What is indicated by the green 510 at the bottom of the attitude indicator?

The green 510 at the bottom of the attitude indicator signifies the radio height, which is the height above the terrain directly below the aircraft. This measurement is crucial for pilots, especially during approaches and landings, as it provides accurate information on the aircraft's altitude above ground level, allowing for safer positioning and better situational awareness in relation to the surface.

Radio height is typically provided in feet and is particularly useful in low-altitude flight, such as during approaches in poor visibility or in mountainous terrain. Understanding this reading can help pilots execute safe vertical maneuvering close to the ground.

The other options pertain to different measurements that do not align with the function of the indicated value. For example, barometric altitude refers to altitude based on atmospheric pressure, while altimeter settings in hectopascals reflect air pressure used to adjust altimeter readings, neither of which corresponds to the specific function of the green 510 indicator. Wind speed and direction in degrees magnetic involve the aircraft's horizontal movement and are not relevant to the vertical height measurement indicated by the radio height reading.