Conventional control columns operate through independent cable systems to control the two dual Hydraulic Servo Units. Each servo unit contains two hydraulically independent control valves. Each of the 4 servo units are supplied by a separate hydraulic system. Thus, two dual control and 4 hydraulic actuators provide maximum protection against system failure. Feedback links between the servos and the stabilizer signal position of stabilizer to the servos.
Stabilizer Servo Interconnect (Auto Transfer to System B.)
A mechanical interconnect device between the A/B and D/C stabilizer servos prevent shutdown of all 4 servos simultaneously. The unit allows only paired servos to be depressurized at any onetime. The mechanical device gives the B Servo priority. Since the Ram Air Turbine (RAT) will supply pressure the the B Hydraulic System in case of loss of all other sources, it is logical to have a mechanical interlock which gives the B Stabilizer Servo priority.
The separate control paths from the Captain's and Co-Pilot's stations are coupled at both the forward and the aft ends of the aircraft by the pitch disconnect mechanism and the aft coupler. The Pitch Disconnect can be separated manually by using the T-handle in the center console. The Aft Coupler is disconnected electrically and automatically if both servos on one side are turned off. A pitch monitor is used to continually monitor the Pitch System for possible jams. Each Stabilizer Servo has two switch lights located on the Primary Flight Control System (PFCS) panel which display a loss of hydraulic pressure or a jam condition by illuminating INOP.
Bungee cartridges, one in each control path and one int he Aft Coupler path act as solid links during normal operation. If either control path should jam, Pilot forces exerted on the control path will apply breakout forces to the affected bungee. Switches attached tot he bungee will sense the breakout and signal the monitor system the fault condition which then illuminates the PUSH lights on the PFCS.
Pitch Trim/Feel
The Pitch Feel and Trim Force System is comprised of two units, one in each cable path which provides artificial feel forces and trim inputs for the Pilot and reaction forces to the autopilot. The Captain's path has an electric unit one the Co-Pilot's path has a mechanical unit. These feel/trim unit are coupled with a Trim Synchronizing Shaft so any inputs into one side will be transferred to the other. Manual trim inputs are made to the mechanical trim unit in the Co-Pilot's side cable path. Electric trim input go to the two electric motors in the Electric Path. These inputs are kept synchronized by a shaft connecting the two units. Loss of one channel reduces Electric Trim Rate to half normal rate.
Mach Trim
The Mach Trim system is a two-channel system as well, one is active - the other one is standby. The system provides speed stability compensation in the pitch axis trim when operating under manual control in the high speed region. Engagement of the autopilot does not disable the Mach Trim system function. Mach Trim can be overridden by electric trim, manual trim and autopilot trim inputs. Mach Trim remains in the existing condition should both channels fail.
The two channel switches located in the FECS overhead panel provide the Mach Trim control and fault annunciators.
Mach Feel
The Mach Feel system is a two-challen active-standby system which operates together with stabilizer trim angle and Mach number to provide pitch control force changes in stabilizer effectiveness as a function of Mach.
The system normally controls one of two electrical motors which are integrated into the pitch feel/trim mechanism. The pitch control force gradients are scheduled from Mach data derived from the two Central Air Data Computers (CADC). Engagement of the autopilot does not remove the function of the Mach Feel system.
The meter on the Mach Feel panel indicates position status of the Mach feel system, calibrated in Mach. The feel system position meter also provides pilot indication of proper system operation. The indicated Mach number should match the Mach number on the Airspeed Indicator.
Autopilot Servo
The two autopilots powered by hydraulic systems A and B provide pitch inputs to the Control Path through the Aft Cross Tie. Trim follow-up is brought the mechanical Feel/Trim Unit. Therefore, the Stabilizer servos receive the input signals the same way from the autopilot as they would from pilot inputs. The Electric Trim Thumbwheel on the control column, however, is de-energized when the autopilot is in operation, except in Control Wheel Steering (CWS) mode when then Pilot applies the necessary force to the control column.
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