National Airport Research Articles

Neural Networks Analysis of Airfield Pavement Heavy Weight Deflectometer Data

The Heavy Weight Deflectometer (HWD) test is one of the most widely used tests for assessing the structural integrity of airport pavements in a non-destructive manner. The elastic moduli of the individual pavement layers predicted from the HWD deflection measurements through inverse engineering analysis are effective indicators of pavement layer condition. The primary objective of this study was to develop a tool for backcalculating non-linear pavement layer moduli from HWD data using Artificial Neural Networks (ANN) for rapid structural evaluation of airfield pavements. A multi- layer, feed-forward backpropagation ANN which uses an error-backpropagation algorithm was trained to approximate the HWD backcalculation function. The synthetic database generated using an axisymmetric pavement finite-element program was used to train the ANN. Using the ANN, the Asphalt Concrete (AC) moduli and subgrade moduli were successfully predicted. Apart from the moduli, an attempt was made to predict the critical pavement structural responses using ANN models. The final product was used in backcalculating pavement layer moduli and predicting subgrade deviator stresses from actual field data acquired at the Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF).

PREDICTING CAPACITIES OF RUNWAYS SERVING NEW LARGE AIRCRAFT

This paper presents a simplified approach for predicting the allowable load repetitions of New Large Aircraft (NLA) loading for airfield runways based on Non‐Destructive Test (NDT) data. Full‐scale traffic test results from the Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF) were used to develop the NDT‐based evaluation methodology. Four flexible test pavement sections with variable (unbound layer) thicknesses were trafficked using six‐wheel and four‐wheel NLA test gears until the test pavements were deemed failed. Non‐destructive tests using a Heavy Weight Deflectometer (HWD) were conducted prior to the initiation of traffic testing to measure the pavement surface deflections. In the past, pavement surface deflections have been successfully used as an indicator of airport pavement life. In this study, the HWD surface deflections and the derived Deflection Basin Parameters (DBPs) were related to functional performance of NAPTF flexible pavements through simple regression analysis. The results demonstrated the usefulness of NDT data for predicting the performance of airport flexible pavements serving the next generation of aircrafts.

Multifactor Model for Predicting Delays at U.S. Airports

Traffic flow management (TFM), in coordination with airline operation centers, manages the arrival and departure flow of aircraft at the nation's airports on the basis of airport arrival and departure rates for each 15-min segment throughout the day. The management of traffic flow has become so efficient in the United States that approximately 95% of delays now occur at the airports (not while airborne). Inefficiencies in traffic flow occur when nontraffic flow delays (e.g., carrier, turnaround, aircraft swapping, and local weather delays) are superimposed on the traffic flow delays. Researchers have correlated these nontraffic flow delays at airports with sets of causal factors and have created models to predict aggregate delays at airports on the time scale of a day. To be consistent with the way traffic flow is managed, a model of causal factors of delays in 15-min increments would provide the basis for improving the efficiency of TFM. The development of multifactor models for predicting airport delays in 15-min epochs at 34 operational evolution plan airports is described. The models are created by using multivariate adaptive regression splines. The models, generated by using historic individual airport data, exhibit an accuracy of 5.3 min for generated delay across all airports and 2.1 min for absorbed delay across all airports. A summary of the factors that drive the performance of each airport is provided. The implications of these results are discussed.

Airport Taxi-Out Prediction Using Approximate Dynamic Programming

Flight delay is one of the pressing problems that have far-reaching effects on society and the nation's economy. A primary cause of flight delay in the National Airspace System is high taxi-out times (time between gate push-back and wheels-off) at major airports. Accurate prediction of taxi-out time is needed to make downstream schedule adjustments and for better departure planning, which could mitigate delays, emissions, and congestion on the ground. However, accurate prediction of taxi-out time is difficult because of uncertainties associated with the dynamically changing airport operation. A novel stochastic approximation scheme based on reinforcement learning (RL) is presented for predicting taxi-out times in the presence of weather and other departure-related uncertainties. The prediction problem is cast in a probabilistic framework of stochastic dynamic programming and solved by using approximate dynamic programming approaches (particularly RL). The strengths of the method are that it is nonparametric, unlike regression models with fixed parameters, it is highly adaptable to the dynamic airport environment since it is learning based, it is scalable, it is inexpensive since it does not need highly sophisticated surface management systems, and it can effectively handle uncertainties because of the probabilistic framework. Taxi-out prediction performance was tested on data obtained from the FAA Aviation System Performance Metrics database on Detroit International and Washington Reagan National Airports. Results show that the root-mean-square prediction error calculated 15 min before gate departure time is on average 2.9 min for about 80% of the predicted flights.

