What transponder code indicates lost communications?

The transponder code that indicates lost communications is 7600. When a pilot experiences a failure in radio communications with Air Traffic Control, they are required to set their transponder to 7600 to alert ATC of the situation. This code signals that the aircraft is not able to communicate with ATC, and it helps controllers maintain situational awareness regarding the aircraft's status.

Understanding the importance of this code is crucial for maintaining safety in aviation operations. Setting the transponder to 7600 allows other aircraft and ATC to recognize that the pilot is still flying the aircraft and following standard protocols, but simply cannot communicate. This signals to controllers that alternate procedures may be necessary, such as providing visual separation or making adjustments to air traffic flow to accommodate the situation.

In contrast, the other codes serve different purposes: 7500 indicates a hijacking situation, 7700 signals an emergency that requires immediate attention, and 121.5 is the frequency used for emergency communication, not a transponder code. Understanding these distinctions is vital for effective communication and response in various flying scenarios.