What happens when Mode C altitude is within ±200 feet of the limit of block altitude?

When Mode C altitude is within ±200 feet of the limit of block altitude, the system indicates that the altitude provided by the Mode C transponder is valid. This typically means that the altitude reported by the aircraft aligns closely enough with the designated block altitude range to be considered reliable. Establishing confidence in the altitude reading is crucial for safe air traffic management, especially in busy airspaces where separation and appropriate altitude assignments are necessary.

In the context of air traffic control, accurate altitude information assists controllers in ensuring safe vertical separation between aircraft, as well as in maintaining situational awareness when managing traffic flows, particularly around altitude limits.

The other options do not accurately reflect the consequences of Mode C altitude readings near block altitude limits. For example, flight type VFR refers to Visual Flight Rules and doesn't specifically relate to altitude validity per se, while the checks on controller-entered altitude might be necessary in some situations, they do not directly pertain to the validity of the Mode C altitude. Disregarding the altitude by controllers is also contrary to the protocols intended to ensure safe operations within controlled airspace.