1. There are three kinds of civilian transponders used in U.S. airspace:
2. 2. Mode A
   a. A Mode A transponder, when requested by the air traffic control radar beacon system (ATCRBS), transmits a four-digit squawk code to ATC.
3. 3. Mode C (Automatic Altitude Reporting)
   i. This type of transponder converts your airplane’s altitude in 100-ft. increments to coded digital information, which is transmitted in the reply to the interrogating radar facility. A Mode C transponder provides this information in addition to transmitting the four-digit squawk code.
   ii. If your airplane is Mode C-equipped, you must set your transponder to reply Mode C (i.e., set function switch to ALT) unless ATC requests otherwise.
4. 4. Mode C is required when flying
   1. At or above 10,000 ft. MSL, except in that airspace below 2,500 ft. AGL
   2. Within 30 NM of a Class B airspace primary airport
   3. Within and above a Class C airspace area
   a. Into, within, or across the U.S. ADIZ (Air Defense Identification Zone)
5. 5. Mode S (Selective)
6. 6. Mode S (Selective) transponders are designed to help air traffic control in busy areas and allow automatic collision avoidance.
   i. Mode S transponders allow TCAS (Traffic Alert and Collision Avoidance System) and TIS (Traffic Information System) to function.
7. 7. Mode S transponders broadcast information about the equipped aircraft to the Secondary Surveillance Radar (SSR) system, TCAS receivers on board aircraft, and to the ADS-B system.
   1. This information includes the call sign of the aircraft and/or the transponder’s permanent unit code (i.e., not the four-digit user-entered squawk code).
   a. These transponders also receive ground-based radar information through a datalink and can display that information to pilots to aid in collision avoidance.

8. The military has multiple kinds of transponders, and the military type that corresponds to civilian Mode A and civilian Mode C is referred to as military Mode 3. You may see FAA questions that refer to Mode A/3 or Mode C/3. The “3” is referring to military transponders, so just think “Mode A” or “Mode C.”

9. Code 1200 is the standard VFR transponder code.

10. The ident feature should not be engaged unless instructed by ATC.

11. Certain special codes should never be engaged (except in an emergency), as they may cause problems at ATC centers:
a. 7500 is the hijacking code.
b. 7600 is the lost radio communication code.
c. 7700 is the general emergency code.
d. 7777 is the military interceptor code.