(c)
The arm lever is equipped with a switch that transmits a discrete signal to the SAM when the trim levers are engaged. This synchronizes the Mach/speed trim function to the stabilizer position commanded by the manual control system.
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(d) When the electric trim switches are engaged, both trim channels are activated, and the stabilizer is driven at full-rate trim. The STCM responds to electrical trim commands by applying hydraulic pressure to the motors on the actuator. A hydraulic brake on the actuator assembly is released by the same hydraulic pressure. A hydraulic pressure signal from the elevator feel computer modulates the flow rate of fluid to the actuator via a rate valve within the STCM. The flow rate is reduced at high speed and at aft center of gravity. This varies the trim rate between 0.1 and 0.25 degree/sec for one active STCM, and between 0.2 and 0.5 degree/sec for two active STCMs. Between high and low trim rates, the rate varies linearly as a function of elevator feel pressure. Actuation of the manual electric trim switches overrides all other electric trim modes, except when in Autoland mode.
(2) Cutout, Asymmetric and Stabilizer Limit Switch Activation
(a)
Each control column is coupled to a set of four switches that enable electric trim commands to be interrupted (solenoid valve voltage removed) by column action. A column forward action opens the switch in the airplane nose-up stabilizer command circuit. A column aft action opens the switch in the airplane nose-down command circuit. Limit switches remove solenoid valve voltage when stabilizer position limits are reached. Two swithes are implemented such that if the columns are operated asymmetrically, as would occur after a system jam, both trim modules remain operative. This allows full rate trim capability after a jam.
(b)
The forward and aft control column cutout switches and the nose-up and nose-down limit switches operate as follows: 1) A control column forward movement from neutral greater than
4.0 degrees opens the nose-up ARM inputs to the STCMs. If the leading edge stabilizer position is more than 14.5 units, the limit switch opens the nose-up ARM input to the STCM.
2) A control column aft movement from neutral greater than 2.7 degrees opens the nose-down ARM inputs to the STCMs. The limit switch opens the nose-down ARM input to the STCM if the leading edge stabilizer position is less than +3.25 units (flaps retracted) or +0.2 units (flaps extended).
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3) Two asymmetric switches on each column are also wired in series with the up and down ARM channel output commands from both SAMs. Either set of trim switches are provided even when one column is jammed past the cutout switch interrupt point. The asymmetric switches allow up to 24 degrees left to right elevator asymmetry for computed airspeed less than 194 |3 knots. The switches allow up to 8 degrees asymmetry for computed airspeed greater than 210 |3 knots.
(c) Stabilizer mechanical stops are at +4 and -11.5 degrees. These limits control the maximum range that the stabilizer may travel.
H. Configuration
(1) All airplanes have SAM -116 installed.
2. Operation_________
A. Functional Description
(1)
Automatic Stabilizer Trim Block Diagram (Fig. 5)
(a)
The purpose of the Flight Control Computers (FCCs) in the stabilizer trim system is to provide trim commands when at least one channel of the autopilot is engaged. All three FCCs may be used; any one when in the cruise mode, or two or more when in the autoland mode. The left and right FCCs provide inputs to their respective SAMs. The center FCC provides inputs to both SAMs. Only one SAM is engaged at any time, with the first FCC engaged selected as the autotrim source. Priority for simultaneous FCC engagement is left, center, right.
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