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BAKFAA - Computer Program for Backcalculation of Airport Pavement Properties

The layered elastic program LEAF can also be run independently to compute pavement response for arbitrary gear geometries. The gear geometry and load configurations are entered by hand into an external ASCII file called LEAFAircraft.Ext. A header section in the file gives the required format for data entry. The file is read when LEAF starts. LEAF must therefore be closed down and restarted if changes are made to the aircraft data file. Pavement response output data appears in a separate text box for examination and cutting and pasting into external documents.

BAKFAA is a 32-bit program and requires Windows 95/98, NT, or later. The program is written in Microsoft Visual Basic 6.0. Service Pack 4 must be installed if BAKFAA is run under Windows 2000.

Unzip the file, copy all of the files into a directory on a hard disk. Then run setup.exe.

DOS Program Compilation

• 3DAAP: The Three-Dimensional Airspace Analysis Program (3DAAP) is software developed for use as an analysis tool to study airspace constraints to vertical development on and off the airport property boundary. It provides a set of tools to construct, depict, and analyze airspace restricting surfaces including Federal Aviation Regulation (FAR) Part 77.25 Imaginary Surface analysis, obstruction shadow diagramming, line of sight analysis and navigational aid siting criteria. It also provides different methods to determine conflicts within the airspace surfaces and objects or structures on the ground.
• Airport Design (for Microcomputers): This augments Advisory Circular (AC) 150/5060-5, Airport Capacity and Delay, including Changes 1 and 2, AC 150/5300-13, Airport Design, including Changes 1 through 4, and AC 150/5325-4A, Runway Length Requirements for Airport Design.
• GEO83: This is a collection of geodetic calculation programs, using the North American Datum of 1983 (NAD83), for individuals involved in airspace studies. GEO83 is composed of input information for new users and five geodetic programs. Both written and graphic descriptions for the programs are provided below. The INPUT/OUTPUT data and the results calculated by the programs are boxed in the examples for easy identification.
• MU41A: A program to estimate runway friction.

Spreadsheet Compilation

• FAA Flexible Pavement Design, AC 150/5320-6D
• FAA Rigid Pavement Design, AC 150/5320-6D

COMFAA

The FAA does not officially support the program as a standard nor sanction any use of the results.

A help file is included which gives brief information on the capabilities of the program and how to use the various features. It would probably be useful to print the help file for reference as there is no other documentation describing operation of the program. An additional report describes the procedures used to compute pavement strength and thickness.

The external aircraft file contains the landing gears used as examples in the ICAO pavement design manual. English units are used throughout.

The ACNs are computed using the International Civil Aviation Organization (ICAO) methodology. It is not an official FAA standard, specification or regulation, nor is it intended as a substitute for official guidance on reporting ACNs contained in ICAO publications. It is believed that the ACNs computed by this program are generally consistent with those reported by ICAO for specific aircraft, but in the event of conflict, the latter shall be considered authoritative.

In 1997, ICAO's ACN-PCN Study Group recommended that an interim alpha factor of 0.72 at 10,000 coverages be used for computing ACN for 6-wheeled landing gears.

By default, the ACN values for 6-wheel aircraft gear configuration including the Boeing B-777 airplane are computed using this interim modified alpha factor. The standard ACN cutoff for rigid pavement stress computation is 3 times the radius of relative stiffness (rrs). This gives inconsistent results with large complex gear configurations such as the C-17 (high-strength ACN higher than low-strength ACN). An option is therefore provided to change the cutoff. This sometimes leads to numerical problems and the numerical procedure may not converge.

Flexible pavement thickness design with COMFAA follows the same methodology used to produce the thickness design charts published in FAA AC 150/5320-6. That is, for a given subgrade CBR and a given number of coverages for the design aircraft, total pavement thickness is computed by the "CBR" method. The user has to determine the design aircraft and make the conversion from aircraft departures to design aircraft coverages. Conversion of layer thicknesses with appropriate equivalency factors must also be done by the user.

