Preparing for new large aircraft

The next generation of aircraft creates new pressures for an airport’s pavements, consequentially engineers at the FAA’s Research and Development centre are continuing to develop systems to predict and assess requirements.

In 1927, the Ford Motor Company built one of the world’s first paved runways at Ford Airport in Dearborn, Michigan. With no aviation experience and no airport pavement design specifications, engineers built this and other early runways using pavement thicknesses similar to those of early highways. In fact, until World War II, airport engineers based concrete pavement design on the anticipated loads imposed by the trucks refuelling the airplanes, rather than the airplanes themselves.

For many years after the war, airport pavement research and technology benefited from advances in highway research, as well as from Department of Defense research supporting military aircraft and airfields. “Added complications in today’s design estimates stem from the facts that aircraft weigh far more than they did when the basic relationships were established and that landing gear layouts are far more complicated, with many more wheels per gear and more gears per aircraft,” explains Dr. Gordon Hayhoe, Manager of the FAA’s National Airport Pavement Test Facility.

“During takeoff, landing and taxiing, the aircraft pavement must support as much as 600 tons of aircraft, luggage, fuel and passengers, all of which is concentrated on the relatively small contact area of the plane’s tyres,” adds Dr. David Brill, FAA’s airport pavement research engineer.

Weighty consequences

The next generation of large civil aircraft will include models that will weigh up to 1.3 million pounds and have complex, multiple-wheel, multiple truck landing gear systems. For example the Boeing 777, which entered commercial service in June 1995, has two six-wheel main landing gears to support a gross weight of up to 777,000 pounds. The six-wheel gear loads applied to airport pavements by the Boeing 777 and the new Airbus A380 are quite different from the loads applied by the older generation of commercial airplanes.

In engineering parlance, airport pavements (runways, taxiways and aprons) may be either flexible or rigid, depending on the type of material. Flexible pavements are constructed of asphalt, while rigid pavements consist of concrete slabs on a prepared sub-base. At larger airports, both types of pavement generally include a stiffened, or ‘stabilised’, base layer to provide additional support.

As aircraft get bigger and heavier, with new kinds of landing gear, added stresses and strains are exerted on airport pavements. Complex wheel load interactions within pavement structures can contribute to the premature failure of the pavement structures and must therefore be considered in pavement design analyses. When traditional, empirically-based pavement design methods are used to analyse loads from the new generation of landing gear, they require pavements that seem unrealistically thick. Hence, new heavy aircraft may require airports to overlay, reinforce, or even rebuild runways to adapt to heavier loads.

Assessing the load

To assess pavement requirements for heavier aircraft and in an effort to create more cost-effective, longer lasting airport pavements, FAA researchers are developing criteria and methods for the design, evaluation, performance and serviceability of airport pavements. Prior to the first flight of the Boeing 777, the existing FAA design standard for airport pavements – FAA Advisory Circular AC 150/5320-6D – had been used for more than 25 years. However this standard could not accurately assess damage to airport pavements as a result of the complex gear loads of the Boeing 777.

To better predict wheel load interactions and to provide the airport community with a pavement design methodology addressing the needs of new, heavier aircraft, FAA researchers have developed a design program called LEDFAA-Layered Elastics Design (LED) method. As a result of this research effort, in 1995, the FAA issued a new Advisory Circular, AC 150/5320-16, implementing LEDFAA as a new standard for the design of airport pavements intended to serve the Boeing 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’s researchers have also developed the LEDFAA program package with a user-friendly graphical interface. The program minimises user input variables and the design thickness of the airport pavement is automatically computed.

In 2003, the FAA further modified the LEDFAA program to accommodate the latest commercial jet aircraft, in particular the Airbus A-380-800 and to incorporate software and hardware compatibility with new operating systems. The new Advisory Circular, designated AC 150/5320-6D-Change 3, can be downloaded from the FAA’s website at http://www.faa.gov/arp/engineering/software.cfm. AC 150/5320-6D-Change 3 specifies that LEDFAA can be used as an alternative FAA design standard for airport pavements handling all traffic mixes, not just those intended to serve the B-777 and replaces AC 150/5320-6D and AC 150/5320-16.

Rigid considerations

However the use of a layered elastic model to represent a jointed rigid pavement does have some deficiencies; it can accommodate multiple layers for support but not the joints. So the FAA developed another program, called FEDFAA, for rigid pavement thickness design. In this program a 3D finite element model (3-D FEM) is used to model the rigid pavement structure with joints.

The 3D-FEM method can handle greater detail and more complex characterisations of construction materials than the LED method. It is particularly useful for modelling rigid pavements, because the slab edges are often the critical components in them. In addition, the 3D-FEM method can incorporate nonlinear and non-elastic material models. Lastly, the 3D-FEM method is also capable of more accurately simulating the structural response of airport pavements to complex loading configura­tions, including aircraft with six or more wheels per gear.

FAA researchers successfully completed in-house alpha testing of FEDFAA in August 2003. The primary goal of alpha testing was to verify that FEDFAA performs new rigid and rigid overlay design computations properly. The program also incorporates the flexible design procedures from LEDFAA. FEDFAA is available for download from the FAA website. This program should be of interest to pavement engineers and designers, civil aviation authorities, aircraft industries, civil engineering faculty, and others with an interest in civil aviation infrastructure.

Currently, the FAA is seeking feedback from users as part of the design procedure development process concerning FEDFAA and suggestions for how it could be improved. To this end, an online response form has also been posted to facilitate feedback from users. Once all feedback is completed and implemented in the final design procedures, there will only be one design program to handle any aircraft on any pavement type anywhere in the world.

Future work

During the next year, researchers will continue to improve the 3D-FEM model to reduce the computation time and make it more suitable as a tool for airport pavement thickness design. They will also aim to develop a revised fatigue model for rigid pavements based on existing full-scale test data and new full-scale data from the National Airport Pavement Test Facility. Finally scheduled for this year are full-scale traffic tests on overlaid rigid pavements, including rigid on rigid and asphalt on rubblized rigid.