Originally Posted by
GordonR_Cape
safetypee
I asked a question earlier in the thread, but it got lost. Do pilots ever look at the stabiliser trim analog scale on the pedestal during flight (after takeoff)?
You are asking a related question. My followup questions: Is there a digital value of stabiliser trim available on the flight control computers? Is this value used, or is trim driven from arbitrary positions by incremental turns of the jackscrew? Are there any software limits on the jackscrew position?
I may have this story backwards, but have not seen it discussed in relation to MCAS (nor in the Tech Log forum).
The only time that stabilizer positioning requires or even benefits from reference to the absolute position of the stabilizer is when on ground, preparing for takeoff. When in air, pilot commanded movement of the stabilizer is completely based on establishing pitch trim such that no column input is needed and the pilot is able to maintain the desired pitch attitude / with the column in its spring centered detent. The cue for the pilot to move the stabilizer when in flight is steady column force needed to balance airplane pitching moments to keep the pitch attitude where it is needed to achieve the desired steady state flight path response. Where the stabilizer ends up to achieve pitch trim is a function of CG, weight, thrust, speed, Mach number, flap position, speedbrake position, gear position, and anything else that might impose a steady pitching moment on the airplane.
There are limits on the stabilizer travel both in the software that is used to provide automatic stabilizer control and in the pilot trim input paths via electric wheel mounted trim switch and manual cranking of the mechanical trim wheel. These are designed to allow for the range of stabilizer positions that are needed to be able to achieve pitch trim throughout the flight envelope including the allowable range of loading and flap/speedbrake/gear configurations.