Aircraft subject to unlawful interference. Pilots may squawk 7500 to silently alert ATC of a hijack situation. Triggers immediate emergency protocols.

Radio communication failure. Aircraft is unable to receive or transmit on assigned frequency. ATC will use light gun signals or alternate methods.

Catch-all emergency. Mayday, medical emergency, engine failure, fuel emergency, anything requiring priority handling. All ATC facilities give priority.

Standard Visual Flight Rules code in the United States. Used by pilots not in contact with ATC, operating under VFR.

Conspicuity code for VFR flights operating in European airspace per ICAO recommendation.

Reserved for VFR aircraft operating in proximity to Los Angeles Class B airspace.

Glider operations not in contact with ATC (USA).

IFR flight equipped with Mode S, where ATC has not assigned a discrete code. Used in Europe.

IFR aircraft entering an ATC area from outside, before being assigned a discrete code.

Aircraft on military training routes (MTR) below 10,000 ft, not in contact with ATC.

Unmanned aircraft that has lost its command and control link. Assigned so controllers can identify an aircraft that will now fly its programmed lost-link profile rather than respond to instructions.

NORAD aircraft on air defense missions.

NORAD exercise / training mission.

Reserved for military interceptor operations. Civilian pilots must not squawk this.

Aircraft involved in aerial firefighting operations, including air tankers, scoopers, and lead planes.

Penetration of Air Defense Identification Zone; operations under DVFR (Defense VFR) flight plan.

Search and rescue (SAR) operations.

Active parachute drop operations.

Aerial surveying operations.

Law enforcement aircraft on operations.

United Kingdom allocation, reserved for helicopter Royal Flights. Fixed-wing Royal Flights use a separate code.

Reserved for SSR Mode S transponder test purposes.