Cars drive along the road, and airplanes fly along the "invisible roads" in the sky known as air routes.

Have you ever wondered how these routes were determined initially?

As the number of routes increases, how are new routes established? These routes, officially called air traffic lines, dictate the specific direction of aircraft flight, landing, and stopping points. They also determine the route width and flight altitude according to air traffic control needs, maintaining order and ensuring flight safety.

Categories of Air Routes

Air routes can be roughly divided into three categories: international routes, domestic routes, and regional routes.

- International Routes: These routes connect two or more countries, facilitating international transportation. A flight is considered international if its departure, stopping, or terminating station is on foreign territory.

- Regional Routes: These routes connect regions with special status within a country, such as routes between mainland China and Hong Kong, Macao, and Taiwan.

- Domestic Routes: These routes are within a country and are subdivided into trunk routes, feeder routes, and local routes.

Establishing a New Route

When news reports mention the opening of a new route, it implies that specific technical requirements and criteria have been met. Opening a route involves selecting a flight path that meets the aircraft's performance requirements and ensures continuous contact with ground control throughout the journey. This is a technical and meticulous job.

Safety and Regulation

Safety is paramount in aviation. For safety reasons, international civil aviation authorities require civil flights to adhere to routes designed and published by air traffic control (ATC). These routes specify the flight direction, distance, and time separation between aircraft in three-dimensional space. When designing these routes, authorities consider factors such as alternate landing airports, facilitating emergency landings in case of mechanical problems.

Efficiency and Economic Considerations

While safety is prioritized, air route design also considers efficiency to maximize economic benefits. For instance, utilizing great circle routes (the shortest path between two points on a sphere) and leveraging jet streams can expedite flights and save fuel.

Distance to Alternate Airports

Current international civil aviation regulations generally stipulate that the distance from any point on an aircraft's flight path to the nearest alternate airport should not exceed 120 minutes of flight time for a single-engine aircraft. This regulation ensures aircraft can safely land before reaching a point of uncontrollability, known as the “Airworthiness Regulation Limit” or ETOPS (Extended-range Twin-engine Operational Performance Standards) Regulation Limit.

Air Traffic Control Restrictions

In addition to route planning, ATC plays a crucial role in maintaining safe distances between aircraft. Controllers manage airspace by issuing instructions that may require detours, occasionally deviating from the ideal or most direct route.