What color is the background for mandatory signs in aviation?

The correct answer is red. In aviation, mandatory signs are utilized primarily on airport taxiways and runways to convey critical information, such as prohibitions or instructions that must be followed by pilots. The color red signifies mandatory actions, indicating that a pilot must comply with the instruction presented on the sign.

The use of red for mandatory signs is a universally accepted standard in aviation, aligning with the International Civil Aviation Organization (ICAO) guidelines. It serves to grab the pilot's attention and underscore the importance of adherence to the instructions, which may involve stopping or avoiding certain areas to ensure safety while operating aircraft.

Other colors have specific meanings in aviation signage; for example, yellow is often associated with cautionary information and ground traffic control, while green typically indicates information or guidance for taxiing. White is used for informational signs that provide further guidance, such as runway distances or airport facilities.