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I was recently reading about Alaska Airlines Flight 261, a crash where understanding it in detail requires a bit of a deep dive into the trimmable stabilizer. On that particular aircraft (MD-80), it was electrically actuated using an electric motor and jackscrew.

I'm curious how this works on larger aircraft like the DC-10 and 747. Both of these suffered well-known loss-of-control incidents (UA232, JAL123) where the crew lost control of most or all flight controls due to catastrophic hydraulic damage (with these larger planes being simply too big to have purely mechanical fallback, they were designed with hydraulic redundancy that failed in the extreme cases of these incidents.) In UA232's case, no flight controls were available, while in JAL123's case, only an electric alternate system for the flaps was usable.

Presumably, this means that the stabilizer on these larger aircraft was hydraulically powered as opposed to electrically. My question is, how? Did they use a jackscrew mechanism that was driven by a hydraulic motor instead of an electric motor? Or did they use a different sort of mechanism altogether?

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    $\begingroup$ You've picked two (UA232 and JAL123) quite specific incidents. In both of them the tail cone got extensively damaged and any line passing through it would have been damaged, no matter if it had been hydraulic or electric and no matter how redundant it had been. $\endgroup$
    – sophit
    Feb 1 at 19:16
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    $\begingroup$ Alaska Airlines was actually a tale of grease, and the pitfalls of intermixing clay based and lithium based greases. It's a long and complicated story that affected the entire aircraft industry, because nobody was aware of the problems of intermix and there were no limitations, and the MIL-SPEC confusion made things worse. $\endgroup$
    – John K
    Feb 1 at 19:20
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    $\begingroup$ Added Wikipedia links to the 3 crashes mentioned for those who aren't already intimately familiar with them. Not everybody walks around with encyclopedic knowledge of airline crashes and investigations rattling around their brain pan... $\endgroup$
    – FreeMan
    Feb 2 at 13:53

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Affirmative, it's hydraulic. NASA-CR-159010 https://ntrs.nasa.gov/api/citations/19810018578/downloads/19810018578.pdf is a good reference: "The stabilizer control system uses a linear, ball-screw actuator, driven by two hydraulic motors..." With all or most of the vertical tail gone and other roll issues they really needed help in the roll and yaw axes, also. Furthermore, while a usable stabilizer could have helped with pitch trim, it would have been challenging to say the least to use it to maneuver to a safe landing.

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