What do mandatory signs indicate at an airfield?

Mandatory signs at an airfield are designed to convey important information regarding runway and critical area access. Specifically, they are red with a white legend and indicate areas that require particular attention, such as entrances to runways or other critical sections of the airfield. This color scheme ensures high visibility and alerts personnel to critical operational areas where strict compliance with airfield rules is necessary for safety and regulation.

The use of red signifies warning or prohibition, which is appropriate for areas where unauthorized access could lead to dangerous situations. By clearly marking these critical access points, mandatory signs help maintain the orderly flow of aircraft operations and mitigate the risk of accidents.

Other options describe different types of signage used at airfields. For instance, yellow signs with black text indicate parking areas, blue signs denote taxi routes, and green signs assist with runway direction. These options do not fulfill the criteria for mandatory signs, which specifically highlight critical airfield access points.