# Analytical Model for Enhancing the Adoptability of Continuous Descent Approach at Airports
## 3 CDA Adoptability and Aircraft Descent Times
### Factors Impacting CDA Adoptability and Aircraft Descent Times  
There are a number of factors that could impact the nature of aircraft arrivals at airports. Such factors could be operational, meteorological, planning, technological, or related to airspace structure and procedures design. We discussed some of these factors in Section 2 and discuss others briefly in the following subsections. Technology factors (e.g., the level of Air Traffic Management automation at an airport) are beyond the scope of this work. Other factors, however, such as traffic at neighboring airports and wind speed and direction, can be managed by reducing aircraft stacking space and increasing the minimum separation distance between aircrafts. The Airport Arrival Rate (AAR) states the hourly capacity of airplane arrivals at an airport, and thus, it is critical to our model. The aircraft fleet mix, or more generally fleet mix, refers to the ratio of various aircraft types that, based on wake turbulence categories, make up the total arrival traffic that operates at an airport. Fleet mix is essential in airport planning to determine the likely average landing speed and separation requirements on final approach, which are important factors that affect the AAR and, in turn, our model. Generally speaking, and from the perspective of runway capacity, which is defined as the expected number of landings that can be performed per hour on a runway, a relatively homogenous fleet mix, consisting of two dominant aircraft classes, is more favorable than a heterogeneous fleet mix. The aircrafts' separation requirements determine the maximum number of aircrafts that can navigate each part of the airspace or can use a runway system per unit of time. The separation requirements for an aircraft landing on the same runway specify the minimum separation in longitudinal distance, or time, that must always be maintained between two aircrafts operating consecutively on the runway. These requirements are also specified for every possible pair of classes and every possible sequence of movements [35]. Table 2 exhibits the ICAO's minimum wake turbulence separation standards [36], and apparently, the larger the separation required, the lower the AAR. Furthermore, the more heterogeneous the fleet mix at an airport, the more influence there will be on AAR and our model. These separation distances are based upon SDA being used. If CDA is used, then the minimum separation distances are the same when the leading and trailing aircrafts are from the same weight turbulence category but longer than with SDA when the aircraft weight classes differ. Furthermore, the air traffic controller will be inclined to use longer minimum separation distances with CDA because of the greater challenge of controlling aircrafts using CDA. Among the usually considered weather conditions at airports, such as cloud ceiling and visibility, wind speed and direction are the most influential conditions on ATM operations in general and on approach operations, in particular. The two components of the wind, headwinds and tailwinds, have a significant impact on AAR. In fact, wind speed and direction dictate the availability and orientation of runways at any given time. Adverse wind conditions can reduce AAR due to the increased complexity of merging arrival traffic streams and separating aircrafts as they descend and change heading under intense or varying winds. Specifically, winds aloft may result in a phenomenon called compression, in which the separation between pairs of arriving aircrafts decreases rapidly as they descend to the final approach [37]. The results from applying our model indicate it captures the effect of wind speed. In general, airport and airspace constraints refer to limitations that hinder airport capacity by creating difficulties for arrival aircrafts, largely due to airspace consideration.