Characterization of NAPTF subgrade soils for mechanistic-based analysis and design of airport flexible pavements

The National Airport Pavement Test Facility (NAPTF) was constructed to generate full-scale testing data to support the development of advanced airport pavement design procedures. The NAPTF material properties were characterized using test pit data obtained prior to full-scale heavy gear trafficking as well as using test data obtained from trenches opened after the completion of traffic testing. In this paper, the post-traffic subgrade characterization test results obtained from NAPTF subgrade trench sections were evaluated and compared with the pre-traffic test pit data. The combined data were used to evaluate the most commonly used correlations amongst subgrade soil properties. Subgrade moduli backcalculated from Heavy Weight Deflectometer (HWD) test data were compared with the laboratory test results and were also correlated to field California Bearing Ratio (CBR) values. Regression analyses indicated that subgrade resilient modulus is significantly related to the unconfined compressive strength. These results may be used in the mechanistic-based analysis and design of airport flexible pavements subjected to New Generation Aircraft (NGA).

Mechanistic-empirical evaluation of airfield test pavements

The impact of new-generation aircraft (NGA) such as the Boeing 777 and Airbus A380 on airfield pavement performance is of serious concern to airfield authorities worldwide. Recognising the need to develop advanced pavement design procedures based on sound theoretical principles, and with models verified from appropriate full-scale NGA test data, the Federal Aviation Administration constructed the National Airport Pavement Test Facility (NAPTF). In this paper, a mechanistic-based approach is presented to predict the allowable six-wheel and four-wheel NGA gear passes for airfield flexible pavements based on full-scale traffic test results from the NAPTF. A finite-element pavement structural model that can account for the non-linear stress-dependent behaviour of pavement geomaterials was used to compute the mechanistic responses of NAPTF test sections. Both the subgrade deviator stress and subgrade stress ratio were found to be good estimators of airfield pavement rutting performance. Although the results are applicable specifically to the six-wheel and four-wheel NGA gears used at NAPTF, the method can easily be extended to any prototype NGA gear loading with additional data from such full-scale traffic tests.

The privatization of Argentine airports

The significant increase in air traffic in Argentina during 1990s highlighted a need to upgrade the country's airport infrastructure. In the ideological and political–economical context of the period, the heavy investment required could not be carried through without private capital and thus a National Airport System was developed with an accompanying regulatory agency. This article looks at the origin, characteristics, and operations of this system.

Rebound and residual in situ pavement displacements measured during NAPTF performance testing

The FAA's National Airport Pavement Test Facility (NAPTF) was constructed to generate full-scale test data needed for the development of advanced airport pavement design procedures. In this paper, the comparative effects of six-wheel Boeing 777 (B777) and four-wheel Boeing 747 (B747) simulated landing gear loading on the multi-depth deflectometer (MDD) responses of two NAPTF low-strength subgrade flexible test sections are discussed. The results showed that the major contribution to surface rebound MDD displacements was the subgrade layer in both the test sections. Transverse response distributions showed that, in the vicinity of wheel loads, the net average residual MDD displacement was contractive, whereas at small offsets away from the wheel loads, the displacement was dilative which led to a continuous contractive/dilative permanent deformation pattern. The majority of the surface residual response came from the structural layers above the subgrade.

