You may have heard of the infamous code 7500 which airliners transmit when they are being hijacked, common amongst movies and TV shows, but what do squawk codes like this actually mean and why are they used? Here, we’ll look at their history and some common squawk codes which have become well-known within the aviation community.
What are Squawk codes?
Squawk codes are what air traffic control (ATC) use to identify aircraft when they are flying. They are unique four-digit numbers and range from 0000 to 7777; some of which are fixed values signifying specific scenarios (see below), others being randomly generated by ATC. At the dawn of commercial aviation, the position of an aircraft was tracked using radar, and aircraft would show up as anonymous dots on ATC radar display. However, as air travel became more popular and the skies more crowded, this method became ineffective and even dangerous since there were too many planes and a means of distinguishing one plane from the next was needed.
This is where squawk codes came in. There are 4096 unique combinations aircraft can use to identify themselves. When the aircraft enters an ATC’s airspace, the ATC generates a squawk code for the aircraft, and radios the pilot to enter it into their aircraft’s transponder. The aircraft transponder then continually transmits this back to the ATC, and the squawk code will be displayed next to the aircraft’s dot on the tracking screen.
Famous Squawk codes
As well as the codes used to identify aircraft, there are three squawk codes which aren’t randomly generated and are used to communicate to ATC without the use of speech. They tend to be used in emergencies, and work by the pilots changing their squawk code away from their designated one, signaling to ATC what is happening on board. These special codes are standardized by the International Civil Aviation Organization (ICAO), so seeing an emergency code in Denver means the same thing as seeing the code in Delhi.
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. What happens after this code is transmitted varies, but usually, security forces tend to get involved. The process which is depicted in movies and TV is an escort by military aircraft, but other things happen such as authorities waiting upon arrival.
Can’t talk now, sorry
The second emergency squawk code is 7600, showing ATC that the aircraft has lost verbal communication. This could mean that it can still hear ATC and yet not respond, in which case the ATC will direct the pilot to speak with them through the Ident button. This is a small button on the transponder which causes the aircraft to flash on the controller’s screen and therefore can be used as a means of talking through non-verbal communication. Interestingly, if the pilot can’t hear ATC or speak to them when near the airport, they can often communicate with the aircraft through lights. These come from a light gun situated on the tower, where ATC shines lights at an aircraft in colors of either red, green or white to signify different things when the aircraft has lost communication. These light gun signals mean different things depending on whether the aircraft is on the ground or in the air, and their values are taught to pilots during training.
And finally, the last emergency code that can be squawked is 7700, which can be used for general emergency. An aircraft may even be directly asked to squawk 7700 after speaking to ATC verbally so that they can recognise them and give them priority over others. Squawking 7700 gives the pilot the responsibility to do essentially anything to ensure the safety of those onboard, regardless of the rules. Squawking 7700 also notifies all nearby ATC in the surrounding areas of the situation with the flight.
Squawk codes are a key part of managing flights everywhere, and without them, the skies would be a lot more chaotic and unorganized. So, the next time you are on Flightradar24 and looking at an aircraft’s details, check the squawk code to find out what’s going on onboard. It’s most likely that it will just be a random number, but, in rare cases, there could actually be something significant happening.