What does H-12 signify in relation to an aircraft's status?

H-12 signifies the status of an aircraft's transponder code, indicating that the aircraft has an improper code displayed. This can occur when the assigned code does not match the one in use by air traffic control or when the aircraft is transmitting a code that is not recognized or valid. Correctly identifying and addressing an improper code is crucial for effective communication and management of air traffic, as it can impact the aircraft's identification and tracking in radar systems.

Understanding that H-12 refers specifically to transponder issues helps ensure that air traffic controllers are alerted to potential problems with an aircraft's radar returns, allowing for timely corrective action. The other options pertain to different aircraft statuses, but they do not accurately describe the significance of H-12.