By Aaron Flodin

Important Terms

  • ATC - Air Traffic Control
  • Transponder - A radio transmitter in aircraft used to identify the aircraft to ATC
  • Squawk Code - A code given by ATC to radar identify aircraft. This code is entered into the transponder aboard the aircraft.
  • Data tag - The information displayed on an ATC's screen

Welcome to the VATSIM PRC Transponder and Squawk Code information guide. In this short tutorial we will cover the basics of using your transponder. By the end of this course you should understand the reasons why we use a transponder, the terminology related to its usage, and when transponder use is necessary.

The function of the transponder is to transmit a signal to a ground radar station. Signals from this radar station provide ATC with information such as aircraft identification, altitude, etc.

The ATCRBS, or Air Traffic Control Radar Beacon System, was developed to keep extraneous echoes off the controller's scope. The "beacon" is what we call a transponder. The ground station sends out an "interrogation" and the transponder in the aircraft replies. Because there are many different aircraft in the sky at one time, ATC's needs a way to differentiate between the aircraft that they are responsible for. This is where discrete transponder codes come in to the picture.

Using your transponder
In most flight simulator aircraft your transponder is located in the same pop-up window as your radio stack (this is by default SHIFT+4). When you open the panel, at the top you will see your communications and navigation radios all with their Primary and Standby frequencies, these are all great, but what we're after is that little tiny one towards the bottom with the 4 numbers in it. The numbers should be 1200. Right below and next to these numbers you will see several Buttons; we'll go through the uses of these buttons below.

STBY- The standby function of the transponder makes it so the controller doesn't have to see all of the information about your flight on his screen. The situation where this is most often used is when you're on the ground at an airport. With so many planes in a tight space, the controllers screen can get very crowded. If you hear a controller ask you to squawk Standby, this is what they mean. (Don't forget to turn it on when you are at the runway, ready for takeoff)

ALT- The ALT key on your transponder turns on the altitude reporting or "Mode C" function of your transponder. This key can also be labeled "NORM". If you hear a controller tell you to "Squawk normal" or "Squawk Mode Charlie" This is what they mean.

IDENT- This function on your transponder puts a special mark on the controller's screen that lets him know that it is you who are "Identing". This is one of the methods controllers use to identify aircraft.

OFF- Pretty obvious eh.

As we have discussed earlier, transponders are capable of transmitting a discrete code that helps ATC identify the aircraft, this is what the 4 numbers are for. If ATC gives you a squawk code, for example

ATC: "Warrior N149FA Las Vegas approach, Reset (or recycle) transponder, squawk 0123 and ident"

You would change the numbers in your transponder to "0123" and then press your ident key. This action will do two things: one, on the controllers screen they will see your aircraft change from a little blip to a full data tag and be able to track your aircraft, two, that special signal that we talked about earlier, will show up on their screen to show that you have idented, this is so the controller knows for sure they have started tracking the correct aircraft.

Transponder use in the VFR environment
By now you have learned the difference between IFR and VFR flight operations. If you need a refresher please go read "VFR or IFR?". We will now look into how your transponder can be used in VFR flight operations. As a VFR pilot you are responsible for your own terrain and traffic separation, but that does not mean that you are completely on your own. One of the many services provided to VFR aircraft is called "flight following". Using "flight following" a controller will assign you a discrete transponder code and be able to see and track you on his radar scope. You as a VFR pilot are still responsible for separation, but the controller is able to issue traffic point outs and other advisories while you are enroute. The example below depicts a normal VFR aircraft requesting flight following.

ACFT: "Las Vegas Approach, Warrior, 20 miles east of Boulder City, request flight following to Henderson"

ATC: "Warrior N149FA Las Vegas approach, Reset transponder, squawk 0123 and ident"

ACFT reads back: "0123 and Ident, N149FA"

You can read more about flight following in VFR Radar Services

In addition to using your transponder to receive radar services from ATC, your transponder must also be used simply to broadcast your altitude in some cases. As a general rule, anytime you are airborne on VATSIM, have your transponder turned on and be squawking Normal (Mode C). One of the few exceptions to this is military operations and formation flying, you can read more about that in the SpecOps section of the PRC.

When you are not in contact with ATC you should be squawking 1200, the default transponder code for all VFR aircraft. While not always required, Squawking Mode C all the time can help ATC route aircraft around you even if you are not in contact with them.

Transponder use in the IFR environment
In the IFR environment your transponder is your link to the outside world, you can be in the clouds for hours on end without worrying about running into anything because you know that ATC is tracking your aircraft's transponder. Most of the time, you will only adjust your transponder when you receive your IFR clearance, when you change to mode C on takeoff, and when you switch back to standby after you land.

After you depart, most of the time, the code that you were originally assigned is yours for the rest of the flight. When you first start flight simulator, your transponder should be on standby and the numbers should indicate 1200. After you call ATC and receive your clearance, immediately enter your assigned squawk code in to your transponder. Leave your transponder on standby for taxi and as soon as you are cleared for takeoff switch it to "Mode C" or "NORM"

Special Transponder Codes
In addition to being used as a method of identification, your transponder can also be used to transmit a signal for help to ATC. I have listed some of the codes below:

  • 7700 - Emergency, this will be an indication on the controller's screen that your aircraft has experienced an emergency. Just as a reminder, it is OK to declare an emergency on VATSIM just as long as you ask the controller for permission first, if they decline the emergency, you must disconnect..
  • 7600 - Radio Failure, this code is not used very often on VATSIM because we have both Text and Voice communications. If you would like to simulate lost comms procedures, make sure to check with your controller first.
  • 7777 - Military Interceptor Operations, consult the VATSIM Special operation regulations before engaging in any Military Interceptor Operations.
  • 7500 - Hijack, this code is a part of the tutorial for informational purposes only. Simulating hijackings is ABSOLUTELY NOT ALLOWED on the VATSIM Network. Should you set your transponder to this code you will automatically be removed from the network. Please do not "Try to see if it works", Users simulating a hijacking are subject to disciplinary action up to and including permanent suspension of their PID.

VFR Squawk Codes of the world

  • 1200- VFR flight, this is the standard squawk code used in North American airspace when no other has been assigned.
  • 7000- VFR standard squawk code for most of European airspace.
  • 0021- VFR squawk code for German airspace (5000 feet and below)
  • 0022-VFR squawk code for German airspace (above 5000 feet)


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