1.GENERAL
A. The automatic flight control systems (AFCS) consist of three independent systems: digital flight control system (DFCS), yaw damper system, and autothrottle system. These systems provide automatic airplane stabilization about the pitch, roll, and yaw axes and control the airplane with selective guidance from radio, heading, flight management computer, and air data computer inputs.B. The DFCS system is a two-axis (pitch and roll) system which operates the elevators and ailerons to automatically maintain altitude, airspeed and/or guide the airplane to designated locations and make automatic landings. Control functions are also translated into flight director commands for display on the pilots' attitude director indicators (ADI's)[electronic attitude director indicators (EADI's)]. thereby providing the pilots' flight attitude commands during manual operation or allowing the pilots to monitor autopilot operation. Automatic stabilizer trimming relieves sustained elevator loads which might be incurred due to fuelburnoff.C. The yaw damper systems operate the rudders to correct any periodic yaw oscillations (dutch roll).D. An autothrottle system automatically maintains selected airspeeds or Mach during cruise conditions and maintains selected engine thrust settings when making flight director controlled take-offs or autopilot/flight director controlled landing approaches by adjusting engine thrust levers.
2. Digital Flight Control System
A. The DFCS incorporates two separate channels (A, B). Each channel controls the pitch and roll axes and provides mach trim and speed trim control. Flight director commands and flag logic are connected to the captain's ADI [EADI] (system A) and the first officer's ADI [EADI] (system B). DFCS control of the airplane is enabled by engaging channel A or B switch on DFCS mode select panel. When dual channel option is provided, channels A and B can be engaged simultaneously when making fail-passive automatic landings.B. The Mach Trim System provides automatic respositioning of the elevators as a function of Mach number. As the airplane enters the Mach tuck region, the elevator is repositioned to provide a new neutral in an upward direction which is proportional to the increase in Mach. The Mach Trim System operates with or without the autopilot system engaged. The Mach Trim System operates in conjunction with the elevator hydraulic power control units and stabilizer/elevator neutral shift mechanism.Each flight control computer (FCC) includes a Mach trim computer with automatic switching to other system if failure occurs.C. The speed trim system actuates the stabilizer trim to maintain positive speed control. The system may be required during takeoff or go-around with low gross weight, aft CG and high thrust. The system is operational when autopilot is not engaged, stabilizer is not trimming, and control column displacement switches are not actuated. Each FCC contains speed trim computer with automatic monitoring and automatic switching between systems (when no faults). The monitoring automatically selects the operational channel when a single channel failure occurs.D. Each DFCS channel consists of an FCC, an ADI [EADI], and two A/P actuators. A mode select panel controls autopilot/flight director systems and the autothrottle system. An AFC accessory unit (or Integrated Flight System Accessory Unit) and automatic stabilizer trim servo are also installed as well as elevator spoiler, flap position, and stabilizer position sensors.E. Autopilot/flight director disengage warning consists of autopilot warning lights and wailer operation when certain autopilot functions are not correct and/or the autopilot is disengaged. ADI [EADI] pitch and/or roll bars are biased out of view and/or the flight director flag (or computer flag) is biased into view as an indication of certain flight director malfunctions.
3. Yaw Damper System
A. The yaw damper system is a full-time, series-connected, stability augmentation system. Airplane periodic oscillations (dutch roll) are detected with a rate sensor in a yaw damper computer. The rudder is displaced at the proper time to dampen out any dutch roll before it can significantly affect the flight path of the airplane. The yaw damper system and actuators are connected such that no rudder feedback is applied to the pedals, thereby allowing the system to operate independently without interfering with pilot initiated rudder commands.
4. Autothrottle System
A. The autothrottle system automatically positions all thrust levers to maintain a computed engine thrust level during take-off or go-around, and a selected or FMC computed airspeed (IAS or Mach) during cruise. This system also computes and maintains a safe airspeed during holding patterns and initial approach manuevers. The autothrottle system also retards the thrust levers during automatic landing during flare. The autothrottle system consists of a computer which operates the thrust levers through integrated servo assemblies.
