What does it mean if an aircraft has an assigned beacon code that is not being received?

If an aircraft has an assigned beacon code that is not being received, it indicates that the aircraft is untracked. This situation arises when the transponder in the aircraft, which transmits the assigned beacon code, is either malfunctioning or turned off. As a result, air traffic control is unable to receive the aircraft's position and identification information, thus rendering it as untracked.

In air traffic operations, continuous tracking through radar or transponder signals is crucial for maintaining safe separation and communication with aircraft. If a transponder is not functioning properly, the aircraft cannot be integrated into the traffic flow, leading to potential safety concerns. This scenario emphasizes the critical need for proper functioning of onboard systems to ensure that aircraft remain in communication with air traffic control.