Rigid pavement thickness design with COMFAA follows the same methodology used to produce the thickness design charts published in FAA AC 150/5320-6. That is, for a given modulus of subgrade reaction and a given number of coverages for the design aircraft, total pavement thickness is computed by the Westergaard edge stress method with FAA failure criteria. The user has to determine the design aircraft and make the conversion from aircraft departures to design aircraft coverages. Conversion of support layers to effective modulus of subgrade reaction must also be done by the user.

FEAFAA

FEAFAA 1.0 is not an official FAA standard, specification or regulation. FEAFAA has been compiled to run on personal computers operating Windows 98 or higher, with a recommended minimum of 256 megabytes of RAM. Users running Windows 2000 Professional may need to install Service Pack 3.

The download includes two utility programs, HexStressReader.exe and ShellStressReader.exe, which are useful for interpreting the 3D finite element output data (which can be voluminous). The use of these utility programs is described in the program help file under "Output Data Arrangement." It is also possible to view the mesh using the data file pltdata.dat. This is an ASCII file containing 3D finite element mesh data that can be read or converted by common postprocessing programs.

LEDFAA

• Setup.exe
• Setup.lst
• LEDFAA1.cab
• LEDFAA2.cab

Copy all of the files into a directory on a hard disk. Then run setup.exe.

The Alternate SG check box allows a flexible pavement design (or Life) to be run based on the vertical strain at the top of the layer below the arrow on the left side of the pavement structure. The procedure is:

1. get into the Structure window
2. check the Alternate SG box at the bottom left of the structure
3. press the Modify button
4. click to the left of the picture of the structure (in the margin) to move the arrow so that it sits to the left of the layer above the layer where the vertical strain criterion will be applied
5. return to design mode
6. if a design is run then the layer pointed to by the arrow will change thickness to satisfy the strain criterion in the layer below
7. if Life is run then the life for the strain criterion in the layer below will be calculated.

See the help file under Running the Program - Options for information on Batch, No AC CDF, and No Out Files.

LEDFAA 1.3 includes the Airbus A340-500 and A340-600 aircraft in the aircraft library. The same methodology is used as for other aircraft with two wing gear and one belly gear (MD-11 and A340-200 for example) except that the A340-500/600 has a dual-tandem belly gear instead of a dual-wheel belly gear. The methodology is for the A340-500/600 to be split into two separate aircraft for design; one with the two wing gear and the other with the one belly gear. The two aircraft (for design) are shown separately in the aircraft list.

LEDFAA 1.3 includes the Airbus A380-800 and A380-800F aircraft in the aircraft library. Inclusion of these aircraft required changes to the design methodology implemented in version 1.2 because of the different number of wheels in the wing and body gears (four in the wing and six in the body). The main difference is in the flexible design procedure as follows:

1. 1. Vertical strain at the top of the subgrade is computed at multiple points with all of the wheels in the main gear contributing to the computed strain (16 wheels for the B-747 and 20 wheels for the A-380). In version 1.2, subgrade strain is computed for the B-747 with four wheels, representing only one of the gears, contributing to the computed strain.
2. 2. Response under the wing and body gears is evenly split into two areas. The maximum response in the wing gear area is used to compute the pass-to-coverage ratios and cumulative damage factors (CDF) for the wing gears. The maximum response in the body gear area is used to compute pass-to-coverage ratios and CDFs for the body gears. These two sets of CDF values are then added to the CDFs calculated for all of the other aircraft in the mix and the result used in the normal thickness design procedure.

The design procedure for rigid pavement design is unchanged from that in LEDFAA 1.2. The difference between the number of wheels in the wing and body gears of the A-380 is accommodated by treating the aircraft as two different aircraft in the design mix. One of the design mix aircraft is a dual tandem with the same configuration as the A-380 wing gears and the other is a dual-tridem with the same configuration as the body gears of the A-380. The separation into two design mix aircraft is transparent to the user.

