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In some instances of an aircraft suffering a catastrophic failure of its primary flight controls, the loss of control force on the elevator allows said elevator to drop down under its own weight, causing an immediate crash.

Examples:

In other loss-of-flight-controls accidents, however, the elevator either stays stuck at its position immediately prior to the control failure, or floats in the airstream:

Why does elevator dropdown occur during some, and only some, loss-of-flight-controls accidents?

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    $\begingroup$ Hi Sean, for the first two where does it say an elevator dropdown occurred? Note for the LOT it says it was the horizontal stabilizer, not the elevator. $\endgroup$
    – user14897
    Commented Sep 16, 2018 at 11:47
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    $\begingroup$ @ymb1, the article says horizontal stabilizer, but it clearly means the elevator, because the foreplane dropping with gravity would cause pitch up and also because it is then mentioned that using manual trim could have worked (but there wasn't enough time to recover). $\endgroup$
    – Jan Hudec
    Commented Sep 16, 2018 at 14:19
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    $\begingroup$ An elevator wouldn't 'drop down', even if all mechanical links to it were snapped. First, aerodynamic forces are much greater than its weight. Second, control surfaces, especially on aircraft with [hydro]mechanical linkage (such as IL-62), are normally weight balanced. That said, the behaviour of the airplane with relaxed vs. stuck control will be different, but neither necessarily leads to immediate pitch down. $\endgroup$
    – Zeus
    Commented Sep 17, 2018 at 1:49

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The mechanical or hydromechanical control system transmits the forces both ways, potentially amplified in the later case. When the pilot trims the plane so he is not applying any force on the control column, all the forces on the elevator are balanced and this includes gravity.

So when the control links are broken, the elevator will return to its trimmed position. In that condition, the plane will maintain speed with pitch, oscillating around the equilibrium in a Phugoid oscillation.

In the later four cases, the planes were likely trimmed, and had engine power left, so they remained controllable for some time.

In the LOT 007 case the aircraft was almost certainly not trimmed – the pilot could just hold the control column and not bother with the trim – and it lost most power, so it pitched down immediately.

The THY 981 either for some reason was not trimmed, or maybe the stabilizer moved a bit as the control links severed. I don't think there is enough information to tell for sure.

Also in the LOT 5055 case it is not clear the plane did not initially pitch down as the trim was still operating (until the spreading fire burned the power supply to it) and they had enough altitude to regain control by using it.

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  • $\begingroup$ Even if the elevator starts out in a trimmed position, it won't stay there once the hydraulics/cables/servotabs/whatnot holding it in trim are suddenly no longer functioning. $\endgroup$
    – Vikki
    Commented Sep 18, 2018 at 3:09
  • $\begingroup$ @Sean, that's what I am saying: “elevator will return to its trimmed position”. The point is that that position will not be flying straight, so the aircraft will pitch. Up, or down, depending on the position of the trim. $\endgroup$
    – Jan Hudec
    Commented Sep 18, 2018 at 5:08

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