# 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 Time  
Before we present the details of our model [33], we briefly discuss the concept of acceptance, and rejection, in the context of landing operations at an airport and, particularly, with CDA operations. In general, our queueing model assumes that at a given airport, the air traffic controller will either accept all CDA requests from aircraft arrivals to approach and land using CDA over a specified brief time period, say 15 min, or reject them all. Before the queueing model details are calculated, we first calculate the CDA Adoptability Factor (CDA_AF). This factor is a function of \(\lambda_{\mathrm{ss}}\), the average arrival rate of the aircraft that requests CDA at an airport (which our model assumes is all the aircrafts arriving at  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Comparison Criteria** & **CDA** & **SDA** \\ \hline \multirow{3}{*}{Operational Benefits} & Reduces noise, emissions, and & \\  & flight time and improves fuel & \\  & efficiency. & \\ \hline \multirow{3}{*}{Facilitation} & Tactical radar vectoring, & \\  & published arrival procedures & \\  & (STAR), or a combination of & \\  & these. & \\ \hline \multirow{3}{*}{Approach Type Based on Vertical Navigation} & Performance path computed by & \\  & the FMS using idle or near-idle & \\  & thrust from the TOD point to & \\  & the first waypoint. & \\ \hline \multirow{3}{*}{Sequencing and Separation of Air Traffic} & Requires more spacing between & \\  & aircrafts during radar vectoring & \\  & and early sequencing that may & \\  & require advanced sequencing & \\  & tools. & \\ \hline \multirow{3}{*}{Impact on Airport Capacity} & \multirow{3}{*}{May reduce airport capacity.} & No reduction in \\  & & airport capacity \\ \hline \multirow{3}{*}{Descent Initiation} & The pilot initiates descent from & \\  & a TOD point that is as close to & \\  & the airport as possible. & \\  & & airport than with CDA. \\ \hline \multirow{3}{*}{Aircraft Performance: Airspeed} & Smooth speed profile, although & \\  & the pilot may occasionally & \\ \cline{1-1}  & adjust speed at the controller’s & \\ \cline{1-1}  & request to account or to balance & \\ \cline{1-1}  & the rate of descent. & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the Differences between CDA and SDA.  
the TMA's stacking space due to the advantages of continuous descent), and _AAR_, the Airport Arrival Rate, which is defined as the dynamic parameter that specifies the number of arriving aircrafts that an airport can accept during any consecutive 15-min period of time [34]. As shown from the equation below, CDA_AF represents the ratio of \(\lambda_{ss}\) to _AAR_:  
\[\text{CDA\_AF}=\frac{\lambda_{ss}}{AAR} \tag{1}\]  
If the value of CDA_AF during the brief time period is high (e.g., over 100%), then there is no need to continue the analysis. It is obvious in that case that CDA will not be permitted during that time period. Conversely, if the value of CDA_AF is low (e.g., under 10%), then it is obvious that CDA will be permitted during the time period. The queueing calculations are performed only for those periods of time when the value of CDA_AF does not make it obvious whether or not CDA should be permitted during the time period.