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Figure: G450 AFSC big picture, (FSI G450 MTM, figure 22-1)

[G450 MM, §22-00-00, ¶1.A] The autoflight system functions as a completely Automatic Flight Control System (AFCS). The AFCS is dual system, fail-operational, for both the Autopilot (AP) and the Auto Throttle (AT) subsystems. This is accomplished by hosting each system in a different modular avionics unit with two lanes in separate processors. The purpose of the AFCS is to provide Flight Director (FD), AP yaw damper and trim functions. The autoflight system receives computed lateral (roll) and vertical (pitch) steering commands from the FD system for the autopilot and for display on the Primary Flight Display (PFD). In the event the AP is disengaged, the pilot can manually fly the steering commands presented on the PFD. The AP and AT systems make up the subsystems of the autoflight system.


[G450 MM, §22-10-00, ¶3]

  1. Autopilot Manual Control Input — The AP manual control input system provides the flightcrew with the ability to input new data and change existing data in the Automatic Flight Control System (AFCS).
  2. Autopilot Roll and Heading Control — The AP roll and heading control system automatically steers the aircraft to ensure turns to a new heading are coordinated, stable and smooth. The system also holds the aircraft to a manual heading or returns it to a preset heading when necessary. The AP roll and heading commands can be supplied electronically by the primary Flight Management System (FMS) or manually by the flightcrew.
  3. Autopilot Pitch Control — The AP pitch control system controls the aircraft when climbing or descending to a different altitude to ensure a smooth and safe altitude change. This system also holds the aircraft at, or returns it to, a commanded altitude and pitch angle. This includes changes in altitude under the direction of the AFCS or the pilot. The AP pitch control system also compensates for external influences such as wind, turbulence, and temperature along with aircraft changes such as fuel usage to maintain commanded pitch attitude.
  4. Autopilot Yaw Control — The AP yaw control system supplies aircraft stabilization around the vertical axis. The Yaw Damper (YD) supplies dutch roll damping and turn coordination for manual or AP commanded turns. The YD rudder position command is influenced by inertial and air data parameters.
  5. Autopilot Digital Interface — The AP digital interface system consists of a Control Area Network (CAN) bus to interface between the actuator input / output processor modules, in the Modular Avionics Units (MAU) and the servos and actuators. All data transmitted to and from the MAU modules is put on the ASCB-D by the network interface module making the data available to other aircraft systems. The CAN bus is based on the CAN industry standard and uses controller integrated circuits that operate at 500 kHz. The CAN bus is bidirectional and uses the same wire and harness construction as the ASCB-D.
  6. Stall Warning and Protection System — The stall warning and protection system supplies a dual stage stall warning to the pilot. The first warning level alerts the pilot to a potential aircraft stall condition. The second warning level applies a pitch down force to the control column.

Where is the autopilot?

The autopilot isn't so much a "black box" as it is a bunch of computers on cards, servos, and interfaces. The primary elements of the autopilot reside in "Actuator Input / Output (AIOP) Modules" in the MAUs. Each AFCS has two AIOP modules which are identified as "Lane A" and "Lane B" respectively.


Figure: G450 AFSC modules, (FSI G450 MTM, figure 22-3)

How does the autopilot move the flight controls?

Dual servos on single brackets are used to control aileron and elevator flight controls when the autopilot is engaged. A servo is basically a DC motor attached to a internal "servo loop" and a motor micro-controller.


Figure: G450 AFSC servo, (FSI G450 MTM, figure 22-6)

The elevator pitch trim servo is in the tail compartment just aft of the #1 battery.


Figure: G450 AFSC elevator trim servo, (FSI G450 MTM, figure 22-7)

The rudder actuator is electrically controlled and hydraulically actuated. Two hydraulic servo valves receive commands from two channels of the AFCS for yaw dampening and turn coordination.


Figure: G450 rudder actuator, (FSI G450 MTM, figure 22-8)

How does the autopilot get feedback from the flight controls?

Figure: G450 surface position RVDTs, (FSI G450 MTM, figure 22-10)

[FSI G450 MTM, §22, pg. 23]

  • There are seven surface positions that support the AFCS system. These RVDTs are used for the following surfaces (Figure 22-10):
    • Ailerons (2) — Left and Right Wing tip
    • Elevators (2) — Vertical Stab doghouse
    • Elevator Trim (2) — Left and Right Vertical Stab
    • Rudder (1) — Aft Tail Compartment
  • These RVDTs provide position feedback information to the AFCS system for position comparison data to the servo actuator position feedback data. The system also uses this data to present the surface position on the flight controls synoptic page and it sends this information to the flight data recorder.
  • Exceptions to the above statement are the elevator trim RVDTs and the rudder RVDT. The elevator trim position data is the only source of position feedback for the trim surfaces. The TM-260 elevator trim servo does not provide position feedback to the AFCS based on surface position. The rudder RVDT is only used for the rudder limit message presented by the crew alerting system. Position data from the rudder comes from the rudder actuator LVDTs.

Auto Throttle


Figure: G450 Auto Throttle System, (FSI G450 MTM, figure 22-76)

[G450 MM, §22-30-00]

  • The Auto Throttle (AT) operates as a regulator of engine power from takeoff to landing. It automatically controls the throttles to maintain the correct speed or thrust for the selected mode of operation. The system uses processors, AT servos, and limiters to control the position of the throttle levers.
  • The AT Full Authority Digital Engine Controller (FADEC) interface system provides interfaces between the two AT systems, two Electronic Thrust Trim Systems (ETTS) and a dual thrust rating selection.
  • While engaged, the AT system automatically sets the throttle levers to control the aircraft thrust throughout the flight. The AT system changes the engine thrust to keep the aircraft within thrust and speed limits.
  • During dual operation, the AT systems operate in synchronization with the ETTS to ensure only one AT system controls the throttle servos. The AT systems also operate in synchronization with the ETTS to ensure only one AT system controls the electronic trim of each engine by the FADEC.
  • An AT servo is installed on each throttle lever. The AT servos supply automatic control of the aircraft throttles. The servos also allow the pilot to manually override the aircraft throttles during each of the AT flight modes.
  • [FSI G450 MTM, §22, pg. 110]

  • The AT provides speed and thrust envelope limiting. Thrust envelope limiting is based on the active Engine Pressure Ratio (EPR) rating while speed envelope limiting is based on minimum speed limits as well as placard and structural speed limits. AT thrust envelope limiting is provided while the AT is engaged in closed loop thrust control. AT speed envelope limiting, as well as thrust envelope limiting, is provided while the AT is engaged in speed mode.
  • The AT can provide control during a single engine out condition. The AT removes power from the throttle servo of the affected engine and allows manual repositioning of that throttle without disengaging the AT. The AT disengages when an engine out is detected only when in AT HOLD mode during takeoff.
  • The AT active mode annunciators, including system failure and status annunciators, are displayed on the PFD. System failure messages are displayed on the EICAS. The system also incorporates a monitoring system that verifies the following:
    • System validity
    • Servo response
    • Pilot-override conditions
  • The system monitors ensure that all AT parameters that are required for AT control are on-line and valid. It also detects an engine-out condition, thrust reverser deployment, and throttle quadrant internal monitor faults.
  • The servo monitor compares the servo response to the commanded response, to verify the integrity of the servo control system.
  • The pilot-override monitor detects when the pilot moves the throttle levers while the AT is engaged. If throttle motion is detected, the AT automatically disconnects.


* FSI G450 MTM, FlightSafety International Gulfstream G450 Maintenance Training Manual, August 2008

Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013

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Revision: 20161218