Special Transponder codes:

Squawk codes also enable the controller to easily locate a aircraft in an urgency or an emergency situation before being contacted by the pilot.
Transponder codes are usually assigned by the controllers except for those 4 dedicated to specific situations :

• 7700 - Emergency, this will be an indication on the controller's screen that your aircraft has experienced an emergency.

• 7600 - Radio Failure, this code is not used very often on IVAO 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.

• 7500 - Hijack, this code is a part of the tutorial for informational purposes only. Although terrorism and war exist in the real world we in IVAO do not allow the simulation of any kind of aggression or armed violence on our network. This rule can be found here http://www.ivao.aero/rulregs/pilots.htm.

• 2000 - default transponder code for an aircraft which has not received any instructions from air traffic control units to operate the transponder.
  Note: In some countries, a non-discrete squawk such as 1200, 7000, .... may be used by VFR's in uncontrolled airspace. Check the country regulations of the Division for more information.

• 1000 - non discrete code used in Mode S airspace where the flightplan is not correlated using a squawk code but using the aircraft identification.

What's a squawk code?

To make it possible for an ATC to know "who is who and where" on his display, aircraft are equipped with a device called a "transponder". The transponder transmits a 4-digit code as a reply to an interrogation by a radar station. The 4 digit code is called a squawk code. Each digit of a squawk varies from "0" to "7" only (octal numeral system). There can never be an "8" nor a "9" in a transponder code. By combining 4 numbers from "0" till "7" ("0000" - "7777"), 4096 different squawk codes are available.

Pilots will select a particular "non-discrete" transponder code such as "2000" to show their presence to ATC before they have been able to make contact. A "non-discrete code" is a set of 4 numbers that may be used by several aircraft in the same area.

By making a particular aircraft squawk a specific "discrete" code, the controllers can easily see "who is who" amongst other aircraft. "Discrete code" means that a particular squawk code has been assigned to 1 aircraft only by ATC.

The squawk code is also used as a primary means to "correlate" or link a flight plan to a specific aircraft. If a pilot has filed a Flight Plan and received a discrete squawk code, Flight Plan information such as aircraft callsign, aircraft type and wake turbulence category are shown in the aircraft label on the ATC's Situational Data Display (SDD).

Transponder settings

When squawking "stand-by", controllers can still see the aircraft blip on their screens but without any label with flightplan information. Stand-by is required when on apron, before departure and after landing. That way, the pilot does not cause any disturbance in the ground traffic, as all the extra information can clutter the controller's screen. On IVAO, stand-by is simulated by setting the transponder "OFF".

Set the transponder "ON" when entering the active runway for departure until vacating the runway after landing at the end of your flight.

Squawk IDENT can also be requested by ATC. This highlights the aircraft label on the controller's screen so that the controller can easily identify the traffic if he/she has a doubt. IDENT is accessed with the little button in the IvAp window and only should be pushed when requested by a controller only.

Transponder Equipment Modes

Always mention in your Flight Plan what information the transponder equipment on board of the aircraft is sending. If the transponder is "not working properly", select the letter that corresponds to the remaining equipment capability.

Mode "A"

This transponder sends only the 4-digit squawk code that was set by the pilot to the radar antenna. In this case no altitude information (mode C) is transmitted when a radar station interrogates the transponder.

Mode "C"

This type of transponder sends the 4-digit squawk code but also the pressure altitude at which the aircraft is flying.

Mode "S"

Since traffic has increased a lot over the years and because more ATC Units received radar equipment, the 4096 squawk code combinations are no longer sufficient to give different squawks to all aircraft. Besides Mode "A" and "C" information, this generation of transponders also sends the aircraft identification, selected heading, selected altitude, ... and much more. The additional information can only be decoded by an ATC system that is adapted to do so.Mode S stands for "selective interrogation" where every transponder is interrogated seperately by each radar station.

Source : IVAO Academy , http://academy.ivao.aero/node/49