1. Primary Model Category:

System-wide model of air traffic flows and delays

2. Summary:

The National Airspace System Performance Capability (NASPAC) is a fast-time simulation model that may encompass large regions of airspace and a large number of airports. The simulation ÒfliesÓ individual aircraft through daily itineraries (that may include landings and take-offs at a sequence of airports) and provides statistical reports on delays and flow rates observed. The model includes simplified representations of en route sectors, as well as of airports. Some graphical outputs by airport, sector or region can be provided. NASPAC was originally conceived as a macroscopic-level model that would support studies dealing with issues related to strategies for national airport investments and to policy for national and international ATM. However, much detail has been added to it over the years and it may actually be better suited today to answer questions of a more tactical nature, such as the effects on delays of alternative flow management strategies. Several variations and extensions of NASPAC have been developed in recent years by MITRE, for internal use. CENA in France has also developed a version (F-NASPAC) which is better adopted to the European ATM environment.

3. Input Requirements:

The principal inputs to NASPAC, include: demand, in the form of a complete schedule of aircraft itineraries in the airspace region of interest (the demand includes both scheduled and unscheduled flights); capacities of airports and of other ATM resources, such as any modeled fixes and en route sectors; and aircraft performance data.

4. Outputs:

The main outputs of NASPAC consist of estimates of delay and of flows past given points ("throughput") in the system modeled. Delay is reported in the form of "technical delay" (defined as the local delay incurred at any specific point in the system) and of "effective delay" (defined as tthe difference between scheduled and actual times of events, such as the arrival or departure from a gate).

5. Major Assumptions:

The NASPAC simulation is essentially a deterministic one. Given a schedule of operations and a set of resource capacities, the model performs this schedule and then reports associated delays and flows. Modeling of resources is at a low level of detail, in keeping with the model's objectives. For example, an airport's capacity is a single number that represents the acceptance rate of that airport and is not concerned with gate capacity, taxiway capacity or the nuances of the runwau configuration in use.

NASPAC includes a module that attempts to infer the itineraries of individual aircraft from OAG-like airline schedules. This process is, of course, an approximate one.

6. Computational Characteristics:

The NASPAC simulation model is written in SIMSCRIPT II.5 and its pre- processor and a graphics and report-generating post-processor in Fortran, C and Pascal. The model runs in a workstation environment (SUN Sparcstations).

7. Modularity and Flexibility:

The level of detail in NASPAC modeling can be adjusted to some extent to fit the needs of the study at hand. The pre-processor and post-processor consist of a large collection of programs that can be utilized according to need.

8. Status of Model:

NASPAC was originally developed by the MITRE Corporation for the FAA during the late 1980s. After several revisions, the model was transferred to the FAA. NASPAC has also been transferred by the FAA to a small number of national and international civil aviation organizations outside the United States, such as CENA (France) and Eurocontrol, where it is used as a research tool, as well as for support of traffic flow management operations.

The FAA has no current funding or plans for the further development of NASPAC. However, some model enhancements are taking place in connection with specific model applications. For example, CENA has implemented a number of model modifications that make NASPAC more adaptable and appropriate to the European ATM environment.

MITRE has also developed recently at least two other models which can be viewed as simplified alternatives to alternatives to NASPAC. These are Quickpac and AMC (the Aggregate Modeling Capability).

9. Extent of Model Validation:

A number of NASPAC validation efforts have taken place over the years (see, e.g., Cherniavsky, Ellen A. et al., Validation of the National Airspace System Performance Analysis Capability Simulation Model, MITRE Report MTR-89W00170, May 1990). Agreement with field observations has been reported to be reasonably good.

10. Principal applications:

NASPAC has been applied in a number of instances in the United States and in Europe. For example, the model was used to assess the impact on airline delays nationwide of the (then proposed) new Denver International Airport. It was found that the proposed airport would contribute to a substantial reduction of delays on a national scale. Some applications of NASPAC have been concerned with the impacts of alternative traffic flow management (TFM) strategies. The NASPAC database currently includes the entire National Airspace System in the United Staes with emphasis on the 58 busiest commercial airports.

11. Model Availability:

NASPAC is not a commercially available product, but access to it can be obtained through the FAA. Occasional NASPAC-based studies in the United Staes are carried out at FAA Headquarters (supported by CSSI), FAA Technical Center and the MITRE Corporation. NASPAC-based studies in Europe are performed by CENA (France) and Eurocontrol, which also have copies of the model.

12. Information Base for Model Evaluation:

Interview with Steve Bradford (FAA) and William Weiss (CSSI) on 12/18/95.

Presentation by Anthony Zukas (MITRE) on 12/18/95.

Numerous informal discussions with NASPAC developers or users at MITRE, CSSI, FAA and CENA.

13. Summary Evaluation:

NASPAC is the first model to be developed for the express purpose of studying the propagation of delays and congestion through a national or regional ATM system. It can be a useful tool, if utilized properly with a recognition of its strengths and limitations. For example, because the capacity of each airport in a national system can assume several different values, there is typically an enormous number of different combinations of airport capacity values that can materialize on any given day. Thus a very large number of "runs" of NASPAC, each with a different combination of airport capacity values would be needed to obtain good estimates of the mean delay values encountered in the system and of the typical range of these values (e.g., their standard deviation).

Use of the model requires considerable training and significant resources in terms of both costs and personnel. Arrangements must be made with one of the organizations that operate the model. Extensive data are also needed, but databases have by now been assembled both for the United States and for parts of Western Europe to support many types of NASPAC studies.in