Your Mode C transponder has a digital squawk code—a four-digit identifier that ATC sees on their radar screen. In normal operations, you fly a standard code assigned by ATC (e.g., 3651). But when you are in distress, experiencing radio failure, or facing unlawful interference, you set specific emergency codes:

You set squawk 7700 when you declare MAYDAY. This code tells ATC immediately: "This aircraft requires emergency assistance." Set it before or immediately after your MAYDAY call. Most modern transponders have a dedicated emergency button; if yours does, press it and confirm in your transmission that you are squawking 7700. If you must manually dial the code, do so while maintaining control of the aircraft—aviate first, squawk second.

You set squawk 7600 when you lose all transmitted radio communication (transmitter failure or complete failure). This code tells ATC: "I cannot hear you or you cannot hear me, but I am squawking so you can track me visually." See the detailed radio failure section below.

You set squawk 7500 if your aircraft is subjected to unlawful interference (hijack, bomb threat, sabotage threat, or crew incapacitation). This code is a discreet distress signal for law enforcement. If you are in this situation, set the code, remain calm, and do not announce it on the radio. Simply fly the routing and altitude requested by the person or persons who have taken control, and proceed to the nearest major aerodrome. ATC will see the code, alert authorities, and coordinate your safe landing.