3 ms·

Unfortunately I think you may be misinterpreting that section in the manual. From your senario when the trim moves nose down. You apply nose up on the elevator

Unfortunately I think you may be misinterpreting that section in the manual. From your senario when the trim moves nose down. You apply nose up on the elevator which makes the trailing edge of the elevator go up which applies a downwards force on the trailing edge of the stabilizer. In order to move the trim back towards neural you need to lift the trailing edge against this force. If the trim is extreme you may have to relax back pressure to reduce that force. By in trim speed they mean the speed at which the aircraft would naturally settle at given the current trim setting.

mannykannot 8y ago
You have a point, but I am not sure that this is a complete analysis of the situation, because it does not consider the changing moments as the centers of pressure of the wing and stabilizer move in response to the changing airspeed. Furthermore, when one moves the elevator of a subsonic airplane, it does not just change the force generated by the elevator, but also the force generated by the stabilizer it is attached to (by affecting the airflow over it), so that is another potential source of changing moments. Perhaps the phrase "aerodynamically relieve the air loads" means to minimize the aerodynamic force generated by the stabilizer? The stabilizer of a conventional airplane will invariably generate a downwards force when the aircraft is in pitch equilibrium at high speed, but it may be upwards at low speed, even with the C of G within limits. In such cases, the speed at which it has no load will be an intermediate one, and therefore could be either above or below the speed to which the airplane is currently (mis-)trimmed. The quoted passage says it is referring to correcting what it calls a mis-trim, so if your interpretation is correct, then where it says "in-trim speed", it actually means "the speed to which the airplane is mis-trimmed", while the general usage of the term "in trim" means trimmed to fly at the intended speed, and I would take the phrase "in-trim speed" to refer to that speed - the speed that the pilots intend to fly at, or, in other words, the speed that this out-of-trim airplane would tend to fly at when put back in trim. Furthermore, if things are as you describe, then the rule could have been more simply and clearly phrased as "reduce your pressure on the control yoke". Maybe your interpretation is correct, but if so, it seems to me to be an unnecessarily confusing way to describe it.
- cjbprime 8y ago
  For what it's worth, my reading matches VBprogrammer's, though I agree with you that much of the language seems unnecessary. Frustrating to have a question that could be answered quickly by a 737 pilot! (Whether the trim wheel becomes much harder to turn after you set STAB TRIM CUTOUT to cutout, mistrim down, and pull up.)
- VBprogrammer 8y ago
  You make a couple of very good points. And your right, if it were that simple they hopefully would have described it that way. In general pitch trim runaway incidents are easiest to control at low speeds. It gives you more aircraft configuration options available to try and reduces the aero loads general. But perhaps they were just not ruling out either.
  - mannykannot 8y ago
    I have asked around, and the consensus is that you are correct.

mannykannot 8y ago

You have a point, but I am not sure that this is a complete analysis of the situation, because it does not consider the changing moments as the centers of pressure of the wing and stabilizer move in response to the changing airspeed. Furthermore, when one moves the elevator of a subsonic airplane, it does not just change the force generated by the elevator, but also the force generated by the stabilizer it is attached to (by affecting the airflow over it), so that is another potential source of changing moments. Perhaps the phrase "aerodynamically relieve the air loads" means to minimize the aerodynamic force generated by the stabilizer? The stabilizer of a conventional airplane will invariably generate a downwards force when the aircraft is in pitch equilibrium at high speed, but it may be upwards at low speed, even with the C of G within limits. In such cases, the speed at which it has no load will be an intermediate one, and therefore could be either above or below the speed to which the airplane is currently (mis-)trimmed. The quoted passage says it is referring to correcting what it calls a mis-trim, so if your interpretation is correct, then where it says "in-trim speed", it actually means "the speed to which the airplane is mis-trimmed", while the general usage of the term "in trim" means trimmed to fly at the intended speed, and I would take the phrase "in-trim speed" to refer to that speed - the speed that the pilots intend to fly at, or, in other words, the speed that this out-of-trim airplane would tend to fly at when put back in trim. Furthermore, if things are as you describe, then the rule could have been more simply and clearly phrased as "reduce your pressure on the control yoke". Maybe your interpretation is correct, but if so, it seems to me to be an unnecessarily confusing way to describe it.

cjbprime 8y ago
For what it's worth, my reading matches VBprogrammer's, though I agree with you that much of the language seems unnecessary. Frustrating to have a question that could be answered quickly by a 737 pilot! (Whether the trim wheel becomes much harder to turn after you set STAB TRIM CUTOUT to cutout, mistrim down, and pull up.)
VBprogrammer 8y ago
You make a couple of very good points. And your right, if it were that simple they hopefully would have described it that way. In general pitch trim runaway incidents are easiest to control at low speeds. It gives you more aircraft configuration options available to try and reduces the aero loads general. But perhaps they were just not ruling out either.
- mannykannot 8y ago
  I have asked around, and the consensus is that you are correct.