When can a Mode C altitude be disregarded?

A Mode C altitude can be disregarded when it is corrupted or lost because reliable altitude information is crucial for maintaining safe separation between aircraft. If the altitude data transmitted by the aircraft is unreliable—whether due to technical failure, loss of signal, or interference—it does not provide the necessary assurances that the aircraft is flying at the correct altitude. In such cases, controllers must rely on other information and tools to ensure safety and separation, rather than trust an echoing altitude report that may lead to potentially dangerous situations.

In other situations, such as conflicts with block altitudes or when IFR flight is authorized, the system may still have a basis for using the Mode C altitude data because the context allows for standard operational procedures to manage those scenarios safely. Disregarding altitude in those contexts would depend on additional safeguards and procedures, thus underscoring the importance of reliable data.