LEDFAA - Layered Elastic Design


Introduction


The next generation of large civil aircraft is expected to include models that will weigh up to 1.7 million pounds and have complex, multiple-wheel, multiple truck landing gear systems. The Boeing B-777-ER, which entered commercial service in June 2004, has only two six-wheel landing main landing gears to support a gross weight of up to 750,000 pounds. The 1.3-million-pound Airbus A380, which will enter commercial service in 2006, has two six-wheel body gears in addition to two four wheel wing gears, for a total of 20 wheels in the main gear assembly. The complex gear loads applied to airport pavements by these new aircraft types are quite different from the loads applied by the older generation of commercial airplanes. Complex wheel load interactions within pavement structures contribute to premature failure of the pavement structures, and must therefore be considered in pavement design analyses.


The pavement thickness design charts (nomographs) in FAA Advisory Circular AC 150/5320-6D have been used successfully for the past 30 years. These design charts were developed for many types of common aircraft gears, but they cannot accurately assess damage to airport pavement structures due to complex gear loads such as the B-777 and A380. To better predict wheel load interactions and to provide the airport community with a pavement design methodology addressing the needs of the B-777 aircraft, the FAA in 1995 introduced the design program called LEDFAA.


LEDFAA developed out of a 1993 FAA-sponsored feasibility study of Layered Elastic Design (LED) methods for airport pavements. The results of this feasibility study demonstrated that the LED procedure for flexible (asphalt) pavements produces pavement designs that are compatible with conventional FAA design procedures for existing aircraft types. At the same time, LED better predicts the wheel load interactions for the B-777 because the landing gear configurations and layered pavement structures can be modeled directly using the LED procedure. Therefore, the FAA decided to implement the LED procedure as a new standard for designing airport pavements intended to serve the B-777 airplane.


To ease the difficulties of implementing the LED procedure, and to empower design engineers with the required computational tools to perform the numerical computations, the FAA developed the LEDFAA program package. This program automates the LED procedures and provides design engineers with user-friendly graphical interfaces. The program minimizes user input variables and contains built-in error checking procedures on all the input values to minimize operator errors. Once all required input values are specified, the design thickness of the airport pavement is automatically computed.


Pre-release Beta testing of LEDFAA was completed in November 1994. The comments and recommendations received during the Beta test phase were incorporated into the initial release of the program (LEDFAA 1.2). On October 22, 1995, the FAA Office of Safety and Standards, AAS-1, issued a new FAA Advisory Circular AC 150/5320-16, which implemented LEDFAA as a new standard for designs of airport pavements intended to serve the Boeing 777 airplane.


Change 3 to FAA AC 150/5320-6D (May 2004) allows LEDFAA (version 1.3) to be used as an FAA alternative design standard for all pavement thickness designs, not just those intended for B-777 traffic. As part of this change, AC 150/5320-16 was cancelled and its provisions were incorporated into a new chapter 7 of AC 150/5320-6D. LEDFAA 1.3 (see below for latest changes) remains the required design procedure for airport pavements intended to handle triple-dual-tandem (TDT) aircraft, including the B-777 and A380.

Program Development

 

The FAA continues to develop and improve LEDFAA for the airport community. An upgraded version (LEDFAA 1.3), was released in 2004 as part of Change 3 to AC 150/5320-6D. The new version contains many enhancements, including: full 32 bit support for faster computations, full metric support, an improved user interface, and refined performance/failure models for more accurate pavement life prediction. LEDFAA 1.3 also features an expanded aircraft library, including the Airbus A340-500/600, A380-800 and A380-800F, and Boeing B-717, B-737-900 and B-777-200/300ER.

About LEDFAA


LEDFAA 1.3 is a computer program for performing thickness design of airport pavements. It implements advanced design procedures based on layered elastic theory developed under the sponsorship of the Federal Aviation Administration (FAA). LEDFAA handles new and overlay design of both flexible and rigid pavements. The layered elastic procedures implemented in the program are described in Chapter 7 of FAA Advisory Circular AC 150/5320-6D, the FAA thickness design standard for airport pavements. More detailed information on the procedures can be found in the internal help file installed with the program.


The computational core of the LEDFAA 1.3 program is LEAF, a layered elastic analysis program that is written in Microsoft© Visual Basic© and operates in a 32 bit Windows© environment.. LEAF is loaded and executed by LEDFAA as needed and is not normally visible to the user. The remainder of the program is wriiten in Microsoft© Visual Basic© and operates under Microsoft Windows©. LEAF is an improved layered elastic analysis program developed by FAA, replacing the JULEA program that ran in previous versions of LEDFAA.


LEDFAA represents a significant departure from earlier FAA design philosophies. Apart from the design procedures being implemented as a computer program instead of as nomographs, the main change from the user's perspective is that the 'design aircraft' concept has been replaced by design for fatigue failure expressed in terms of a 'cumulative damage factor' (CDF) using Miner's rule. Also, the major material property of the pavement layers is now uniformly expressed as an elastic modulus instead of the previous CBR (California Bearing Ratio) for flexible pavements or k-value for rigid pavements. Formulas for transforming CBR and k-values to modulus values are provided where appropriate in the documentation. Automatic conversion is provided in the program.


It should also be noted that, although layered elastic based procedures are normally considered to be mechanistic, and more rational than the previous procedures, a considerable amount of engineering judgment is still required. Designs produced by LEDFAA should comply with the detailed requirements and recommendations of AC 150/5320-6D. The program does not automatically satisfy all of these requirements and the recommendations in the Advisory Circulars should be followed in the selection of input parameters. It is the design engineer's responsibility to use the program and the advisory circulars in conjunction with each other.

It should be pointed out that the traffic and failure models of LEDFAA and the AC 150/5320-6D design nomographs are fundamentally different, and comparisons between the two sets of procedures are only valid when considering multiple aircraft traffic mixes. Single aircraft comparisons misleadingly indicate a degree of conservatism with the LED procedures which is not present for typical multiple aircraft mixes.


The LEDFAA program is also primarily intended for use in designing airport pavements according to a standard procedure. It is not intended to be used to compare the damaging effects of different aircraft by running single aircraft designs or CDF computations, i.e., ACN type calculations.


Getting The LEDFAA Program Package


The LEDFAA v1.3 program can be downloaded from the FAA Office of Airport Safety and Standards (AAS-1) web site.


LEDFAA13.exe (2.64 MB)

 

Note: The above download file is a self-extracting zipped file requiring WinZip©, PKZip©, or similar software to decompress it.


LEDFAA 1.3 should be used in conjunction with the FAA Standard, Advisory Circular AC 150/5320-6D. This standard is distributed by the FAA Office of Airport Safety and Standards, AAS-1, at the FAA Headquarters in Washington, DC. Advisory Circulars are available at AAS-1's Airport (150-series) Advisory Circulars web site, http://www.faa.gov/airports_airtraffic/airports/resources/advisory_circulars/. The LEDFAA User's Manual that was previously distributed as an appendix to cancelled AC 150/5320-16 is now available as an internal help file in the LEDFAA 1.3 software.


Contact Project Lead: Dr. David R. Brill, ANG-E262


Last Update: 04/03/2012