Transponder codes This allows easy identification of aircraft on radar. Codes are made of four octal digits; the dials on a transponder read from zero to seven, inclusive. Four octal digits can represent up to 4096 different codes, which is why such transponders are sometimes described as "4096 code transponders."
A squawk of 4000 observed on radar by a controller usually meant that the aircraft operating in Special Use Airspace, exceeding 250 knots below 10,000 feet and/or rapidly maneuvering. It also normally means that they are not talking to an ATC facility.
Series 77 — Code 7700 is reserved for recognizing an aircraft in emergency. (Codes 7711 to 7717 and 7721 to 7727 are reserved for SAR operations and code 7777 for monitoring the ground transponder.)
The number 4096 comes from the number of different codes you can make out of 7 different numbers in sets of 4. A mode C transponder is required to enter class A airspace, fly within 30 nautical miles of primary airports in class B airspace, or fly in or above class C airspace.
Emergency codes
The first emergency code that you may have become familiar with is Squawk 7500. This code is used to indicate that the aircraft has been hijacked, and therefore requires urgent emergency support from both security services and air traffic control.
Squawking 7700 in an emergency
The most well know of these is the code 7700. This is used to indicate an emergency of any kind. A pilot will enter this when in an emergency situation - either instructed by ATC after declaring an emergency or without communication if there is no time.
0000 — A generic code that is not assigned and should not be used. 1200 — VFR aircraft. The default code for all flights–if you aren't asked to set anything else, you should set 1200. 7500 — Hijacking. 7600 — Voice radio failure.
Code 7600 is the code for a loss of radio. If a pilot's radio goes out during a fight, they can enter 7600 into the transponder. Then they should proceed to their intended airport following a standard landing plan. Pilots should be extra careful and on alert for other aircraft around them in the sky.
Code 7400 may be displayed by unmanned aircraft systems (UAS) when the control link between the aircraft and the pilot is lost. Lost link procedures are programmed into the flight management system and associated with the flight plan being flown.
Some squawk codes are reserved, such as 7700 (emergency), 7600 (communication failure), 7500 (hijacking), 1202 (glider), 1200 (VFR), etc. One of these, 7777, is apparently used for "military interception." What does this mean in the United States?
Squawk 7000: This is the 'conspicuity code' for VFR aircraft that are not assigned a specific code by ATC. Squawk 2000: This is the 'conspicuity code' for IFR aircraft that are not assigned a specific code by ATC.
Transponders back then (and still today) have the option of numbers 0-7. That's because each number can be represented by only three bits, or three 1s or 0s.
If there's a radio failure associated with an emergency, the aircraft might alternately squawk 7600 (no radio) and 7700 (emergency). This alerts ATC that the aircraft is declaring an emergency and does not have an operable radio.
Famous Squawk codes
The first of these is the code 7500, which signals “unlawful interference,” more commonly referred to as hijacking. This is a situation where squawking is particularly useful, as it allows the pilots to contact ATC discreetly.
For example, "1200" in the USA means that the flight is flying under Visual Flight Rules (VFR) and is not typically in direct contact with ATC. "1200" is a shared code so you may see many aircraft transmitting it at the same time in a given area.
2200. VFR - Operating within a controlled aerodrome circuit. 5000-5777. IFR - Domestic (automated by position)
By international agreement, 2000 is used for aircraft which have not been assigned a transponder code, although in some parts of Europe, 7000 is used for this purpose.
You might have noticed on our squawk box that the number 50 isn't available, that's because 5050 is the designated frequency monitoring code for #Southend.. A #pilot may listen to our #frequency, without communicating directly.
Code 7776 and Code 7777 are reserved for SSR ground transponder monitoring. Are reserved for humanitarian flights.
Hijack (7500), lost communications (7600), and emergency (7700) codes should always be avoided when making routine transponder code changes.
There are 3 aircraft emergency squawk codes that every pilot needs to know. They are 7500, 7600, and 7700. It's not only important to know what the codes are used for, but also how to avoid switching to these codes in non-emergency situations to avoid setting off alarms in ATC facilities.