At intersections, there are always traffic lights. Vehicles and pedestrians consciously respect the rule of “red means stop, green means go” to avoid chaos and traffic accidents. Air traffic follows similar principles.
Unexpectedly Busy Air Traffic
Occasionally, when night falls, looking up at the sky, we may spot red, green, and white dots slowly moving across the horizon. These are aviation lights on aircraft, which help to prevent air traffic accidents.
Although the sky is vast, the speed of aircraft is very high, thus there is still a risk of collisions. To prevent accidents in the air, aside from the designated air routes for civilian aircraft on scheduled flights, pilots must continuously monitor their surroundings—front, back, right, left, and above—to avoid other aircraft. To assist pilots in observing their surroundings and keeping track of other aircraft flying along the same route, planes must illuminate three aviation lights during nighttime: two on the left and right wings and one at the tail. From the pilot’s perspective, the red light is always on the left wingtip, the green light is on the right wingtip, and the white light is mounted at the tail of the aircraft. These three lights can continuously shine or blink.
Once aircraft turn on their aviation lights at night, the visibility for pilots improves significantly. If a pilot sees another aircraft at the same altitude and only notices two colored lights, it indicates that the other aircraft is approaching head-on, posing a collision risk that requires evasive action. If only one light is visible, it means the other aircraft is either to the left or right. If all three lights are seen at once, it indicates that the other aircraft is flying above or below. These two scenarios are not dangerous, allowing the pilot to maintain steady control.
However, the speed of modern aircraft is substantial, and relying solely on lights is not enough. What about in bad weather with clouds and fog? Currently, there is a device known as the Aircraft Approach Indicator that helps pilots detect nearby aircraft. This device is equipped with indicator lights, and at the same time, the aircraft’s radar continuously emits radio waves around it. When another aircraft flies close, the radar waves are reflected back, causing the indicator lights to illuminate. From the different indicator lights, pilots can assess the relative direction and distance of the approaching aircraft.
