1. Aircraft Sequencing Problem Near Terminal Area
Final Term Project Presentation
Sepehr Sarmadi
Dec. 10th 2002

2. System users & objectives
System definition
10,000 ft
50 nm
Terminal area:
A cylindrical-shaped region with varying radius and altitude centered around an airport.
Final approach
An area where aircraft begins its landing maneuver
Runway(s)
Size is determined by the range of the airport’s radar and communication coverage.
System users & objectives
Airlines and passengers
Delay
Pilots
Predictable ATC instructions
Being treated fairly
Air Traffic Controllers
Safety (time and space separations)
Workload
Passenger delays
Crew costs
Aircraft utilization
Fuel costs
Airport managers
Throughput maximization

3. Aircraft Sequencing Problem (ASP)
Definition
The problem of sequencing aircraft near the terminal area to satisfy some of the “system objectives” stated before
Aircraft Sequencing
Aircraft category &
Decision making strategies to compute
FCFSSE (First Come First Served System Entrance)
FCFSRW (First Come First Served Runway entrance)
LLT (minimizing the Last landing Time)
TPD (minimizing Total Passenger -or flight- Delay)

4. ASP components Main challenge:
Finding the static solution to the scheduling problem at any given time
Updating the static solution to take into account the arrival of new aircraft
Main challenge:
Both components must be completed in a few seconds as the nature of the real time ASP demands

5. ASP, DP approaches (1)
ASP is a special case of the famous Traveling Salesman Problem 4 1 2 3 5 6 4 1 2 3 5 6
TSP in a complete directed graph 4 1 2 3 5 6
Hamilton problem Equivalent to LLT version of ASP

6. A time saving technique
ASP, DP approaches (2)
DP algorithm solution time for TSP
(non-polynomial)
A time saving technique
classifying arriving aircraft into a few categories (n) generally based on landing velocity and weight
Solution time
For m1=m2=m3=5, n=3, N=15:
= 0.002
But still DP approaches (or any other combinatorial approach) are computationally prohibitive in practice
“n” is usually larger than 3 (10-15 velocity classes is normal)
For every new entrance, the algorithm should be solved again

7. Dynamic updating for a simple example*
Id No. 1
120
990 2 20
135
930
950 3 55
150
900
955 4
110
1100 5
180
1110
Entrance No.
Update time
FCFSSE
FCFSRW
Min
Delay
Min Blockage 1 2 20
1,2
2,1 3 55
1,2,3
2,3,1
2,1,3 4
110
1,2,3,4
2,3,1,4
2,1,4,3 5
180
1,2,3,4,5
2,3,1,4,5
2,3,5,1,4
2,1,4,5,3
*Source: Dear, Roger, PhD thesis, MIT, 1976

8. Min delay and Max throughput strategies fail in practice
Large solution times, huge number of legitimate permutations to check
Difficulty in actual determination of the “optimal” solution
Global shifting of aircraft positions that cannot practically be implemented
Certain aircraft types might continuously be shifted backwards, causing intolerable delays making no sense to the pilot and to the passengers (note the treatment of aircraft #1 in minimum delay case and aircraft #3 in minimum blockage case)

9. CPS methodology
The only practical solution to ASP must involve a compromise.
“Constrained Position Shifting” methodology is a very successful compromise.
CPS Limits the maximum number of position shifts (forwards or backwards) with respect to FCFS
Is computationally practicable, omits many of the previously legitimate permutations
Avoids “wholesale” updating
Treats individual aircraft and velocity classes equitably
Decreases total and average delay
Increases runway throughput

10. CPS in practice (1)
Almost all major airports around the world still implement the FCFS strategy to avoid additional controller’s workload
The “CTAS” system in Denver and Dallas/Ft. Worth and the “COMPAS” system in Frankfurt include algorithms for assisting controllers do CPS with a maximum position shift of 1 or 2.
CTAS was conceived and is being prototyped at the NASA Ames Research Center. In 1991 it was chosen by the FAA as the future automation system for the terminal area
COMPAS was developed in the early 1980’s in Germany. One of the project goals was to plan an optimal sequence of the arriving aircraft from different arrival routes to make best use of the existing runway capacity

11. CPS in practice (2)*
On February 15, 1999, the Passive Final Approach Spacing Tool (pFAST) began sixteen hour per day operational use (covering over 80% of the arrival rushes) at the Dallas/Fort Worth (D/FW) TRACON.
Interesting results in the first two weeks of operation:
Controller acceptance of the runway advisories was 96.9%
Controllers reported the first week of operations “very positive.”
Rush periods seemed to last shorter
An increase in surface traffic was reported
*Source: NASA website

12. Thank You !