BAKFAA Interface (Click to Zoom)


BAKFAA is an analysis software developed by the FAA for the purpose of quickly obtaining results from falling/heavy weight deflectometer (F/HWD) tests. Its name is a combination of “FAA” and “backcalculation”, the main analysis process it uses. In BAKFAA, the load and deflection data from an F/HWD test are analyzed to obtain the pavement layer moduli. It was first released in 2003 and receives continuous updates. BAKFAA’s main features are as follows:

  • estimation of modulus by backcalculation of FWD data
  • multi-layer analysis (up to six)
  • determination of joint transfer efficiency between slabs
  • support for six FWD manufacturers
  • analysis of pavement response to aircraft gear loading


Backcalculation Variables (Click to Zoom)


BAKFAA, along with other FWD analysis programs, makes use of layered elastic theory: the assumption that each pavement layer is homogenous, isotropic, and linear elastic. These assumptions drastically simplify calculations. BAKFAA uses LEAF (Layered Elastic Analysis, FAA), a function that minimizes error, to perform its iterative backcalculation. In this process, it is assumed that there is a set of layer moduli that cause the best match between the measured deflections and calculated deflections. To perform an analysis, a user inputs the pavement layer information (thickness, material, Poisson’s ratio, interface parameter) and seed modulus for each layer. The seed is the starting value used during iteration and provides a rough estimate for modulus to prevent extreme results. During each iteration of backcalculation, deflections are calculated with the seed moduli, then those deflections are compared to the measured deflections, moduli are adjusted (up or down), and deflections are compared again. If the error is lower than before, further adjustment is made in the same direction. If not, the adjustment is reversed. This process is repeated until a set tolerance for deflection difference is reached or the error cannot be minimized any further.


BAKFAA also has several supplementary functions. LEAF can be run independently, allowing the computation of pavement response for aircraft loading gear. The single LEAF analysis can output stresses, strains, and deflections at any point in the pavement structure. A user inputs pavement geometry, gear geometry (by selection or custom creation), and load configuration. Another function of BAKFAA is calculating joint transfer efficiency, or how well one concrete slab transfers load to an adjacent slab. For this test, the HWD is positioned with one sensor on one slab and the other sensors on an opposite slab.


For more information on the HWD, follow the link to the detailed FAA webpage.



To download BAKFAA, follow the link to the FAA download webpage.




Return to NDT Technology Overview