The overall function of the stabilizer trim system is to maintain the airplane in a trimmed condition and to provide automatic mach and speed stability. The system consists of: (1) several alternative sources of trim command signals; (2) two independent actuation devices which position the stabilizer in response to source commands; and (3) an electronic module which selects a particular trim signal source and also energizes one or the other actuation device.
The three sources of electric trim command signals are: (1) the flight control computer (FCC); (2) the mach/speed trim function within the stabilizer/elevator asymmetry module (SAM); and (3) the manual-electric trim switches on the control wheels.
The selection of the active trim source is done by logic circuits in the SAMs.
The SAMs accept input signals from the FCC, the mach/speed trim function in the SAM, and the control wheel switches. The output signals from the SAM are sent to two STCMs.
There are two independent channels of trim command and actuation. Each channel consists of a SAM, an Stabilizer Trim Control Module, and a hydraulic motor and brake on the stabilizer ballscrew actuator. The two channels are known as trim channel left and trim channel right. When an FCC or mach/speed function is the active trim source, only one trim channel is activated, providing half-rate trim. When the electric trim switches are energized, both trim channels are activated simultaneously, and the stabilizer is driven at full-rate trim. The trim rate varies with airplane speed and horizontal stabilizer position as determined by hydraulic pressure output from the elevator feel computer. The variable hydraulic pressure output from the elevator feel computer is fed to each STCM. In each STCM a rate control valve regulates actual hydraulic pressure available to drive the horizontal stabilizer.