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Started in September 2018
Description GA Autopilot
Contributor(s) Octal450
Status Stable

The IT-STEC55X autopilot is a complex simulation of the S-TEC 55X Autopilot System, developed from the real thing!

Download IT-STEC55X from here!

Installation Instructions


This guide will teach you how to install the IT-STEC55X System to an aircraft.

Before you Begin

Download the latest version of IT-STEC55X. Extract the Archive with WinRAR, 7Zip, or similar. If you are using a custom instrumentation file, make sure it contains the <turn-indicator> and <heading-indicator> item. If you are using a custom systems file, make sure it contains at least one <static> item. For realism, you should add and assign a dedicated static port to the S-TEC 55X. See Configuration. Make sure your electrical system outputs to the correct properties with at least 8 volts:

  • Autopilot Power: "/systems/electrical/outputs/autopilot"
  • Electric Trim Power: "/systems/electrical/outputs/electrim"

Step One

  • Copy the Systems and Nasal folders from the IT-STEC55X folder to the aircraft's folder. Overwrite any conflicts.
  • Open the -set file of the aircraft, and find the <systems> area inside <sim></sim>.
  • Add the following inside <systems></systems>

Now to add the IT-STEC55X controller:

  • Find the <nasal> area, which is outside <sim></sim>.
  • Add the following inside <nasal></nasal>:
  • Finally, add the following anywhere outside <sim></sim>:
  • Copy and Paste from this file, start at line 6.
  • ITAF CONFIG.txt.
  • This is the default IT-STEC55X Configuration. We will be adjusting this later.

Step Two (Optional)

  • Follow this step if you want to use the included dialog.
  • Copy the gui folder from the IT-STEC55X folder to the aircraft's folder. Overwrite any conflicts.
  • You may need to remove old autopilot dialogs for the IT-STEC55X dialog to appear. Look inside the Systems and gui/dialogs folders.

Step Three (Optional)

  • Follow this step if you want to use the included 3D Head Unit.
  • Copy the stec-55x folder from the 3D folder in the IT-STEC55X folder to the aircraft's instruments folder. This is usually Models/Instruments, or Models/Cockpit/Instruments. Make sure you note the location. Overwrite any conflicts.
  • Open your cockpit XML. This will be a different file for most aircraft. You will need to find it on your own.
  • Add the following. Make sure you change the path to EXACTLY MATCH the location of the stec-55x folder. Make sure the case of the letters is correct. For example, Instruments is not the same as instruments.
  • Now you will need to adjust the offsets to position the instrument properly. You will need to do this on your own.
  • The property used for the backlighting on the display is "/controls/lighting/panel-norm". You may change it if you wish.

Step Four

You will need to provide your own Autopilot Master, Electric Trim Master, Electric Trim, Autopilot Disconnect, and CWS Mode switches. This is because they are not located on the S-TEC 55X Head Unit. I have included an Autopilot Disconnect in the AP Dialog for ease of use.

  • Autopilot Master (No Flight Director): "/it-stec55x/input/ap-master-sw" (0 = OFF, 1 = ON)
  • Autopilot Master (With Flight Director): "/it-stec55x/input/apfd-master-sw" (0 = OFF, 1 = FD, 2 = AP/FD)
  • Electric Trim Master: "/it-stec55x/input/electric-trim-sw" (0 = OFF, 1 = ON)

The following bindings MUST be used in order for the following switches to function properly. An electric trim handler for joysticks and keyboards will be added to this guide soon.

  • Electric Trim Up
  • Electric Trim Down
  • Autopilot Disconnect
  • CWS Switch


You have finished installing the IT-STEC55X System. However, I highly suggest to go over the Configuration section below, to set up the settings.


Below is listed each area of the IT-STEC55X Config Module, and what it is used for.

You can view the Config Module here.


These settings will change the behavior of the autopilot, as per real life installations.

  • fd-equipped: If the aircraft is equipped with a flight director, set this to 1.
  • gain-pitch: Adjusts the output gain of the pitch axis.
  • gain-roll: Adjusts the output gain of the roll axis.
  • gain-yaw: Adjusts the output gain of the yaw axis.
  • hsi-equipped-1: If the aircraft uses an HSI or Nav Display for NAV1, set this to 1.
  • hsi-equipped-2: If the aircraft uses an HSI or Nav Display for NAV2, set this to 1.
  • is-turboprop: If the aircraft has turboprop engines, set this to 1. (Reduces turn rate limits)
  • static-port-path: Provide the property path to a dedicated static port to use, do not tie into a selectable system.
  • use-controls-flight: If set to 1, control loop outputs will be locked onto /controls/flight properties when the servos are engaged. See below for more information.
  • yd-equipped: If the aircraft is equipped with the optional yaw damper, set this to 1.


About Control Loop

The IT-STEC55X System is rate based, like the real S-TEC 55X. It uses the FGFS Turn Coordinator for the roll axis. For the pitch axis, the autopilot uses its own static pressure source, for pressure rate, and an internal acceleration sensor. The optional yaw damper uses an internal yaw rate sensor.

About Control Loop Gains

The IT-STEC55X System uses a carefully designed control loop to work seamlessly on almost any GA aircraft with plausible flight dynamics. If however, your aircraft is unusually responsive, or unresponsive, you can adjust the axis gain as needed. This will change the overall response of the system, and can be modified at any time. See the Configuration section above for more info on these gains.

About Control Loop Outputs

The IT-STEC55X System can output to the following generic properties if the "use-controls-flight" setting is 1:

  • Roll Axis: "/controls/flight/aileron"
  • Pitch Axis: "/controls/flight/elevator"
  • Yaw Axis: "/controls/flight/rudder"

If you want to make the autopilot part of a custom flight control system, such as used in the PA28, set "use-controls-flight" to 0, and have your FCS read from the following properties:

  • Roll Axis: "/it-stec55x/internal/aileron"
  • Pitch Axis: "/it-stec55x/internal/elevator"
  • Yaw Axis: "/it-stec55x/internal/rudder"

Note: The Electric Trim system always outputs to the generic property currently. The option to output to another property for trim may be added later.

The following properties can be used in your FCS to determine if the Autopilot should be coupled to the controls or not:

  • Roll Axis: "/it-stec55x/output/roll" greater than -1
  • Pitch Axis: "/it-stec55x/output/pitch" greater than -1
  • Yaw Axis: "/it-stec55x/output/yaw" greater than -1

It must be used as a greater than, because -1 means that the axis is off. -2 means that the CWS mode is active, but the servos are disengaged (excluding yaw). Any mode starting with 0 is an active control mode.

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