3 ms·

I'm just wondering: it looks like the absolute trim angle is not visible anywhere in the cockpit? The trim wheel takes multiple revolutions to adjust trim betw

I'm just wondering: it looks like the absolute trim angle is not visible anywhere in the cockpit? The trim wheel takes multiple revolutions to adjust trim between min/max limits, so looking at the wheel itself, you can only every see changes, not values. Unless you notice the change while it happens, you won't have any indication as to the unusual trim angle? Thinking about that I wonder whether there was even a measurement signal that the computers could rely on to determine the current state of the trim angle in absolute terms. Given how the MCAS did incremental changes without stopping before reaching the mechanical limits, I do imagine that there might have been no hardware feedback to assess the current trim state, thereby making it impossible hardware-wise to limit MCAS to a sensible (and safe) range of angles. [edit] Looking at the lion air crash report [1] p.14 ff it seems like also the black box only recorded trim changes, not absolute trim state. [1] https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-LQP_PRELIMINARY.pdf https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...

VBprogrammer 8y ago
There is a trim position indication. It's to the left and right of the throttles. It's a critical thing to check before starting the takeoff roll. You can see it on the copilot's side here by the trim wheel. http://www.b737.org.uk/images/stabtrim_200.jpg http://www.b737.org.uk/images/stabtrim_200.jpg
- dvdkhlng 8y ago
  Yes, thinking about that, not having any pilot-visible indication would be quite some point of failure at lift-off. Still wondering whether MCAS and other automatation systems have a representation of that position indication. I can't currently come up with an idea of how to implement that in an easy, yet error-resilient way. Usually you would initially drive into a reference position, then integrate the motor's actions to arive at an absolute value, but integration would potentially add up subsequent errors, and obviously you can't re-do the referencing step while in the air.
  - VBprogrammer 8y ago
    A potentiometer (or something fancier) directly attached to the stabilizer would do it. I'm not sure if they do have one or not. They rate of movement on the stab trim is different depending on the source of the trim change so it's possible they have a PWM controller with some feedback.
    - fgonzag 8y ago
      Or just use an angle sensor on the control surfaces? You now have two measurements: What the computer is telling the hardware to set the angle to, what the angle actually is. These two are very different but very important pieces of information.
  - VBprogrammer 8y ago
    FYI It does look like the graphs here indicate "pitch trim position" which I believe is the absolute value. https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-LQP_PRELIMINARY.pdf https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...
coredog64 8y ago
Earlier versions of the 737 (up to next gen) have noisy trim wheels that are difficult to ignore. AIUI, runaway trim has always been a possibility and the trim wheel design helps to surface that it’s happening.

VBprogrammer 8y ago

There is a trim position indication. It's to the left and right of the throttles. It's a critical thing to check before starting the takeoff roll. You can see it on the copilot's side here by the trim wheel. http://www.b737.org.uk/images/stabtrim_200.jpg http://www.b737.org.uk/images/stabtrim_200.jpg

dvdkhlng 8y ago
Yes, thinking about that, not having any pilot-visible indication would be quite some point of failure at lift-off. Still wondering whether MCAS and other automatation systems have a representation of that position indication. I can't currently come up with an idea of how to implement that in an easy, yet error-resilient way. Usually you would initially drive into a reference position, then integrate the motor's actions to arive at an absolute value, but integration would potentially add up subsequent errors, and obviously you can't re-do the referencing step while in the air.
- VBprogrammer 8y ago
  A potentiometer (or something fancier) directly attached to the stabilizer would do it. I'm not sure if they do have one or not. They rate of movement on the stab trim is different depending on the source of the trim change so it's possible they have a PWM controller with some feedback.
  - fgonzag 8y ago
    Or just use an angle sensor on the control surfaces? You now have two measurements: What the computer is telling the hardware to set the angle to, what the angle actually is. These two are very different but very important pieces of information.
- VBprogrammer 8y ago
  FYI It does look like the graphs here indicate "pitch trim position" which I believe is the absolute value. https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-LQP_PRELIMINARY.pdf https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...

coredog64 8y ago

Earlier versions of the 737 (up to next gen) have noisy trim wheels that are difficult to ignore. AIUI, runaway trim has always been a possibility and the trim wheel design helps to surface that it’s happening.