Flight Training By

Why Pilots Say “Squawk” & How Do Aircraft Transponders Work?

Aviation is full of jargon and technical terminology. For new pilots, it can be a bit daunting to understand. Nowhere is this more true or more evident than when talking with air traffic control. You may have heard the term “squawk” before somewhere. But what does it mean?

Table of Contents

“Squawk” is the term used to describe any transmission that comes from the aircraft’s transponder. The transponder is a special radio that communicates with the air traffic controller’s radar system. The radar system uses the information from the transponder to help identify specific aircraft and their altitudes.

What is a Transponder?

The cockpit of even the most basic training aircraft has a stack of radios or avionics. They perform a variety of specialized tasks to help the pilot communicate and navigate. The transponder is probably the most mysterious of those items.

Transponder in a private plane squawking 2000
Hp.Baumeler Transponder in a private plane squawking 2000

A transponder works in the background and does not provide any obvious benefit to the pilot. Only once a pilot starts working with air traffic control does its purpose become clear. 

A transponder only has a few settings, and depending on how advanced it is, it only sends out a few key pieces of information. The only settings available to the pilot are Off, On, ALT, IDENT, and inputting a four-digit code.

Once a setting is made on the transponder, there is no visible result inside the cockpit. The results appear on the controller’s radar screens.

How Do Aircraft Transponders Work?

To understand the importance of transponders and how they work, first, one should know a little about air traffic control radar.

Like all radar systems, the controller’s radar sends out a beam of electromagnetic energy in a specific direction. The beam spins like a lighthouse. The transmission tower is capable of “listening” for radio waves that are bounced off of solid objects and returned to the station. 

Since the beam is directional, it can figure out the bearing to an object and its distance from the radar station. It then presents a blip on the controller’s radar screen, which is known as a primary return. These are objects that are not necessarily identified, and they could be any solid object. 

Transponder
Meggar Transponder

By watching the primary return on the radar scope move, a controller can gauge how fast it is moving and in what direction. But the radar provides no way to tell two aircraft apart from one another, and it gives no way of telling what altitude those planes are flying. 

If left to primary radar alone, a controller would not know whether two converging aircraft are at the same height and in danger of collision, or if they are separated by thousands of feet vertically and perfectly safe.

Transponders solve both of these problems. They transmit on a special frequency that is picked up by all air traffic radar facilities within range. They send out the four-digit code that the pilot has set, and if they are a Mode-C unit, the aircraft’s pressure altitude. This extra information, when connected to a primary return, is known as a secondary return on the controller’s scope.

For a good idea of what the radarscope looks like from the controller’s point of view, check out Chapter 4 of the FAA’s Aeronautical Information Manual.

Transponder Types and Modes

The most basic type of transponder only transmits the programmed four-digit code. These Mode-A transponders are sometimes called 4096 transponders since they can send 4,096 different numerical codes. Note, the individual digits on the transponder are only programmable between zero and seven. 

If the transponder is set to “On,” then it is only transmitting Mode-A information. This setting should only be used if your Mode-C altitude report is inaccurate, and the pilot is requested to “stop altitude squawk.”

In ALT mode, a Mode-C transponder transmits the four-digit discrete code, as well as the aircraft’s pressure altitude. The pressure altitude is then corrected to true altitude by the radar system, removing any errors that might be caused by one aircraft having set the incorrect altimeter setting. This is the typical setting for a transponder.

The transponder also includes a momentary button labeled IDENT. Pressing this button forces the aircraft’s secondary return to flash on the radar screen for a few seconds. This is a useful tool for controllers who can then quickly identify a specific aircraft without having the pilot change the code.

If a controller instructs a pilot to “Squawk IDENT,” they want the pilot to simply press the IDENT key momentarily and then wait a few seconds. 

New Technologies Replacing Radar

Newer transponders are now Mode-S, meaning they have the function of sending out the four-digit discrete code, the aircraft’s pressure altitude, as well as receiving data-link communications from ATC.

