FAA Transponder Requirements Explained

By Pilot Institute
Posted on April 6, 2023 - 4 minute read

The Federal Aviation Administration (FAA) has established strict regulations that ensure the safety of air travel. This includes strict requirements on transponders, which are an essential part of an aircraft’s avionics system.

Below, we’ll explain everything you need to know about transponder requirements. This will include a discussion about what transponders are, what they’re used for, and the FAA requirements for transponders.

What Is a Transponder in an Aircraft?

A transponder is an electronic device that transmits a signal in response to a specific type of incoming signal (known as an interrogation). In aviation, transponders are generally used to transmit information about an aircraft’s altitude, speed, and identification to Air Traffic Control (ATC) radar systems.

ATC will assign a transponder code to a particular aircraft (called a “squawk” code) used to identify the aircraft. This allows ATC to use the signals transmitted by the aircraft’s transponder to track the aircraft’s location and ensure that it remains safely separated from other aircraft in the airspace.

What Are Transponders Used For?

There are a variety of purposes transponders are used, such as:

  • Traffic Collision Avoidance System (TCAS): TCAS is a safety system that uses transponder signals to detect and avoid potential collisions with other aircraft.
  • Air Traffic Control (ATC): ATC uses transponder signals to track aircraft and ensure safe separation between them both in flight and on the ground.
  • Search and Rescue (SAR): Transponder signals can be used to locate aircraft in distress, allowing for faster response times and better rescue outcomes.
  • Flight Data Analysis: Transponder data can be used to analyze flight data and identify potential safety issues or operational improvements.

What Are the Three Types of Transponders?


There are three types of transponder modes commonly used in aircraft: Mode A, Mode C, and Mode S transponders.

Mode A Transponders

Mode A is the oldest and most basic type of transponder mode. It was developed in the 1940s and transmits a four-digit code to ATC radar systems. The code is manually set by the pilot using a selector switch in the cockpit.

Mode A transponders do not provide altitude information and are primarily used for identification purposes (because they only transmit a code).

Mode C Transponders

These transponders transmit both the four-digit code and the aircraft’s altitude or flight level to ATC radar systems. The altitude information is obtained from an altitude encoder, which is a separate piece of equipment that is typically installed in the aircraft’s avionics system. The altitude information allows ATC to accurately track the aircraft’s position.

Mode S Transponders

Mode S transponders were developed in the 1970s (and rolled out in the 1990s). They transmit a unique identification code for the aircraft, as well as altitude and other information, such as the aircraft’s speed and heading.

Mode S transponders are able to receive external information such as:

  • Information from other aircraft.
  • Information through a data link to receive information about airspace restrictions and weather – is known as a Flight Information System Broadcast (FIS-B).
  • Display of other aircraft transmitted by ground stations – known as Traffic Information Service Broadcasts (TIS-B).

What Are the FAA Requirements for a Transponder?


The FAA has established strict regulations regarding transponders to ensure the safety of air travel.

These regulations are outlined in 14 CFR § 91.215. Let’s break the requirements down in detail.

When Must an Aircraft Have a Transponder?

A transponder is not required unless an aircraft is operating:

  • In Class A, Class B, or Class C airspace.
  • Above 10,000 feet Mean Sea Level (MSL), excluding airspace below 2,500 feet Above Ground Level (AGL).
  • Within a 30 nautical mile radius of the relevant primary airport in class B airspace (This 30 nautical mile area is known as the “Mode C Veil”).
  • Above, and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet MSL.
  • Class E airspace over the Gulf of Mexico at and above 3,000 feet MSL within 12 nautical miles of the coastline of the United States.

What Are the Transponder Requirements for Different Classes of Airspace?

Different classes of airspace have different transponder requirements. Here’s a breakdown:

  • Class A, B, and C airspace: All aircraft must be equipped with an operable Mode C transponder.
  • Class D airspace: No transponder is required unless otherwise specified by ATC (Pilots only require two-way radio communication in this class of airspace).
  • Class E airspace: Transponder requirements vary depending on the altitude of the aircraft:
    • Below 10,000 feet MSL (mean sea level): A transponder is not required unless the aircraft is within 30 nautical miles of a Class B airport.
    • At or above 10,000 feet MSL: A transponder with altitude reporting capability is required.

What Are the Transponder Equipment Requirements?

In addition to the airspace and altitude requirements, the FAA also has specific equipment requirements for transponders.

The transponder must:

  • Be an ‘operable coded radar beacon transponder’; and
  • Either have:
    • Mode 3/A 4096 code capability, replying to Mode 3/A interrogations with the code specified by ATC; or
    • Mode S capability, replying to Mode 3/A interrogations with the code specified by ATC and Mode S interrogations

Please see 14 CFR § 91.215 for the complete requirements. You are allowed to deviate from the provisions with approval from the ATC under certain conditions.

Where Can I Find More Information about Transponder Requirements?

The FAA’s website is a great place to start for information about transponder requirements.

You can search for information by topic, such as “transponders,” or by regulation, such as “14 CFR § 91.215” The FAA also publishes advisory circulars (ACs) that provide guidance on specific topics related to aviation, including transponders.


It is important to ensure that you have a solid understanding of transponder requirements and how to operate your aircraft’s transponder.

Failure to comply with these requirements can result in serious consequences, including loss of your pilot’s license and penalties.

Now that you know when a transponder is required, check out this article to learn about ADS-B requirements – they are equally as important!

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