X in space B4 in a full data block indicates

X in space B4 in a full data block indicates

• A. Mode C altitude is corrupt or lost
• B. Data block missing
• C. Incorrect frequency
• D. Communication failure

Answer: (A)

In a full data block, certain fields carry a validity indicator for the data they hold. Seeing an X in the position that corresponds to the Mode C altitude means that the altitude value reported by Mode C is not valid—it's corrupt or has been lost. The X acts as a flag saying, in effect, “this piece of data cannot be trusted.” This is why the marking points to the altitude information specifically: Mode C altitude is the data item tied to that field, and the X signals its status as invalid. If the data block were missing entirely, you wouldn’t see a specific field flagged; you’d have no block or a failure to receive data. A wrong frequency or a general communication failure would present as different indicators, not as a single field invalidation.

In a full data block, certain fields carry a validity indicator for the data they hold. Seeing an X in the position that corresponds to the Mode C altitude means that the altitude value reported by Mode C is not valid—it's corrupt or has been lost. The X acts as a flag saying, in effect, “this piece of data cannot be trusted.”

This is why the marking points to the altitude information specifically: Mode C altitude is the data item tied to that field, and the X signals its status as invalid. If the data block were missing entirely, you wouldn’t see a specific field flagged; you’d have no block or a failure to receive data. A wrong frequency or a general communication failure would present as different indicators, not as a single field invalidation.