This includes, in some locations, some traffic information service (TIS), as well as limited weather information via the Flight Information Service-Broadcast (FIS-B). This data can then be shown on the plane’s multi-function display.

The FAA’s NextGen program is aimed at eventually replacing ground-based radar systems with satellite-based air traffic control. To do that, aircraft need to be outfitted with new technologies, namely Automatic Dependent Surveillance-Broadcast, or ADS-B, transponders. 

This technology effectively upgrades the standard transponder with one that receives the aircraft’s GPS position and transmits that out to both air traffic control and other aircraft. To see more about the FAA’s new transponder requirements, check out their ADS-B website at https://www.faa.gov/nextgen/equipadsb/.

ADS-B out transponders have been required in some airspaces in the US since January 1, 2020. Aircraft can still operate with legacy transponders outside of these airspace areas, and they can receive a waiver should they need to operate in an ADS-B required airspace. 

Where are you Required to Have a Transponder?

The Federal Aviation Regulations spell out exactly where aircraft are required to have a transponder. If your plane doesn’t travel in these areas, you don’t need to own one. But most small planes do occasionally venture into these spots, in which case they must have one on board.

Cessna ARC RT 359A transponder and BendixKing KY197 VHF communication radio
Public Domain Cessna ARC RT 359A transponder and BendixKing KY197 VHF communication radio

According to the Aeronautical Information Manual, if you have a functioning transponder onboard the aircraft, it must be switched on during flight. This makes logical sense, considering it helps air traffic control separate other traffic from you. This is even true if you are VFR and not receiving separation services. 

Transponders are required above 10,000 feet MSL, within 30 nautical miles of a Class B airport, and inside and above Class C airspace. Additionally, ADS-B is required at or above 3,000 feet MSL within 12 NM of the US coastline when over the Gulf of Mexico.

Special Transponder Codes

The ability to set the four-digit code provides the ability for an air traffic controller to positively identify a specific aircraft. If a controller needs to track a particular flight, a discrete code is assigned. 

A discrete transponder code is a code that is only given to one aircraft at a time, so ideally, no one else should be squawking it. All instrument flights have their own discrete codes, as do most aircraft that are in contact with a Class B or C approach controller, or transiting a special flight rules area. 

There are also several codes for general use that every pilot should know. Even if you are not in an area that requires having a transponder installed on your aircraft, you are required to operate the transponder if you have one anyway. This makes it easier for controllers to do their job since it gives them the ability to separate their IFR traffic away from non-participating aircraft.

Squawk Codes Every Pilot Should Know

Here is a list of squawk codes that every pilot operating in the United States should know. Other countries may have others, but the emergency codes listed here are ICAO international standards.

1200

1200 is used by VFR aircraft who have not been told to squawk anything else. This is generally the default setting that should be left in your transponder on training and VFR flights. If you are transiting a Class B or Class C airspace, the controller may assign you a discrete code. This does not change the fact that you are operating under visual flight rules, and once the controller is done, they will instruct the pilot to “squawk VFR.” The pilot should then return the transponder code to 1200.

7500

7500 is an international code for hijacking and should be avoided unless you really need to use it. Since the transponder is silent and discrete, this code is established as a sly way for the pilot to signal for help. If in ATC contract, the controller may ask, “Confirm Squawking 7500?” An aircraft squawking 7500 can expect to be intercepted and met by law enforcement upon landing.

7600

7600 can be put into the transponder to indicate that an aircraft has lost their voice communications. This displays a message on the controller’s scope that says NORAD, or no radios. The controller will continue to broadcast voice instructions to the pilot since it is possible that the pilot can hear the controller but not transmit. If that is the case, the controller may ask the pilot to IDENT. If the pilot complies, the controller has established two-way communication, albeit somewhat crudely.

7700

7700 is the international transponder equivalent of saying, “Mayday!” It means an emergency is occurring onboard, and you require immediate assistance. 

author photo
Matt C.
Matt C. is an FAA Airline Transport Pilot and Certified Flight Instructor.

Similar Content