What does a black diamond symbol with a white letter "C" inside of it indicate?

A black diamond symbol with a white letter "C" inside indicates expanding circling minimums. This symbol is used on instrument approach charts to inform pilots that the circling minimums for a particular procedure may be higher than standard values. It alerts pilots that they need to be cautious as the required visibility or altitude may require special consideration during the approach phase.

In this context, understanding the implications of circling minimums is crucial for pilot safety, especially when operating in potentially challenging conditions. It provides guidance on the required altitudes and visibility to ensure a safe transition from the final approach to landing. The clarity provided by this marking aids pilots in assessing the risks associated with the approach and making informed decisions based on their aircraft capabilities and current weather conditions.