# Analytical Model for Enhancing the Adoptability of Continuous Descent Approach at Airports
## 2 Preliminaries and Process Description
### Structure of Airspace around Airports  
Terminal Maneuvering Area (TMA). TMA refers to the designated area of airspace managed by air traffic control services around major airports that have high volumes of traffic. Normally, TMA airspace is designed in a cylindrical configuration, including all altitudes centered around the geographical coordinates of the airport. Geographical positions that define the boundaries of the TMA, known as entry fixes, are considered as entry points to the TMA (although each fix includes all altitudes and thus may be conceptualized as a line), and the arriving aircraft enters the TMA airspace via entry or arrival fixes. When crossing the TMA boundary over one of these entry fixes, the responsibility for the separating aircraft will be handed over usually from a controller at the air traffic control center responsible for separating the en route aircraft (e.g., Air Route Traffic Control Center \"ARTCC\") to a controller at the air traffic control center responsible for separating the aircraft approaching an airport (e.g., Terminal Radar Approach Control \"TRACON\"). A simplified structure of a TMA is illustrated in Figure 1.  
As an arriving aircraft nears an entry fix, the air traffic controller may clear the pilot for the approach or, depending on traffic congestion and the separation and sequencing method used, may place the aircraft in a holding pattern. While aircraft separation aims to have the controller apply and maintain the separation dist  
Figure 1: Typical structure of a TMA (top view).  
aircrafts for safety purposes, aircraft sequencing aims to have the controller organize a stream of aircrafts to provide an orderly sequence of continuous traffic flow towards the final approach path. In practice, there are a number of aircraft-sequencing methods for approach traffic management, but generally, all of them fall under two broad categories: procedural control (published procedures with altitude change and speed instructions) and radar vectoring (controller-generated instructions in terms of headings, altitude, and speeds to optimize traffic flow in order to maximize the number of aircrafts with the least average delay). Today, radar vectoring is one of the main methods to achieve efficient sequencing for aircrafts flying towards the final approach path.  
Once an aircraft has been cleared by the controller to approach the airport or to leave a holding pattern, the aircraft approaches the merging fix. However, as the aircraft approaches the merging fix, it flies in the stacking space, the space that the controllers use from the available terminal airspace to stack arriving aircrafts, that is, orderly align aircraft arrivals for approach. In the stacking space, the controller manages air traffic and enhances airspace capacity by stacking arriving aircrafts using techniques such as minimal speed adjustments and path-stretching. This efficient management of air traffic flow enables the controller to bring together aircrafts that have crossed entry fixes from different directions to be stacked and merged at the merging fix. For instance, an aircraft may enter the TMA through one of about 12 entry fixes and then proceed to one of about four merger fixes as they get closer to the airport. The merging fix provides a transition for arriving aircrafts from the stacking space to approach, as it connects traffic from different directions into one merger fix to create one stream of aircraft arrivals to follow a standard published arrival procedure. This way, arrivals from several directions can be accommodated, and traffic flow is efficiently managed within a congested airspace. To safely merge arriving aircrafts, the controller synchronizes aircrafts based on joining window time on the air route leading to the merging fix considering sufficient spacing for other aircrafts to fit into the air traffic stream and while maintaining, at least, the minimum required separation between aircrafts.