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ATC-pie logo
Tower viewing, following a departing aircraft
Tower viewing, following a departing aircraft
Developed by Michael Filhol
Initial release February 1, 2015
Latest release 1.8.8 (April 1, 2023)
Written in Python (Version 3)
OS Any
Platform Qt5
Development status Active
Type ATC client
License GNU GPL v3

ATC-pie is a free (libre) air traffic control simulation program with strong ties to FlightGear. It features:

  • solo sessions with AI traffic (incl. voice instruction recognition and pilot read-back);
  • "multi-player" network sessions (FlightGear and FSD protocols supported);
  • tutorial sessions for teacher supervision of an ATC student.

It is designed to support a maximum range of ATC situations (roles, equipment...), at any world location and for every session type above. All control positions are possible, whether airport-based (TWR, APP, GND...) or en-route (CTR). Equipment may include radar screens, data link, etc. or be limited to binoculars and a view of the airfield.

Its essential goal is realism. It simulates many tasks of real-life ATC such as:

  • strip racks and sequence management;
  • coordination with neighbouring controllers (handovers, voice phone calls...);
  • radar monitoring and identification of traffic;
  • vectoring and course/level conflict anticipation;
  • flight plan operations;
  • CPDLC...


Visit the ATC-pie screenshot category for more.

Detailed feature list

Sessions and environments

Session/connection types:

  • solo simulation (AI traffic)
  • FlightGear network connection (FGMS protocol)
  • FSD connection (as served by commit bc7d43, latest available in Dec. 2022)
  • teaching service (spawn and simulate traffic visible to a connected student)
  • student session (control traffic simulated by teacher)

Location modes (available for all sessions):

  • airport (AD): positions such as TWR, GND, APP, DEP at a selected airfield
  • en-route centre (CTR): free positioning of radar, no base airport or runway-related options
Feature support by session type
Solo FlightGear FSD Tutoring (teacher/student)
ACFT traffic AI aircraft generated according to RWY capacities, ACFT equipment, intentions... connected flight sim pilots created and simulated by teacher
ATCs and coordination virtual ATCs depending on assumed positions connected ATC clients (full ATC-pie interaction, interoperability with other software) teacher-configured ATCs
Voice radio voice recognition for instructions (mouse-only also available) and synthesis for pilot read-back FGCom-mumble integration teacher simulates pilots
ATC phone lines N/A integrated Mumble connection teacher simulates ATCs
Flight plans local entries only interface with the FlightGear de facto data base by Lenny64 available from network (NB: protocol does not support open/close and only pilots can file/amend FPLs) shared virtual online system
CPDLC interpreted subset of instruction messages integrated (supported by MP IRC) integrated interface with Hoppie's ACARS network full simulation by teacher (with WILCO assistance to execute instructions)
Weather randomised and evolving real world METAR retrieval fetch from server or retrieve real world METAR controlled by teacher
Other specific features
  • aircraft type and airline choice with custom appearence in tower view
  • configurable airspace rules and traffic density, incl. uncontrolled distractors
  • exact aircraft rendering in views (incl. control surfaces, etc.)
  • legacy stand-alone FGCom alternate possible (although deprecated)
  • frequency tuning system for radio text chat
  • text ATIS repeatedly sent through radio chat on recorded frequency
  • traffic snapshots and recall to repeat situations with the student
  • individual aircraft freeze

ATC surveillance

Radars and tracking:

  • SSR mode capability selection (none/A/C/S)
  • primary radar toggle
  • traffic identification assistant
  • position/track vs. strip assignment mismatch warning system
  • route/vector conflict anticipation
  • separation incident alarm
  • runway occupation/incursion detection

Tower view in airport mode (rendered by FlightGear):

  • view of airport, aircraft, weather, time of day
  • start internal process or use externally running instance
  • control panel to orient/zoom view or follow aircraft
  • additional views can be connected (for multiple camera angles)


  • radio direction finding (RDF) and integration to radar
  • multiple weather (METAR) station monitor

Traffic management

Strips and racks:

  • user-defined strip racks with configurable colours (for linked radar contacts) and ATCs to receive from
  • runway boxes with automatic RWY separation timers
  • loose strip bays with customisable backgrounds

Flight plans and routes:

  • flight plan system (file, edit, open, close, publish/retrieve online)
  • world route suggestions, presets, analysis, radar drawing and world map view
  • departure clearance assistant
  • automatic strip printing for expected departures or arrivals (from FPLs)

