Auto Flight

- James Albright (a former G450 driver)

Updated: 2016-12-18

"Auto flight," not too long ago, was a pretty simple subject and warranted itself a single chapter in the Air Transport Association (ATA) system, that being Chapter 22. But in this day and age auto flight systems are deeply integrated with the rest of the avionics. So here we shall look at the mechanics and leave everything else for other chapters, such as G450 Avionics.

 

[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.

Everything here is from the references shown below, with a few comments in an alternate color.

 Autopilot

[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.

 

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 an internal "servo loop" and a motor micro-controller.

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

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.

 

Auto Throttle

[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.

 References:

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