting hundreds of aircraft into service before the simulators were available to airline pilots looks rather bad in hindsight. I would hope that Boeing internally had simulators for the MAX while they were testing it - along with a borrowed supply of pilots who were competent but had no inside information. Even if they didn't, a pencil and paper exercise might have been useful: 1. AOA sensor fails at takeoff (A), the pilots receive an unreliable airspeed warning and the stick shaker activates. The pilots now mistrust their sensors and want to keep their speed up and the nose down. 2. At about 1000 feet above ground the pilots level the aircraft, raise the flaps and start the unreliable airspeed checklist. MCAS starts trimming intermittently. 3. The pilot flying is having to pull further back on the stick and trim correctively while the crew work through their checklist. At this point the crew need to avoid any over-focus, task saturation or over-stimulus from the many warnings and indicators and ask themselves why the aircraft is trimming the way it is. They have a fairly narrow window of time in which to do this and then disable the trimming with the plane in a well trimmed state. (A) That this happened twice in relatively quick succession might be due to a common problem. Even if such a problem is found and fixed the AOA vanes and pitot tubes are delicate sensors sticking out of the nose of the aircraft; the system should be robust to failures in these sensors.
rellui 8y ago
From the way you describe it, if the stabilizer reaches the point where human force is insufficient to bring it back then they're toast since MCAS is tied into the electronic stabilizer. How did this scenario get missed during testing? You'd think they would do extra testing around the new changes.
- msbarnett 8y ago
  > How did this scenario get missed during testing? You'd think they would do extra testing around the new changes. Seems like systemic issues. Reportedly Boeing's submitted risk assessment of the MCAS was based on a single adjustment applied from it, not repeated cycles of application based on a failure of the AoA to recover to what it saw as "level". The FSB noted the change to the cutout configuration as a difference, but didn't analyze how that could change anything during Autopilot or Speed Trim runaway situations. They didn't even make any note of the MCAS as a difference at all (possibly they were not informed of it?). I suspect this all comes down to the fact that Boeing started with a conclusion -- The MAX series must not require new simulator time -- and that conclusion was the lens through which they analyzed (or didn't) everything else.
hencq 8y ago
> This is extremely well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. They went for option 1 in this case, with the MCAS kicking in 5 seconds later. > This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. Yeah, and even that is understating it in my opinion.
- msbarnett 8y ago
  > They went for option 1 in this case, with the MCAS kicking in 5 seconds later. Yeah, they basically had to go with option 1. They were only 1,000 feet above the ground, the maneuvering room to let the nose dip, crank, dip, crank, etc until properly trimmed just wasn't there.
fgonzag 8y ago
Layman question here: Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the trim trying to pitch the plane down while the pilot is trying to pitch up with the stabilizer? Or should we limit the trim so it never actually opposes the pilots intentions? (Maybe set it to a neutral position and disengage auto function if this condition is detected?)
- msbarnett 8y ago
  I'm not really an expert but my understanding is that the stabilizer will always out-muscle the elevator due to the physics of their respective positions on the aircraft. In yet another change in the MAX series, there used to be what was called an aft column cutout switch which did more or less what it sounds like: engage when the stick was pulled back, cutting out autopilot commands to the stabilizer, so that it never attempted to outmuscle the pilot's input commands to the elevator. This was removed in the MAX. Possibly this was to meet certification requirements: if the MCAS was needed to push down on the nose to meet the requirement that "During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning." If the MAX tends to pitch nose up at a certain point, the MCAS is intended to create a counter force against the pilots commands as it approaches that, rather than have the pilot pulling back at some point suddenly buck the nose upwards, then the MCAS needs to be active when the pilot is pulling back on the stick. But it does argue that a LOT more thought needed to go into MCAS failure modes if it was necessary for it not to be cut out when the pilot is operating elevators.
- Gibbon1 8y ago
  I think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input. I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane. I'm now wondering is the MACS is needed because the tendency to pitch up also exceeds the pilots control inputs which is why it had to be on the elevator and have that much authority. Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down.
treis 8y ago
>less great at a height of 1,000 feet. Am I reading the chart on page 27 wrong? Because it looks like they reached a maximum height of 7,000 feet. Also looks like they were continuing to slowly climb until they re-enabled electric trim. EDIT TO ADD: Page 12 also says: >about 32 seconds before the end of the recording, at approximately 13,400 ft Which is a bit under 6,000 feet higher than the airport.
- msbarnett 8y ago
  Looks like you're right. But apparently the option 2 maneuver (relax pressure on stick, then crank) hasn't been covered in a Boeing 737 manual since the 200 series in the 1980s. So even given the extra room, there was no documentation covering that possible recovery scenario.
- ambicapter 8y ago
  A lot of people are thinking they maxed out at a height of ~1,000 ft because that is what the flight tracker website last showed.

