What is indicated when a flashing white signal is shown for aircraft in flight?

In air traffic control, a flashing white signal shown to aircraft in flight indicates that the signal is not applicable and has no meaning for those aircraft. Flashing white signals are primarily used for visual flight rules (VFR) operations in ground or non-flight scenarios, and when displayed in flight, they serve no directive to the aircraft. The indication is essentially a signal to disregard any further action since it does not relate to air navigation or control in the airspace for those in flight.

Understanding this aspect of signals is crucial for pilots and air traffic controllers, as it helps them recognize what information is relevant to their operations. The choice of “not applicable” is significant because it avoids confusion and potential misinterpretation of instructions during flight.