Under which condition is Mode C (vertical) separation not applicable?

Mode C (vertical) separation is a standard protocol used in air traffic control to ensure that aircraft maintain an adequate vertical distance from one another, based on their transponder data. The use of Mode C is predicated on the reliability of the Mode C altitude information being transmitted by aircraft transponders.

When Mode C information is invalid, it means that the altitude data being reported by the transponder is either incorrect or unavailable. In this situation, maintaining vertical separation based on faulty data could lead to dangerous proximity between aircraft, as air traffic control cannot accurately determine the vertical position of the aircraft. Therefore, without valid Mode C data, vertical separation cannot be guaranteed, making it an inappropriate condition for relying on this mode of separation.

While the other options may involve different dynamics in air traffic management or the use of visual references, they do not directly impair the fundamental understanding and functionality of Mode C separation as does the invalidity of Mode C information.