4 ms·

potential for an electrical short to move the jet’s horizontal tail uncommanded I think this is a reference to the stabilizer, because the elevators are contro

by cmurf 7y ago

potential for an electrical short to move the jet’s horizontal tail uncommanded I think this is a reference to the stabilizer, because the elevators are controlled by cable connected to the yoke and are hydraulically powered. Since stabilizer (trim) can overpower elevator force, uncommanded changes in this control surface could be really bad, depending on how a short manifests into control surface movements. I can't assess the relative probabilities: a short happening vs the fix inducing some other problem. But I do wonder whether there's another way to mitigate it.

Gwypaas 7y ago
Today it's mitigated by the pilot. Runaway Stabilizer is a memory-item. [1] This is the same memory item Boeing thought would mitigate MCAS, since that is essentially a runaway stabilizer trim, although in retrospect behaving in a very different manner making it much harder to diagnose. [1]: http://www.b737.org.uk/runawaystab.htm http://www.b737.org.uk/runawaystab.htm
- kds3 7y ago
  Runaway Stabilizer / MCAS cases can be mitigated by the pilots (they can use cutoff switches and do manual trim), but not this kind of electrical short: > Furthermore, the electrical power in that wire could circumvent the cutoff switches in the cockpit that, in the event of such a stabilizer runaway, are used to kill electrical power to the tail. Theoretically, the pilots could be unable to shut it off.
  - mlyle 7y ago
    It's not a great mitigation, but the last thing on runaway stabilizer's memory list is to "grasp and firmly hold" the stab trim wheel.
    - SAI_Peregrinus 7y ago
      The electric motors can apply more power than a human can resist. So that's not exactly helpful.
      - userbinator 7y ago
        I believe there is a clutch mechanism. There are videos on YouTube of pilots doing this (in a simulator.)
      - mlyle 7y ago
        The system is engineered so that an operator holding the trim wheel wins, via a clutch mechanism.

by cmurf 7y ago

Gwypaas 7y ago

Today it's mitigated by the pilot. Runaway Stabilizer is a memory-item. [1] This is the same memory item Boeing thought would mitigate MCAS, since that is essentially a runaway stabilizer trim, although in retrospect behaving in a very different manner making it much harder to diagnose. [1]: http://www.b737.org.uk/runawaystab.htm http://www.b737.org.uk/runawaystab.htm

kds3 7y ago
Runaway Stabilizer / MCAS cases can be mitigated by the pilots (they can use cutoff switches and do manual trim), but not this kind of electrical short: > Furthermore, the electrical power in that wire could circumvent the cutoff switches in the cockpit that, in the event of such a stabilizer runaway, are used to kill electrical power to the tail. Theoretically, the pilots could be unable to shut it off.
- mlyle 7y ago
  It's not a great mitigation, but the last thing on runaway stabilizer's memory list is to "grasp and firmly hold" the stab trim wheel.
  - SAI_Peregrinus 7y ago
    The electric motors can apply more power than a human can resist. So that's not exactly helpful.
    - userbinator 7y ago
      I believe there is a clutch mechanism. There are videos on YouTube of pilots doing this (in a simulator.)
    - mlyle 7y ago
      The system is engineered so that an operator holding the trim wheel wins, via a clutch mechanism.