Radar tools:

  • convenient mouse input for instructions (vectors, taxi...) and CPDLC integration
  • approach spacing hints (estimated touch-down time difference, sequence optimisation suggestions)
  • quick point-to-point heading and distance measuring tool
  • direct text annotation of radar screen
  • flag/unflag (highlight) radar targets


With aircraft:

  • voice radio with 8.33 kHz frequency spacing, multiple radio transmissions and monitoring
  • ATIS recording and reminder alarm (see dialog with pre-filled notepad)
  • controller-pilot data link communication (CPDLC), incl. DEP clearance delivery, multi-element messages...
  • text radio chat with preset messages, auto-completion, predefined and custom aliases (context-sensitive replacements), sender blacklist

ATC coordination:

  • strip exchange (handovers)
  • CPDLC authority transfers
  • telephone lines and switchboard (direct voice communication)
  • text messaging (private channels and general ATC chat room)
  • "who has?" requests


Misc. tools:

  • world airport, map navpoint and AD parking position browsing/indicating
  • aeronautical unit conversion calculator
  • custom alarm clocks with quick keyboard timer start
  • general and location-specific notepads restored between sessions


  • multiple window workspace (radar screens, strip racks and bays) saved by location
  • floatable/dockable panels and toolbars (see screenshot) and layout save/restore
  • notification system combining selectable sounds, status bar messages and time-tagged history
  • customisable style and colours

Data sources:

  • airport and navigation data sourced in the X-Plane format (old world-wide default file set provided but custom imports recommended)
  • editable aircraft data base (ICAO designators, cruise speeds, WTC, etc.)
  • custom radar background images and hand drawings (EuroScope/VATSIM/IVAO "sector file" conversion tool included)
  • ground elevation maps (can be generated automatically with a provided script if FlightGear terrain data available)
  • manual magnetic declination input

Interoperability with other software


OpenRadar is another stand-alone program able to connect to FlightGear networks. ATC-pie and OpenRadar's philosophies differ in several ways:

  • OpenRadar's basic processing unit is the FGMS callsign, whereas ATC-pie's is the strip;
  • OpenRadar's concept of handover is based on a shared notion of aircraft ownership, whereas ATC-pie allows any controller to pull out a strip and write a callsign on it;
  • in OpenRadar, a handover must be acknowledged by the receiver for the sender to lose ownership and for all neighbouring users to see it complete, whereas ATC-pie considers that a strip sent is gone and assumed to land on the receiver's rack, without anybody else necessarily to know.

For most interactions to work in FlightGear sessions while respecting both approaches as much as possible, the following principles and restrictions apply to strip exchange between the two programs:

  • ATC-pie users can only hand over strips to OpenRadar that are linked to a radar contact;
  • aircraft under ATC-pie control are not shown as "owned" to OpenRadar users;
  • handovers from ATC-pie will fail if an OpenRadar user is claiming ownership on the linked radar contact;
  • when sending to ATC-pie controllers, OpenRadar users will see their transfers acknowledged straight away, unconditionally.

Callsign handover policy:

  • OpenRadar to ATC-pie: FGMS callsign will appear on the strip, as if the sender had filled the detail herself;
  • ATC-pie to OpenRadar: callsign resolved for the receiver, sender's entry will reappear next time ATC-pie handles the strip;
  • pie-to-pie handovers through OpenRadar's service: strip detail preserved, whether present or absent.

Features not supported by OpenRadar:

  • wake turbulance category on strips (but detail preserved for ATC-pie clients later receiving the strip);
  • ATC text messaging;
  • ATC phone lines;
  • CPDLC transfers.

Who-has requests are fully supported.


Euroscope is a popular program to control on VATSIM, a flight simulation network whose protocol is historically based on FSD. It has been increasingly tailored for VATSIM, although for a long time it allowed also to connect to "plain" (non-VATSIM) FSD servers. Operability outside of VATSIM is now discontinued all together, but older versions of Euroscope are still around and connecting to FSD networks. ATC-pie is able to interact with them in FSD sessions, but only to a limited extent:

  • sending a strip to Euroscope will result in a loss of all strip details but the callsign (which must be connected), the only information left to the recipient being the FPL details for that callsign if any (strip changes made after FPL data retrieval are therefore lost);
  • receiving a strip from Euroscope is supported, but the sender will see the hondover pending (never "assumed");
  • who-has requests will remain unanswered by Euroscope;
  • there are no integrated phone lines to Euroscope clients.