FAA Airfield Capacity Model
(ARO; 7/29/96)
1. Model Category
Airport Capacity.
2. Summary
The FAA Airfield Capacity Model is an analytic computer model which calculates the (maximum throughput) capacity of a runway system given continuous demand. Given data on the runway configuration and operating procedures in use, it estimates the hourly capacity for 15 common airfield configurations ranging from a single active runway to four active runways. The model was initially developed in the late 1970s by a consortium that included Peat, Marwick, Mitchell and Company and McDonnell Douglas Automation and further modified by the FAA with support from the MITRE Corporation. It was last modified in February 1981.The model approximates single runway capacity using logic based on the fundamental concepts of the classical Blumstein model and its extensions. For more complex configurations it uses modules (models) that extend the analysis. Combinations of these base modules are then used for even more complex configurations.
3. Inputs
The input is a single text file with information on: runway configuration in use and the type of operations (arrivals, departures or both) assigned to each runway; aircraft mix on each runway; ATC separation requirements between operations on each runway; aircraft characteristics, such as final approach speed, runway occupancy times for arrivals and departurees; length of final approaches; and weather inputs (ceiling and visibility) to determine flight rule conditions. Standard deviations of those inputs which are treated as random variables (e.g., runway occupancy times) are also required.
4. Output
The model estimates the capacity per hour of the runway system for any specified arrival-departure ratio. Increments of 10% are used, if desired, to obtain a capacity "envelope" that consists of 11 points ranging from (100% arrivals, 0% departures) to (0% arrivals, 100% departures).
5. Major Assumptions
The FAA Airfield Capacity Model assumes that each of the 15 common configurations it can analyze can be viewed as a combination of four fundamental configurations: single runway, closely-spaced parallel runways, intermediate-spaced parallel runways and intersecting runways. For each of these four fundamental configurations it includes a module which computes that configuration's capacity. These modules are, in turn, based on a single- runway model that computes (i) the "all arrivals" capacity of the runway, (ii) the "all departures" capacity and (iii) the capacity of the runway when departures are inserted between arrivals, without reducing arrival capacity. The capacity for other mixes of arrivals and departures is then computed by interpolating among these three points.All random variables in the model are assumed to be normally distributed and a 5% probability of violation of separation requirements is used in determining spacing of runway operations, using these normal distributions.
An implicit assumption is that taxiways and gates have little impact on determining airfield capacity. Another implicit assumption is that many airports operate with one of the fifteen runway configurations that the model analyzes and therefore the model will be useful. Both of these assumptions are substantially true in practice.
6. Computational Characteristics
The code is written in FORTRAN and is available for IBM machines, running on just 200 KB of memory. The UserŐs Guide contains a full description of the methodology used and clear instructions on how to run the program. Provided that the relevant information is available, it takes little time to prepare the input file and run the program. No graphical interface is available.
7. Modularity and Flexibility
Almost all variables are specified by the user, so the model can handle most situations. Since the output is a simple number, using the FAA Airfield Capacity Model in combination with other software packages is straightforward.
8. Status of Model
This is considered a "mature" model, with no changes planned.
9. Extent of Model Validation
Model validation took place in the 1970s. For selected configurations and cases, it was determined that the model provides adequately accurate estimates of airfield capacity.
10. Principal Applications
The model was used in the preparation of the FAA Handbook of Airport Capacity and Delay in 19. It has also been used in connection with a number of airport studies in the late 1970s and early 1980s, but its use seems to be limited today.
11. Model Availability
The FAA Airfield Capacity Model can be obtained from William J. Swedish, CAASD, The Mitre Corporation, 7525 Colshire Drive, McLean, Virginia 22102.
12. Information Base for Model Evaluation
The model was obtained and exercised at MIT. The following report was also reviewed:Upgraded FAA Airfield Capacity Model Supplemental UserŐs Guide, William J. Swedish, The Mitre Corporation, McLean, Virginia 22102, February 1981.
13. Summary Evaluation
The FAA Airfield Capacity Model can be a useful tool for policy-level studies that require quick approximate estimates of the sensitivity of airfield capacity to various changes in the most common operating parameters of airports (number and configuration of runways, aircraft mix, separation requirements, runway occupancy times, etc.) The model, however, can be improved significantly, particularly with respect to the logic for inserting departures between two arrivals on a runway. Because the model's logic is not particularly good in this respect, the model's estimates of capacity for cases in which a runway handles approximately the same numbers of arrivals and departures will often not be particularly accurate. The FAA Airfield Capacity Model could also be strengthened by including in it some of the features rthat exist in the single-runway analytical capacity model that was developed recently by LMI (see review in this volume). In fact, a new model that combines the single-runway logic of the LMI model with the extension to multiple runways featured in the FAA Airfield Capacity Model could be a very useful tool that would provide instantaneously estimates of runway system capacity with limited data requirements.
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