Landing Gear Factors for Pavements with FAA Ratings

Abstract

Many airports in the United States and around the world list their runway pavement ratings according to the Federal Aviation Administration (FAA) system of Dual (D), Dual-Tandem (DT), and Double Dual-Tandem (DDT). This system has been in place for many years, and it has enabled airport authorities to determine allowable gross weights by a method that is easy to understand and use. In recent times, however, the development of larger widebody aircraft by the manufacturers has resulted in pavement loadings that cannot be adequately expressed by this type of rating, and so most ratings are now represented by the Aircraft Classification Number/Pavement Classification Number (ACN/PCN) system. Rather than develop new ratings for the 777 and other widebody aircraft, the FAA has gone ahead with a new design system, but left the current ratings in place. The ACN/PCN system will eventually take over as the rating of choice, but complete implementation appears to be far into the future, at least for U.S. airports. This has left the airport authority and many consultants with no guidance as how to evaluate an FAA-rated pavement which is traversed by aircraft such as the 777 or those with other unusual or non-standard gear configurations. The result is that many pavement loading analyses may be both inadequate and misunderstood. For example, the Boeing 777 has a 6-wheel gear, and it is not defined by the FAA system. Also, an Airbus A330 has a similar, but much larger wheel geometry than the standard FAA gear, and even though its pavement loading characteristics are superior to the FAA Dual-Tandem (DT), proper pavement loading credit may not be given. In the military sector, the C-17 has a six-wheel gear that is unlike any other arrangement. This paper proposes a method that adjusts the standard FAA rating to account for a variety of wheel sets such as those found in the larger widebody aircraft. Factors are developed that adjust the FAA rating for any widebody gear, including the 777. The method is easy to use, requiring only a use of a factor to arrive at an allowable gross weight. Pavement characteristics such as thickness, modulus, subgrade type and strength are not part of the standard FAA rating, and they are not required to utilize this method.