Governance structures and the market orientation of airports in Europe's peripheral areas

This paper investigates the relationship between governance structures and the market orientation of airports. The focus is on Europe's peripheral areas, where airports tend to be publicly owned and operated as an independent entity or as part of a regional or national airport system. This paper finds that market orientation is significantly higher at independently operated airports compared to system-operated airports and that independent governance structures, market opportunities and competitive intensity have a significant positive effect on airport market orientation.

Pavement Deformation Characteristics under Repeated Aircraft Loading with Fixed Wander Sequence

Abstract Full-scale traffic testing was conducted at the Federal Aviation Administration’s National Airport Pavement Test Facility (NAPTF) using Boeing 777 (B777) and Boeing 747 (B747) simulated test gears on flexible pavement test sections until they were deemed to be failed. An inertial profiling device was used to measure the transverse surface profiles at frequent intervals during NAPTF trafficking. Results from the multi-depth deflectometers (MDDs) installed within the test pavements were used to monitor the cumulative permanent deformations. The MDD results showed that the sequence of repetitive aircraft gear loads with different wander positions used at NAPTF led to contractive/dilative permanent deformation behavior, especially in the unbound pavement layers. The magnitudes of MDD residual responses were dependent on the position of the traffic gear, the previous traffic history, and the direction of traffic (east/west). These results suggest that reliable prediction of permanent deformation in airport flexible pavements is a highly challenging task owing to the complexities arising from aircraft loading sequence, gear offset and gear offset sequence, and climatic conditions.

Use of Nondestructive Test Deflection Data for Predicting Airport Pavement Performance

Surface deflections using nondestructive tests (NDTs) were measured prior to and throughout the traffic testing at the U.S. Federal Aviation Administration’s National Airport Pavement Test Facility (NAPTF). The first series of traffic tests involved repeated loading of six-wheel Boeing 777 and four-wheel Boeing 747 test gears on two different lanes until the pavements were deemed failed. The NAPTF structural failure criterion was defined as at least 25.4mm(1in.) surface upheaval adjacent to the traffic lane. A predetermined wander sequence was applied. Two low-strength subgrade and two medium-strength subgrade flexible pavement test sections were tested. Transverse surface profiles were measured periodically to monitor the progression of permanent deformation in pavements. Deflection basin parameters derived from NDT surface deflections were related to pavement rutting performance. An airport pavement functional failure criterion, defined in terms of number of traffic load repetitions to reach specific rut depth levels, was used in characterizing the structural response-performance relations.

Behaviour of airfield pavement subgrades under load test

The US Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF) was constructed to provide full-scale pavement response and performance data under new generation aircraft loading for development and verification of airport pavement design criteria. During the first series of traffic tests, four flexible pavement test sections (two low-strength subgrade and two medium-strength subgrade test sections) were subjected to six-wheel and four-wheel gear loading to assess the relative severity effects. Trench sections were opened at the completion of traffic testing for conducting forensic investigations. In this paper, the effect of full-scale accelerated heavy aircraft gear trafficking on the NAPTF unbound aggregate and fine-grained subgrade layer material properties (referred to as geomaterials) is discussed. The results indicate that the in situ dry densities and moisture contents have decreased in the crushed stone base as a result of trafficking whereas the dry densities of the sub-base (grey quarry blend fines) layer have increased. The low-strength subgrade California Bearing Ratio (CBR) has increased as a result of trafficking, whereas the medium-strength subgrade CBR has decreased. These results can be used to develop input parameters for NAPTF pavement structural modelling and analysis.

The long march of Pan-Arab media: a personal view

Early in the summer 2006 I booked a flight to Beirut scheduled for the 12 July that would take me home after thirty years abroad. Little was I to know that the date displayed on my ticket would be inscribed in Lebanese history. But being well aware of Lebanon’s acute vulnerability in the volatile politics of the Middle East, I kept to myself the excitement I was feeling about returning home. Mid-morning July 11, a work colleague walked into my office asking if I had heard the latest news. Two Israeli soldiers had been kidnapped by Hizbullah, he told me. Thinking about my scheduled July-12 flight to Beirut, I rushed into the TV monitoring room at the Pan-Arab Research Center where I work. I was eager for the very latest news. The first report I saw featured the Israeli Army chief’s threat to take Lebanon back thirty years. With the painful experience of civil war and Israeli invasions that I share with most Lebanese, I took these threats literally. The first images of war that I witnessed were the flashing shells hitting Beirut airport’s runway. This was an agonizingly familiar image. Our leaders have let the vulnerability of Beirut airport be enshrined in our national psyche: images of the national airport succumbing to the Israeli Army go as far back as 1967 and have been replicated in the subsequent series of attacks against Lebanon. I have particularly strong visual memories of war as my home village lies just 350 meters above Beirut airport and is an ideal place for a panoramic view of all Beirut and the Mediterranean Sea beyond. These images of war remain vivid; nothing can erase them from my memory.

