Aviation Theory

INTRODUCTION

The function of the transponder is to transmit a responding signal to a secondary ground radar which provides ATC with additional information such as aircraft identification, altitude, etc. The theory of radar operation is beyond the scope of this lesson, but, since the radar environment is key to simulated ATC, the use of the transponder is an important topic.

USAGE

The transponders in most civil aircraft are 4096 code transponders. When a code is selected and the transponder is on, it is referred to as "squawking" and the code is called the "squawk code". The highest number that may be selected is 7777.
The international emergency code is 7700. Two other codes have special emergency meanings. Code 7600 is used for communication radio failure and code 7500 is the international hijack code. In actual flight, these three codes set off automatic alarms in ATC facilities and should therefore never be selected, even momentarily, such as when changing codes. The code 1200 is used for VFR flights in the United States and 7000 is used for VFR in Europe. IFR flights use a code specified by ATC in their clearance.

As to flying on IVAO and VATSIM networks: since the SB/PC environment is entirely a radar environment, transponder use is of benefit to both controllers and pilots.

Radio terminology used in reference to transponders is also important in SB/PC so that pilots and controllers will be working on the "same page". For example:

ATC: "...(callsign) squawk 4021"
Pilot response: "...(callsign) squawk 4021", and select 4021 on the transponder.

A similar transmission would be to "squawk code 4021 and ident". This means to select the proper code and press the IDENT button on the transponder.