What a squawk code is
Every aircraft flying in controlled airspace carries a transponder. When it is interrogated by radar it answers with a four-digit code, and because each digit runs only from 0 to 7, there are 4,096 possible codes. Air traffic control assigns a code to each flight so that the blip on the controller's screen carries a label, and the crew dials it into the transponder. "Squawk" is simply the verb: a controller says "squawk 4521" and the pilot sets 4521.
Modern transponders (Mode S) also broadcast the aircraft's unique 24-bit address and, with ADS-B, its GPS position, which is what lets public trackers show the squawk alongside the aircraft's position. That is how an emergency watch page like ours works: it is reading the same broadcasts that volunteer receivers pick up.
The three emergency codes
- 7700, general emergency. The crew is telling everyone that something is wrong and they need priority. It covers anything from a medical emergency in the cabin to a pressurisation problem, an engine issue, smoke indication, or low fuel. Controllers clear the airspace around the aircraft, offer the nearest suitable airport, and alert the airport's rescue services.
- 7600, radio failure. The aircraft can no longer communicate by voice. Controllers switch to procedures that assume the crew is following the last clearance and the published lost-communication rules, and they may use light signals from the tower on arrival. The aircraft is almost always flying perfectly normally.
- 7500, unlawful interference. Hijacking or a similar threat on board. This is the one code crews do not set casually, and when it appears ATC treats it with the utmost seriousness. It is also rarely seen on public trackers, and when it is, a mis-set code is the most common explanation.
An easy way to remember the three: 7500 "taken", 7600 "can't hear", 7700 "going down" is the old mnemonic, though "going down" overstates it: 7700 means "I need help now", not "I am crashing".
What actually happens when 7700 appears
Squawking 7700 is a request for priority, not a report of a crash. A typical 7700 ends with the aircraft landing normally at the nearest suitable airport, sometimes at its own destination, with fire trucks standing by as a precaution. Medical diversions are the single most common cause. Many events never make the news at all because nothing dramatic happened.
Seeing 7700 on a tracker tells you a crew has declared an emergency; it tells you nothing about how serious it is. It is also worth knowing that codes are occasionally selected in error while a pilot is changing to an assigned code, and corrected within seconds, so a brief flash of 7700 or 7500 that disappears is usually just that.
Other codes you will see
- 1200 is the standard VFR code in the United States and Canada for aircraft flying under visual flight rules without a specific assignment; in most of Europe the equivalent is 7000.
- 2000 is used when entering controlled airspace from an area without radar service, until a controller assigns a code.
- 7777 is reserved for military interceptor operations in some countries and should never be set by civil aircraft.
- Everything else is an assigned, local code with no meaning outside that controller's airspace.
Watching emergencies live
Our emergencies page lists every aircraft currently squawking 7700, 7600 or 7500 in the open ADS-B feeds, with the type, registration and a small map. Because it is assembled from volunteer receivers it is best-effort rather than official, and an aircraft over an ocean or far from any receiver will not appear. If you are trying to understand an incident you have heard about, the squawk is one clue; the aircraft's altitude trace and whether it turned back or diverted will tell you more.