A number of other changes have been made to the program either for ease of use or for improved functionality. These changes include:

1. Replacement of the layered elastic computational program Julea with the internally prepared program Leaf.
2. A full 32-bit implementation which is compatible with all current Microsoft PC operating systems.
3. The default setting for data output files (*.out) is for them not to be written. Double click on the gray area of the Structure Window and uncheck "No Out Files" for the data files to be written to the working directory. (The default setting is not described in the help file.)

LEDFAA 1.3 is an interim release of the computer program, FAArfield, now in preparation. FAArfield includes a 3-D FEM model for rigid pavement response calculations. The flexible design procedure will remain unchanged from LEDFAA 1.3 except that the failure model parameters may be adjusted as a result of the compatibility study now underway. Any changes in the flexible design parameters will result in minor differences in computed design thicknesses.

A beta version of FAArfield (called FEDFAA) is shown below.

FEDFAA

Design information is entered by means of two graphical screens, one for the structure and one for the traffic. Default values and ranges for the various input parameters have been set so that the designs produced by FEDFAA are compatible with designs produced by the design procedures in Chapter 3 of AC 150/5320-6D for aircraft up to and including the current generation (B-727, DC-8, B-747, DC-10, etc.). Designs for new generation aircraft having triple-dual-tandem (TDT) landing gear, such as the B-777, are not covered by the design procedures in Chapter 3 of AC 150/5320-6D. Chapter 7 of AC 150/5320-6D, in conjunction with FEDFAA, provides the necessary information for thickness design when TDT aircraft are included in the aircraft mix.

Apart from the procedures being implemented as a computer program instead of as nomographs, the main change in pavement design from the userís perspective is that the ìdesign aircraftî concept has been replaced by design for fatigue failure expressed in terms of a ìcumulative damage factorî (CDF) using Minerís rule. Also, the major material property of the pavement layers is now uniformly expressed as an elastic modulus instead of the previous CBR (California Bearing Ratio) for flexible pavements or k value for rigid pavements. Formulas for transforming CBR and k values to modulus values are provided where appropriate in the documentation. Automatic conversion is provided in the program.

It should also be borne in mind that, although layered elastic based procedures, and 3D finite element models, are normally considered to be mechanistic, and more rational than the previous procedures, a considerable amount of engineering judgment is still required. Designs produced by FEDFAA should comply with the detailed requirements and recommendations of AC 150/5320-6D. The program does not automatically satisfy all of these requirements and the recommendations in the AC should be followed in the selection of input parameters. It is the designerís responsibility to use the program and the advisory circular in conjunction with each other.

A complete description of the design procedures and program structure is not possible within the confines of a userís manual, and these descriptions can be found in other publications. The main intent of the userís manual is to provide sufficient information for operating the program, selecting input data values, and interpreting the output data. Installation of the program is described first, followed by descriptions of the various parts of the program and its operation. Information is then given on the structure data input requirements and how they relate to the design procedures. Final sections provide a short discussion on program running times, with possible strategies for decreasing design time for a given design case, a description of the structure of external data files so that interested users can access the files and incorporate the data in other applications if desired, and a selection of design examples.

The relationship between the layered elastic and 3D finite element based thickness design procedures and the nomograph based thickness design procedures in Chapter 3 of AC 150/5320-6D is not discussed in the manual. However, it should be pointed out that the traffic and failure models are fundamentally different and comparisons between the two sets of procedures are only valid when considering multiple aircraft traffic mixes. Single aircraft comparisons misleadingly indicate a degree of conservatism with the LED procedures which is not present for typical multiple aircraft mixes. The program is also primarily intended for use in designing airport pavements according to a standard procedure. It is not intended to be used to compare the damaging effects of different aircraft by running single aircraft designs or CDF computations, i.e., ACN type calculations.