Rooster61 8y ago

> They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to another flight that lost all hands. I'm trying very hard to look at this objectively, but this is an extremely bad look for Boeing.

daddylonglegs 8y ago
> > They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. > All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to another flight that lost all hands. Additionally an angle of attack (AOA) sensor failure also triggers an unreliable airspeed warning so the pilots no longer trust their airspeed sensors or their AOA sensors while trying to judge pitch and speed. If they go too fast the control forces overpower them, if they go too slow they are more likely to stall. I don't think Boeing gamed out or tested in a simulator the human factors involved in an AOA failure at takeoff, particularly for pilots who - like the Lion Air pilots - are entirely ignorant of MCASes existence.
- Rooster61 8y ago
  I don't remember where I read it, but my understanding is that the MAX simulator does NOT simulate MCAS behavior. EDIT: Found it: https://www.reuters.com/article/us-ethiopia-airplane-simulator-exclusive/ethiopia-crash-captain-did-not-train-on-airlines-max-simulator-source-idUSKCN1R20WD https://www.reuters.com/article/us-ethiopia-airplane-simulat...
  - daddylonglegs 8y ago
    Thank you for that. Putting hundreds of aircraft into service before the simulators were available to airline pilots looks rather bad in hindsight. I would hope that Boeing internally had simulators for the MAX while they were testing it - along with a borrowed supply of pilots who were competent but had no inside information. Even if they didn't, a pencil and paper exercise might have been useful: 1. AOA sensor fails at takeoff (A), the pilots receive an unreliable airspeed warning and the stick shaker activates. The pilots now mistrust their sensors and want to keep their speed up and the nose down. 2. At about 1000 feet above ground the pilots level the aircraft, raise the flaps and start the unreliable airspeed checklist. MCAS starts trimming intermittently. 3. The pilot flying is having to pull further back on the stick and trim correctively while the crew work through their checklist. At this point the crew need to avoid any over-focus, task saturation or over-stimulus from the many warnings and indicators and ask themselves why the aircraft is trimming the way it is. They have a fairly narrow window of time in which to do this and then disable the trimming with the plane in a well trimmed state. (A) That this happened twice in relatively quick succession might be due to a common problem. Even if such a problem is found and fixed the AOA vanes and pitot tubes are delicate sensors sticking out of the nose of the aircraft; the system should be robust to failures in these sensors.

rellui 8y ago

From the way you describe it, if the stabilizer reaches the point where human force is insufficient to bring it back then they're toast since MCAS is tied into the electronic stabilizer. How did this scenario get missed during testing? You'd think they would do extra testing around the new changes.

msbarnett 8y ago
> How did this scenario get missed during testing? You'd think they would do extra testing around the new changes. Seems like systemic issues. Reportedly Boeing's submitted risk assessment of the MCAS was based on a single adjustment applied from it, not repeated cycles of application based on a failure of the AoA to recover to what it saw as "level". The FSB noted the change to the cutout configuration as a difference, but didn't analyze how that could change anything during Autopilot or Speed Trim runaway situations. They didn't even make any note of the MCAS as a difference at all (possibly they were not informed of it?). I suspect this all comes down to the fact that Boeing started with a conclusion -- The MAX series must not require new simulator time -- and that conclusion was the lens through which they analyzed (or didn't) everything else.

hencq 8y ago

> This is extremely well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. They went for option 1 in this case, with the MCAS kicking in 5 seconds later. > This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. Yeah, and even that is understating it in my opinion.

msbarnett 8y ago
> They went for option 1 in this case, with the MCAS kicking in 5 seconds later. Yeah, they basically had to go with option 1. They were only 1,000 feet above the ground, the maneuvering room to let the nose dip, crank, dip, crank, etc until properly trimmed just wasn't there.

fgonzag 8y ago

Layman question here: Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the trim trying to pitch the plane down while the pilot is trying to pitch up with the stabilizer? Or should we limit the trim so it never actually opposes the pilots intentions? (Maybe set it to a neutral position and disengage auto function if this condition is detected?)

msbarnett 8y ago
I'm not really an expert but my understanding is that the stabilizer will always out-muscle the elevator due to the physics of their respective positions on the aircraft. In yet another change in the MAX series, there used to be what was called an aft column cutout switch which did more or less what it sounds like: engage when the stick was pulled back, cutting out autopilot commands to the stabilizer, so that it never attempted to outmuscle the pilot's input commands to the elevator. This was removed in the MAX. Possibly this was to meet certification requirements: if the MCAS was needed to push down on the nose to meet the requirement that "During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning." If the MAX tends to pitch nose up at a certain point, the MCAS is intended to create a counter force against the pilots commands as it approaches that, rather than have the pilot pulling back at some point suddenly buck the nose upwards, then the MCAS needs to be active when the pilot is pulling back on the stick. But it does argue that a LOT more thought needed to go into MCAS failure modes if it was necessary for it not to be cut out when the pilot is operating elevators.
Gibbon1 8y ago
I think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input. I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane. I'm now wondering is the MACS is needed because the tendency to pitch up also exceeds the pilots control inputs which is why it had to be on the elevator and have that much authority. Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down.

treis 8y ago

>less great at a height of 1,000 feet. Am I reading the chart on page 27 wrong? Because it looks like they reached a maximum height of 7,000 feet. Also looks like they were continuing to slowly climb until they re-enabled electric trim. EDIT TO ADD: Page 12 also says: >about 32 seconds before the end of the recording, at approximately 13,400 ft Which is a bit under 6,000 feet higher than the airport.

msbarnett 8y ago
Looks like you're right. But apparently the option 2 maneuver (relax pressure on stick, then crank) hasn't been covered in a Boeing 737 manual since the 200 series in the 1980s. So even given the extra room, there was no documentation covering that possible recovery scenario.
ambicapter 8y ago
A lot of people are thinking they maxed out at a height of ~1,000 ft because that is what the flight tracker website last showed.