Comparative Analysis of the Old MWHGL and the New NAPTF Data

The new generation of aircraft will require more landing-wheels to be packed into basically the same space. A convincing illustration of the move to more densely packed landing gears is the recent Boeing B777, which has six wheels on each of two tridem landing gears. This move increases concern that current pavement-design procedures do not accurately predict load interaction for the type of closely spaced landing gears on the new generation of aircraft. In response to the requirement for developing valid pavement-design procedures, the American Federal Aviation Administration constructed and conducted a National Airport Pavement Test Facility (NAPTF) with the cooperation of the Boeing Company. The data from the NAPTF tests recently made available gave an opportunity to run a comparative analysis with the old multiple-wheel, heavy-gear, load (MWHGL) data, which would contribute to the thickness-design issue in general, and to new large aircrafts in particular. The present paper describes this comparative analysis, which addresses only flexible pavements. Its main conclusions are as follows: (1) The present direct-regression interpretation of the full-scale trafficking tests indicates that the characteristics of the old MWHGL data are identical to those of the new NAPTF data, whereas the previous interpretation (i.e., the alpha-factor interpretation) suggests the opposite; further, the stability of the coefficients obtained in the regression analysis (i.e., the consistency of interpretations) can be regarded as significantly high. (2) All these findings may hint that the present direct-regression interpretation (which eliminates both the load-repetition factor and the need for equivalent single-wheel load calculations) can be considered a possible legitimate method, even one that is more appropriate from the engineering point of view. (3) The design equations based on the direct-regression equations lead to smaller values of total thicknesses than do those derived from the COMFAA program by up to 10% for very weak subgrades.