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atc radar screen

MODE C Transponders

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:

  • 1200 – tells ATC you are flying VFR, this is the default code all transponders should be on when you have not been assigned a code by ATC
  • 7500 – Hijacking, you most likely will never use this code, especially when flying by yourself.  However, should your alter ego fight you for control of the airplane, you may choose to “squawk” 7500.  If you do use this code, plan to be intercepted by military aircraft and escorted to the nearest airfield where you will make some new friends (they’ll be the guys wearing the FBI jackets).
  • 7600 – Communications Emergency (applicable when your radio fails and you are flying in airspace around a towered airport, and need to let them know you have “lost comms” or gone “nordo” (no radio) and will need a light gun signal from the tower.  Your instructor can provide several example scenarios when you might use this code too.
  • 7700 – General Emergency, i.e. engine failure, fire, flight control problem, medical emergency, etc.  If in the middle of trying to save the day you find some downtime, you may consider setting this code in your transponder.  Personally, I feel my efforts would be better spent communicating via radio to ATC EXACTLY what my emergency is and what is going on in the airplane.  However, if time permits, it is a good idea to “squawk” 7700 also.

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.

vfr transponder ground school

Digital Transponder

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:

  • When set to ALT – Reports to ATC the 4 digit code selected and your Pressure Altitude (this is the MODE C function, if told to “stop squawking Mode C” ATC is asking you to turn your transponder to “ON” instead of “ALT”)
  • When set to ON – Reports to ATC the 4 digit code selected
  • When set to STBY – It is in standby mode and simply “warming up” but not relaying information to ATC.
  • OFF – Ya….that one’s obvious I hope.
  • IDENT – Pressing once will make your 4 digit code flash on ATC’s radar screen to draw attention to you.  Only press when directed to do so by ATC

Where is a Transponder required?

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:

  • Within 30nm of a Class B Airport
  • In, below, or above Class B Airspace
  • In or Above Class C Airspace
  • In International Airspace
  • In Class A airspace
  • Above 10,000’msl unless within 2,500′ of the ground as shown below.
  • Often required by the rules pertaining to a SFRA (Special Flight Rules Area).  We’ll talk about SFRAs later on in the Private Pilot Checkride Prep Course.

where you need a transponder vfr

NTSB Case Study

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