(from the ICAO Journal, September 1996)
- Leigh J. Wardle and Bruce Rodway
A major research project has been conducted in Australia over the last four years to overcome some of the limitations of current airport pavement design systems. This has culminated in the development of APSDS (Airport Pavement Structural Design System), a computer program based on layered elastic analysis. One of its unique features is that it rationally takes account of aircraft wander. This is the statistical variation of the paths taken by successive aircraft relative to runway or taxiway centrelines, or to the lead-in lines to parking positions. Increased wander reduces pavement damage by different amounts that depend upon pavement thickness. We believe that our treatment of aircraft wander is more realistic than methods that are based on the simplified ‘coverage’ concept. Most significantly the APSDS model of the full-scale test data generated by the Corps of Engineers over many years has significantly less scatter than other methods of analysis.
The current airport pavement design methods, including that used by the US Federal Aviation Administration are based on the Corps CBR method. The International Civil Aviation Organisation’s (ICAO’s) ACN-PCN system for load rating of aircraft and for publishing aerodrome pavement strength is also based on this full scale data.
The range of pavement structures used in the full-scale tests do not fully reflect many of the structures commonly used today. The tests were limited to relatively thin pavements composed of unbound materials. However, pavements containing thick bitumen or cement bound layers are now common at major airports. In addition, many pavements constructed over weak soils are more than 2 m thick to cater for large numbers of heavy aircraft such as the Boeing 747.
It is now widely recognised that the Corps CBR method cannot adequately compute pavement damage caused by new large aircraft including the Boeing 777, the world’s largest twin-engine aircraft, which entered service in May 1995. The B777 has 6-wheel dual tridem landing gears [CAPTION: Figure 1). Additional problems arise for the proposed future generation ‘super-heavy’ aircraft that have more wheels on each strut and with some struts closely spaced near the centre of the aircraft. The introduction of the Boeing 777 has highlighted the need to revise the current methodology for computing aircraft pavement damage. ICAO recently reconvened the ACN/PCN study group to explore potential improvements to the ACN/PCN scheme. This has lead to minor modifications to the ACN calculation method so that interim ACN values could be assigned to the B777. This has the effect of reducing the B777 damage, particularly for deep pavements (i.e. low strength subgrades).
Figure 1 The Boeing 777 6-wheel landing gear.
The current methods of calculating pavement damage cannot adequately compute pavement damage caused by new large aircraft such as the 777.
Thus, the features of some of the new aircraft and of the typical pavement structures upon which they will operate fall well beyond the empirical database on which the current methodology for calculating aircraft pavement damage is based.
It is important to have an unambiguous, generally accepted methodology for computing pavement damage, to allow airport operators and constructors to adequately design pavements to accommodate new aircraft, and to allow airlines to anticipate airport pavement weight restrictions in planning their operations and in deciding which aircraft to purchase. The availability of such a methodology is also important for aircraft manufacturers, to allow them to design landing gears that will not overstress the world’s existing runways, and yet will optimise aircraft performance.
Although the layered elastic method has been available for more than 20 years, it has not been used as a primary design method for aircraft pavements until recently.
The recently released FAA computer program, LEDFAA (Layered Elastic Design Federal Aviation Administration) is based on the coverage concept. LEDFAA is now an FAA standard intended for use in designing pavements catering for aircraft mixes that contain the new Boeing 777. LEDFAA computes damage caused by the 6-wheeled strut of the B777 that is similar to that calculated by traditional empirical CBR-based methods. This damage is therefore greater than that indicated by the modified ICAO ACN method referred to earlier. Thus it does not address the concern that current methods overstate the damage caused by larger groups of wheels. In its present form it treats only single wheel groups and so does not attempt to allow for interaction effects between wheel groups. LEDFAA adopts taxiway wander and the user cannot specify any other degrees of aircraft wander.
There is presently considerable uncertainty as to the elastic properties that should be assigned to materials within the various pavement layers. The default values used by LEDFAA have been so chosen to produce pavement thicknesses that are similar to those obtained by using the traditional CBR empirical method and layer equivalencies specified in FAA’s Advisory Circular. This is considered to be a transitional measure, and it is expected that modulus values will be changed over time to better model the pavement as performance data becomes available.
APSDS is based on work by Monismith and co-workers in the mid 1980’s and has been jointly developed by Mincad Systems, Federal Airports Corporation and Pioneer Road Services. It does not use the coverage concept. Instead the strain distribution (not just a single maximum strain) at all points across the pavement for a given depth is used to capture the damage contributions of all the aircraft wheels in all their wandering positions. This contrasts with earlier methods which computed only single maximum values of the damage indicators. This additional capability permits rational analysis of the reduction in pavement damage that results from the load spreading effect of aircraft wander.
