If an aircraft's communication fails, which code is appropriate for transmission interruptions?

When an aircraft experiences a communication failure, the appropriate transponder code to transmit is 7600. This code is specifically designated for situations where communication has been lost, helping air traffic controllers identify the aircraft's condition. The use of 7600 signals to controllers that the aircraft is unable to communicate with them, allowing them to take appropriate measures to ensure the safety of the flight.

In contrast, 7700 is used to indicate an emergency situation that may involve a variety of issues, such as a medical emergency or aircraft systems failure. Code 1500 is generally used for VFR (Visual Flight Rules) aircraft operating under specific conditions, while 4000 is not a standard code for communication failure. It's essential for pilots to understand these codes to ensure effective communication and safety in various situations.