Code 7700 highlights an aircraft on every controller's screen
When a flight crew sets an emergency code on the transponder, the aircraft is instantly highlighted on the screens of every controller within range — a single turn of the dial declares an emergency to the whole surrounding sky. Yet according to available examples, the vast majority of such declared "emergencies" end in a calm landing without further complications.
The radar controllers use to guide aircraft doesn't actually see them — it asks them. Primary radar picks up a reflection off the fuselage, but it can't say who's flying or how high. That's why secondary radar is used: it sends a query, and the onboard transponder replies with its code and altitude. The system's roots go back to wartime identification of friendly aircraft in World War II, nicknamed the "parrot" — and the command "squawk" is still used in aviation today: a squawk is the code the transponder is currently transmitting.
The code has four digits, each from 0 to 7 — 4,096 combinations in total, assigned to flights by air traffic control so it can tell the dots on screen apart. Three combinations, however, are reserved worldwide and are not normally assigned to any flight: 7700 means a general emergency, 7600 loss of radio contact, and 7500 unlawful interference on board. The crew sets these themselves by turning the dial — so an emergency can be broadcast even when there's no time to talk.
- 7700
- general emergency
- 7600
- loss of radio contact
- 7500
- unlawful interference you won't find it in our overviews
The moment 7700 appears on the transponder, the flight's label is highlighted on the screens of every control centre within range — no phone calls needed, the whole sky knows at once. The code is usually accompanied by a voice call: "mayday" for immediate danger, "pan-pan" for an urgent situation that doesn't yet threaten lives. The flight gets absolute priority — direct vectors, a clear route, a runway with no waiting — and airport fire crews roll out as a precaution. One thing matters, though: the code describes what the transponder is transmitting, not what's happening on board. Crews set it even when they simply need certainty and clear skies, so the vast majority of "emergencies" end in a calm landing without drama.
Typical triggers
- A medical case on board — the most common reason a flight requests priority landing; with a sick passenger, the aircraft heads to the nearest suitable airport.
- Technical warnings: an engine, landing gear or brake indicator light. Often it's a faulty sensor, but that can only be confirmed on the ground — hence the priority and the fire crews on standby.
- Loss of cabin pressure: the aircraft descends steeply to roughly 10,000 ft (3 km), where breathing without masks is possible. It looks dramatic on the map, but it's a well-rehearsed manoeuvre.
- A bird strike after take-off, a damaged tyre during the take-off roll, or a dwindling fuel reserve after a long holding period.
- With code 7600, the explanation tends to be the most mundane: a radio fault or a mistuned frequency.
A concrete example of such a case is British Airways flight BA825 from Dublin to London: according to karmactive.com, the aircraft squawked 7700 on 1 September 2026 and then made a controlled descent from cruising altitude to roughly 8,000 feet, which the site describes as a standard, orderly descent rather than an emergency dive. According to the same source, the aircraft did not divert to an alternate airport and continued to its planned destination, Heathrow.
One thing is fair to admit: the transponder doesn't transmit the cause. From the data we see the code, the route, the altitude and the times — never the reason. The public only learns that once the airline or an investigating authority announces it. That's why we don't claim to know the cause of specific flights either.
Meanwhile, in the cockpit, the age-old order of priorities applies: "aviate, navigate, communicate" — fly first, then navigate, and talk last. The controller, for their part, clears airspace, offers the nearest suitable airports, and asks about the number of people on board and the fuel remaining. And here lies one of the most common misunderstandings among readers of the map: an emergency aircraft circling has not "lost its way." A large aircraft shortly after take-off is often heavier than it's allowed to be for landing — so it either circles at a safe altitude for an hour to burn off fuel, or (on types capable of it) dumps fuel in a controlled manner. Circles on the map are usually a sign of deliberation, not panic.
Loss of contact follows its own script. An aircraft squawking 7600 continues under pre-established rules — holding its last assigned route and altitude — while air traffic control clears the space around it, since it knows exactly what the aircraft will do. But if the radio silence lasts a long time and the aircraft is heading over densely populated areas, fighter jets can be scrambled to fly up close and attempt to make visual contact.
For an airport, an emergency arrival disrupts a schedule planned down to the minute. The runway is cleared immediately: arrivals go into holding patterns, departures wait on the taxiways. If the aircraft remains on the runway after landing — due to a flat tyre, hot brakes, or an inspection following an overweight landing — the runway can close for tens of minutes and the airport's capacity drops. In Europe, Eurocontrol's network management then steps in: flights still on the ground that are heading into already-congested airspace get their departure times pushed back. And some of the delay can never be made up — an aircraft delayed on its turnaround also departs late on its next route, so a single morning emergency can push back an evening departure at a completely different end of Europe.
Anyone wanting to keep track of this chain can find on map.flights a log of emergency codes with every recorded episode, flight and route; the detail page for a specific flight shows the aircraft's real-time position and delay, and the airport ranking reveals where the wait is currently longest.