Well first, what is a transponder? A transponder is a little box mounted in the panel of your airplane where you can set a 4 digit code that ATC assigns to you. The purpose of this box is to tell ATC where you are and how high you are. Transponders are also often called “pressure altitude reporting equipment”, because it will report your pressure altitude to ATC. This allows ATC to separate airplanes not only horizontally, but also vertically (letting airplanes fly in the same spot but a few thousand feet apart vertically to ensure separation).
A transponder can be programmed with a four-digit code, and the digits are between 0-7. This means there are 4096 possible codes that can be assigned to airplanes at any given time. There are a few default codes we use to communicate with ATC or when they have not assigned us a code (like when we are flying at a non-towered airport, or a Class D airport. Typically only Class C and Class B airports will assign transponder codes). Standard Codes are as follows:
Note: There are two ways to get ATC’s attention without physically making a radio call. When a controller looks at their screen, they see lots of little blips on the radar all with little codes next to each (the squawk codes or transponder codes). Since all of these blips can blend together, you can A: press the IDENT button on a transponder that will make your 4 digit code flash for a few seconds on the controller’s screen to draw attention to your position and altitude, or B: set one of the three emergency codes listed above which will also make your 4 digit code flash on their screen, drawing attention to you and the fact that something is obviously not quite right.
Above are two examples of common transponders you would find in a GA airplane. While the newer digital one may have a few more fancy features like a stopwatch, every “Mode C” transponder you encounter will have the same basic functions:
You are required to have your transponder on and functioning if it is installed in the aircraft. You are specifically required by 91.215 to have a Mode C transponder installed and operating in the following places:
On November 16th, 2000, a Cessna 172 and F-16 Collided in mid-air over Sarasota County, FL. The F-16 was the second aircraft in a flight of two, and being in a flight of two, the F-16 did not have his transponder turned on (which may have provided more of an alert to ATC that the two aircraft were in dangerous proximity to each other). The 172 pilot died, and the F-16 pilot ejected safely. The weather was 10SM and skies clear. The 172 was in a 30-40 degree turn at the time of impact, probably contributing to the pilot’s inability to see the approaching F-16