Squawk 7700: general emergency
The crew has declared an emergency. It covers everything from a sick passenger to an engine problem, and most end as a routine priority landing.
Squawk 7600: radio failure
The crew cannot reach air traffic control by radio. Aircraft follow a published lost-communications procedure for this, and it is usually a failed radio rather than a failing aircraft.
Why 7500 is not listed here
7500 is the code for unlawful interference. We detect it and record it, but we do not publish it. In live transponder data almost every 7500 is a keying error on the way to another code, and printing that phrase next to a real flight number, with real people aboard, on the strength of a mistyped dial is not a trade worth making. Our flight tracking technology guides explain how transponder codes travel from the cockpit to this page, and what is flying over me reads the same broadcasts to name the aircraft above you.