In a full data block, what condition is indicated by an X in B4?

In a full data block, what condition is indicated by an X in B4?

• A. Data block integrity check failed
• B. Mode C altitude corrupt or lost
• C. Radar altitude updates rejected
• D. No error detected

Answer: (B)

An X in the B4 position flags a problem with the altitude information from Mode C: the altitude data is corrupt or not available. The B4 field is used to indicate the integrity of the Mode C altitude, so when it shows an X, you can’t trust the altitude value and must treat it as invalid. This is why the interpretation is that the Mode C altitude is corrupt or lost. Other statuses would show up through different indicators or fields, and a clean, no-error indication would not include this mark.

An X in the B4 position flags a problem with the altitude information from Mode C: the altitude data is corrupt or not available. The B4 field is used to indicate the integrity of the Mode C altitude, so when it shows an X, you can’t trust the altitude value and must treat it as invalid. This is why the interpretation is that the Mode C altitude is corrupt or lost. Other statuses would show up through different indicators or fields, and a clean, no-error indication would not include this mark.