Transponder

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

7600 - Radio Failure

7500 – Hijack

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 separately by each

radar station