Code 7700 when the pilot declares an emergency and the aircraft is not radar identified.
SQUAWK MAYDAY ON 7700.
After radio and radar contact have been established, you may request other than single-piloted helicopters and single-piloted turbojet aircraft to change from Code 7700 to another code appropriate for your radar beacon code environment.
1. The code change, based on pilot concurrence, the nature of the emergency, and current flight conditions will signify to other radar facilities that the aircraft in distress is identified and under ATC control.
2. Pilots of single-piloted helicopters and single-piloted turbojet aircraft may be unable to reposition transponder controls during the emergency.
RADAR CONTACT (position). IF FEASIBLE, SQUAWK (code).