Reserved codes are used in particular situations when an aircraft wants to communicate something to ATC urgently. The most well know of these is the code 7700. This is used to indicate an emergency of any kind.
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Code 7700. This is the code for a general emergency. 7700 can be entered into the transponder for a variety of reasons, and some of the most common include medical emergencies, engine failure, problems with pressurization or landing gear, issues with flight control, and more.
A Squawk 7700 indicates an emergency. This can be an emergency of any kind. Pilots may input it into the transponder themselves – or when instructed to do so by ATC. As a result, ground control will know that the aircraft is dealing with a serious issue and needs help.
It's not very common these days. Squawk (transponder) is primarily for ATC purposes, and if you're already talking to ATC, there's usually no reason to squawk 7700. Modern radar automation systems give the controller the capability of mimicking a 7700 squawk for an aircraft with a simple keyboard entry.
Setting a squawk of 7700 on the transponder shows that the aircraft is presently dealing with an emergency. The pilot can set it proactively or at air traffic control's request following a 'pan' or 'mayday' call.
If the 7500 squawk code is mistakenly selected, the pilot will immediately receive a call from air traffic control asking for confirmation. At this point, the pilot in command or the first officer is able to inform the transponder code has been used by mistake, and that there is no hijacked aircraft.
Red: When an aircraft is displayed as red that means that you are currently following that specific aircraft on the map or that the aircraft's transponder is squawking an emergency code. These codes are 7500 (Hijack), 7600 (Communication Equipment Failure), 7700 (General Emergency)
The Importance of Understanding Squawk CodesThey serve their intended purpose of keeping everyone safe in the air under the guidance of ATC and communication from the pilots. They also work well to provide notice of emergency as referred to above with the squawk codes 7500, 7600, and 7700.
If you're below 18,000, feet you'll start by squawking 7600, then descend or climbing to an altitude that is acceptable for VFR cruising. You'll be expected to land as soon as practical while maintaining visual flight rules. If you're above 18,000 feet (Class A airspace) it gets a bit more complicated.
Very high frequency (VHF) 121.5 and ultra high frequency (UHF) 243.0 are “guarded” frequencies reserved in the aircraft operations band exclusively for emergency communications and notifications.