BAKFAA - Computer Program for Backcalculation of Airport Pavement Properties

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.

Setup requires the files:


Copy all of the files into a directory on a hard disk. Then run setup.exe.
Setup can also be run from the distribution disk.

BAKFAA performs backcalculation of pavement layer modulus values using the FAA layered elastic analysis program LEAF and a downhill multidimensional simplex minimization method. The function minimized is the sum of the squares of the differences between vertical pavement surface deflections measured with a falling weight deflectometer (FWD) and vertical pavement surface deflections computed with the layered elastic program. The minimization routine is a direct implementation of the set of routines in section 10.4 of the book Numerical Recipes in Fortran by William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P. Flannery, Cambridge University Press, 1992. The routines were translated from Fortran into Visual Basic. Constraints are not allowed by the minimization routine and the user must perform this function when necessary by selecting that the appropriate layer modulus values be changeable and setting the modulus values of fixed layers by hand. A version of BAKFAA with settable layer modulus constraints is planned for future implementation.

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.

Documentation for BAKFAA and LEAF can be downloaded from the pavements download area of

Source code for the complete program is also available for download from The source code is provided as additional documentation, for experimentation with the backcalculation and layered elastic routines, and for implementation of LEAF in other applications if desired. LEAF is implemented in the program as an ActiveX dynamic link library. BAKFAA is the host application and the source code for BAKFAA and LEAF is loaded with BAKFAA.vbg.

Further guidance on the use of the program and general FWD-based backcalculation procedures is contained in the FAA Draft Advisory Circular AC 150/5370-11B, "Use of Nondestructive Testing in the Evaluation of Airport Pavements," which can be downloaded from the website The AC will be transferred to the Series 150 AC page ( when reviews and revisions are complete. Brief descriptions of the functions in BAKFAA are given below.

Added the capability to read AASHTO Pavement Deflection Data Exchange 1.0 (PDDX) files.
BAKFAA now reads JILS, KUAB, Dynatest Edition 20, and PDDX formatted files.

Data from PDDX files is read in the units specified in the PDDX file and converted to pounds, inches, and mils for load, plate radius, sensor positions, and sensor deflections, respectively. Output is in English units as indicated in the displays.

Data from other than PDDX files is assumed to be in pounds, inches, and mils.

A full metric option will be provided at a later date.

Changed the routine which reads the Dynatest file to read in fixed format. The previous version would not correctly read a Dynatest file when two numbers had no delimiting space.

Changed the tab order on the main screen for more convenient manual data entry.


Released BAKFAA version 2.0.

Please report any problems or requests for enhancements to the named contact on the downloads website.