Avionics Transponders

Almost all airplanes are required to be equipped with transponders. These communication tools identify your plane to the controllers on the ground and help keep aircraft separated and safe. If you fly, part of your role as a pilot is to be sure you have the right type of avionics transponder for your plane and your flight habits.

What are avionics transponders?

These devices are radio transmitters in airplanes. They are found on the instrument panel in the cockpit and they communicate with ground radar. Transponders may report the plane’s altitude, position, and call sign, depending on the type. This information appears on the radar of air traffic controllers.

What is a squawk code?

This is the 4-digit code a plane is given by the air traffic control system. You enter this code into the transponder on your plane, and it responds with this code when it is contacted by ground radar. Squawk codes use numbers from 0-7.

What types of avionics transponders are available?

Transponders for civil aircraft operate in 3 modes: Mode A, Mode C, and Mode S. Modes are sometimes called interrogation types. If you are flying in busier areas, you will be required to have at least Mode C, and perhaps Mode S, equipment.

Mode A:

This transponder is only capable of sending your four-digit squawk code.

Mode C:

This type transmits the squawk code and the altitude. The altitude may be determined by the transponder itself or may be pulled from a separate altitude encoder, depending on the model. Having the altitude reading available helps the air traffic controllers maintain the proper distance between all planes in the common airspace. Mode C transponders are required if you are flying above 10,000 feet in Class A, B, or C airspace. They are back-compliant with Mode A technology.

Mode S:

This transponder mode transmits the squawk code and the altitude, and it allows for further data exchange. If you are operating in a busy area with several radar systems, a Mode S transponder helps keep your plane from being repeatedly contacted by multiple radar installations. Mode S is necessary if you want to use a collision avoidance system. In the U.S., only planes that have an avoidance system are absolutely required to have a Mode S transponder. Mode S devices are back-compliant with Mode A and Mode C technology.

What are Class 1 and Class 2 avionics transponders?

Transponders are divided into 2 classes based on their power. Class 2 is the lower-powered category. These can only be used when you are operating under visual flight rules below 15,000 feet and 175 knots. Class 1 instruments have more power and are required for higher, faster flight conditions.

What is an ADS-B transponder?

These letters stand for Automatic Dependent Surveillance-Broadcast. It is a system that gives pilots enhanced information by using satellites instead of radar. This equipment allows you to see other aircraft on a display, weather information, temporary airspace restrictions, and other categories of useful information.