Tail structure question
 

I have a question on tail structure that I hope my good friends here will endure.

I've been asked by a friend to explain the apparent upward and downward shift ability that can be seen at the leading edge of some horizontal stabilizers, as evidenced by a slot in the vertical fin which appears to accommodate this movement.

Alas, I am stumped. Be gentle, please.

That is for pitch trim through a wide range of airspeeds and flap configuratiions. The elevator is used for pitch control around this trimmed or "hands off" position.

A broadbrush explanation is that the trimmable horizontal stab addresses long term pitch trim requirements whereas the elevators provide a short term pitch trim.
marrying the two of those together, should you have a "long standing" pitch trim request from the elevators, the stab will be trimmed to null the elevator standing signal - usual disclaimers apply. :)

Sounds like you're talking about a movable horizontal stabilizer. It's aft structure is attached to the vertical stabilizer or aft fuselage structure by hinge fittings attached to each structure and connected to each other by hinge pins. The forward end of the horizontal is attached to a jackscrew(s) which extends upward from the pitch trim actuator driven by hydrualic and/or electrical redundant motors. When the pitch trim switch is operated by the pilot or the autopilot schedules pitch trim, the motor activates and turns the jackscrew in the direction that will raise or lower the leading edge of the horizontal stabilizer, changing the amount of tail downforce being produced. The elevators are hinged to the trailing edge of the stabilizer and are operated by the pilot or autopilot to control aircraft pitch attitude. The pitch trim is operated to relieve the need to hold the elevators deflected to maintain desired pitch attitude. The failure of the jackscrew assembly led to the loss of control of AL 261 a few years ago off the California coast. An improperly assembled jackscrew attachment caused a similar loss of an IAI 1124 Westwind in Pennsylvania also. A jammed stabilizer is a serious problem since full elevator travel cannot overcome the effect of the stabilizer when it is near full travel.

Best,

Westhawk

Many thanks for those very lucid answers. Any day you learn something is a good day.