If Mode C altitude is corrupt, what should be displayed for Field C?

When Mode C altitude information is identified as corrupt, it is essential for operational safety to clearly indicate this issue to avoid any confusion or misinterpretation by air traffic controllers and pilots. Displaying "XXX" serves as a universal signal that the altitude data is not valid or reliable. This visual cue alerts controllers that the aircraft's reported altitude cannot be trusted, prompting them to take appropriate actions, such as verifying altitude through other means or maintaining increased vigilance regarding the aircraft's altitude.

In contrast, displaying a specific altitude value, the last known altitude, or the assigned altitude could lead to misunderstandings about the aircraft's actual position, potentially compromising safety. These alternatives do not provide a clear warning of the corruption in data, thus making "XXX" the preferred choice in these circumstances. This practice ensures that all involved parties are made aware that they should not rely on the altitude information currently being displayed.