AIM Text
- When a distress or urgency condition is encountered, the pilot of an aircraft with a coded radar beacon transponder, who desires to alert a ground radar facility, should squawk Mode 3/A, Code 7700/Emergency and Mode C altitude reporting and then immediately establish communications with the ATC facility.
- Radar facilities are equipped so that Code 7700 normally triggers an alarm or special indicator at all control positions. Pilots should understand that they might not be within a radar coverage area. Therefore, they should continue squawking Code 7700 and establish radio communications as soon as possible.
Source: FAA Aeronautical Information Manual · current edition · paragraph 6-2-2.
Research Notes
AIM 6-2-2 covers Direction Finding Instrument Approach Procedure (DF) — emergency direction-finding service.
DF service: Legacy emergency service where the pilot transmits on a known frequency and a ground station takes a directional bearing. Combined with bearings from multiple stations, the lost pilot's position can be triangulated.
Modern status: Largely superseded by GPS and radar. Still available at some FSS locations for backup emergency use.