FAA William J. Hughes Technical Center Airport Technology R&D Branch, AJP-6310 (AAR-410) COMFAA is a program for computing flexible and rigid Aircraft Classification Numbers (ACNs) and pavement thickness. It is installed by running Setup.exe either with the files on a hard disk or from 2 HD floppies. The program will only run under Windows 95 or NT, or higher. Although the program allows the user to calculate ACN values for any aircraft, it should be remembered that official ACN values are provided by the manufacturer of a particular aircraft. The program is used for pavement thickness design in the procedure required for PCN determination by the technical evaluation method, as described in FAA Advisory Circular 150/5335-5A "Standardized Method of Reporting Airport Pavement Strength - PCN." 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. We would appreciate any comments you may have on the program with regard to errors, features that don’t work properly, features that could be added, etc. 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. June 8, 2004 1. Added the capability to change the size of the main window. 2. Changed metric unit for rigid pavement support stiffness, k, to MN/m^3. 3. Changed the display of the k value to remain continuously visible during rigid pavement thickness design. 4. Changed the display of the CBR value to give two decimal places and to remain continuously visible during flexible pavement thickness design. 5. Added the number of main gears and the single-wheel load to the detailed output display for flexible pavement ACN and thickness design. June 5, 2006 1. Added pass-to-coverage for flexible and rigid. All internal calculations are still based on coverages. Pass-to-coverage was added so that users of the new pavement strength AC (AC 150/5335-5A) can convert departures to coverages for any aircraft in COMFAA, even those defined by the user. Calculation of pass-to- coverage is completely automatic and requires no setup or input from the user. 2. The Boeing and McDonnell-Douglas aircraft libraries have been updated and expanded. 3. The order of listing subgrade strength in the output data table has been reversed. The lowest strength subgrade is now listed first. September 26, 2006 1. Added an additional set of new alpha factors (the '06 Alphas) at 10,000 coverages to match the new ICAO numbers for 4- and 6-wheel gears. 2. Added an additional set of new alpha factor versus coverages curves to match the new ICAO alpha factors at 10,000 coverages. 3. The old ICAO alpha factors are the default. The '06 Alphas can be selected with a check box. An explanatory note is displayed when the '06 Alphas are selected. November 29, 2006 1. Added annual departures for flexible and rigid pavements in the airplane parameter grid. Traffic data is still stored in the airplane libraries as coverages. New traffic data can be stored in the libraries by clicking Coverages or either of the cells for annual departures, but only in the pavement thickness design mode. In ACN mode, the values of annual departures always correspond to the standard 10,000 coverages used to calculate ACN. February 6, 2007 1. Renamed the program COMFAA 2.0 to acknowledge the inclusion of the new alpha factors as an option and the computation of pass-to-coverage ratios. 2. Expanded the internal airplane library to include a full list of Airbus airplanes except for A340-500/600 models. The A340-500/600 models will be included when definitive belly gear load characteristics are provided to the FAA. In the mean time, please consult the Airbus airport planning manual to obtain ACN values or gear dimensions and loading. The gear dimensions and loads can be entered manually into the COMFAA external airplane library if desired. February 2, 2008 1. Extended the input range for concrete flxural strength from 500 to 900 psi to 200 to 2,000 psi.