APSDS is a menu-driven computer program which runs under Microsoft Windows and uses a layered elastic program called CIRCLY. Figure 2 is a typical plot of the damage profile across a pavement.
Figure 2: Sample cumulative damage plot from APSDS.
APSDS calculates the strains for all points across the pavement in order to capture all damage contributions from all the aircraft wheels in all their wandering positions. This contrasts with earlier methods that compute only single maximum values of the damage.
Analyses with APSDS have shown that the coverage concept becomes increasingly inappropriate with increasing depth to subgrade. For example, for a typical 500 mm pavement, taxiway wander reduces damage by 80% of that caused in the channelised, no wander case. This contrasts with a typical 1500 mm pavement where taxiway wander reduces the channelised damage by only 30%.
The user can specify the standard deviation of wander that is appropriate to the particular pavement. Based on empirical data the standard deviation for a taxiway is typically taken as 773 mm and for a runway as 1546 mm.
As APSDS does not use the coverage concept, it was necessary to undertake a detailed study of the original full-scale test data to calibrate the model. APSDS models the lateral distribution of test traffic in more detail than previously attempted. The APSDS model has significantly less scatter than that given by the conventional methods of analysis. This suggests that the treatment of aircraft wander is more realistic than the simplified ‘coverage’ concept used by other design systems.
The emergence of more powerful tools of analysis do not always solve existing problems. Instead they can reveal complexities that previous investigators, working with more limited tools, had not fully appreciated. This has been the case with APSDS. It can compute the subgrade strains, or any alternative damage indicators, that result from any number of wheels in any configuration, and so was used to predict the pavement damage to be expected from very large future generation aircraft. These attempts have demonstrated that current design methods, including APSDS and LEDFAA, cannot predict the damage, and have confirmed that full-scale load testing of the multiwheel landing gears proposed for the new aircraft is necessary. However, these tools are very accessible, fast and powerful, accommodate bound layers and facilitate the introduction and wide use of mechanistic design to efficiently design pavements represented by the Corp’s full scale tests.
The FAA are proceeding with a US$ 50 million pavement research project which includes extensive full scale accelerated tests to quantify the effects of more than four wheels on a strut and interaction effects between closely spaced struts.
The major specifications for the facility are as follows:
- Test track 275 metres long by 18 metres wide.
- Up to 12 independent test pavements along the test track.
- Twelve test wheels capable of being configured to represent two complete landing gear trucks having from one to six wheels per truck and adjustable to vary the distance between the trucks up to six metres forwards and sideways.
- Wheel loads to be independently adjustable up to a maximum of 34 tonnes per wheel.
- Simulate aircraft weighing up to 600 tonnes.
- Full-depth pavements to be accommodated.
APSDS has unique features that will enhance and optimise the analysis and design of airport pavements:
- improved rational quantification of the effect of aircraft wander
- graphical presentation of the damage ‘profile’ across the pavement
- material anisotropy for improved modelling of unbound granular materials and subgrades
The system’s transparency, speed and flexibility enables design specialists to readily change all problem inputs including aircraft wander, aircraft numbers and mass, layer thicknesses and material properties and also the performance models. This allows rapid assessment of the sensitivity to each component input and for all design assumptions.
APSDS gives a better fit to the full-scale test data than previously reported. This suggests that the treatment of aircraft wander statistics is more realistic than the simplified ‘coverage’ concept used by other design systems. APSDS computes the effects of wander at subgrade level and so includes the influence of pavement thickness and properties upon the amount of damage reduction that results from aircraft wander.
Considerable difficulties remain in predicting the impact of new-generation large aircraft on the thickness requirements for flexible pavements. The large wheel groups of the new and proposed large aircraft should cause less surface rutting than predicted by current methods and may not be significantly more damaging to flexible pavements than present generation aircraft. However full-scale testing, as planned by the US Federal Aviation Administration, is required to quantify damage caused by large multiwheeled gears and by interaction of aircraft gears.
The original system concept was developed by Ian Rickards, Technical Manager- Australia, Pioneer Road Services Pty. Ltd. Pioneer financed the initial development and have supported the promotion of APSDS to the international airport design community. The authors wish to acknowledge Ian Rickards for his enthusiastic support and active participation in our research. Permission of the Federal Airports Corporation to publish this paper is gratefully acknowledged.
see our Recent Conference Papers.