Subgrade stress ratios as airfield pavement rutting performance indicators

Rutting is a major distress in airport flexible pavements. Subgrade vertical strain criteria are used in many airport flexible pavement design procedures to consider the development of rutting. Several research studies have identified the limitations associated with these criteria. Design criteria based on subgrade stress ratios (SSRs) are being considered for evaluating subgrade rutting in airport flexible pavements. In this paper, the SSRs based on measured vertical subgrade stresses are related to surface rutting in flexible pavements subjected to repeated trafficking of Boeing 777 and 747 simulated test gears at the US Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF). The results indicated overstressing of the subgrade in two test sections, and this was confirmed by trench study results. A good correlation was obtained between the SSRs and the surface rut depths, supporting the validity of developing SSR-based rutting transfer functions for airport flexible pavements serving the next generation of aircraft.Key words: rutting, subgrade stress ratio (SSR), National Airport Pavement Test Facility (NAPTF), pressure cell (PC), new generation aircraft, airport flexible pavement.

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.

Evaluation of Traffic Induced Subgrade Stresses in Flexible Airfield Pavements

ABSTRACT During the first series of traffic tests at the Federal Aviation Administration's (FAA's) National Airport Pavement Test Facility (NAPTF), the flexible pavement test sections were trafficked using a simulated six-wheel Boeing 777 (B777) landing gear and a simulated four-wheel Boeing 747 (B747) gear simultaneously until the test sections failed. Soil pressure cells (PC) installed within the test sections recorded the load-induced vertical subgrade stresses. The main objective of this paper is to analyze the PC-measured vertical subgrade stresses over the duration of traffic testing and present some interesting findings regarding the comparative effect of B777 and B747 traffic loading on subgrade stress and its relation to the rutting performance of the NAPTF test sections. The results showed that, in general, the subgrade stresses induced by the B777 and B747 traffic loading do not differ significantly.

Multi-Depth Deflectometer Dynamic Response Measurements under Simulated New Generation Aircraft Gear Loading

Abstract In this paper, the comparative effect of dual-tridem (Boeing 777) and dual-tandem (Boeing 747) gear trafficking is studied on the Multi-Depth Deflectometer (MDD) responses of two medium-strength subgrade flexible test sections at the National Airport Pavement Test Facility (NAPTF). Using the MDD response-time histories and in-depth displacements obtained under repeated traffic loading, the results were investigated in terms of rebound (recovered) and residual (unrecovered) displacements in the pavement and in the subgrade. The resilient behavior of the NAPTF flexible pavement sections were characterized by analyzing the MDD rebound response data. The results showed that the rebound surface displacements were significantly influenced by changes in asphalt concrete surface layer temperature. Analysis of transverse distributions of MDD rebound surface displacements revealed multiple-wheel load interaction effects. In general both the rebound and residual (and hence the maximum total) displacements are approximately equivalent for both the dual-tridem and dual-tandem gears.

Assessing Damage to Airport Pavement Structure due to Complex Gear Loads

The Federal Aviation Administration’s National Airport Pavement Test Facility (NAPTF) was constructed to generate high-quality, full-scale accelerated test data from pavements subjected to dynamic simulated heavy aircraft traffic. During the first round of testing, the flexible pavement sections were subjected to dual-tridem Boeing 777 gear and dual-tandem Boeing 747 gear trafficking to study and compare their effect on pavement structural responses and rutting performance. This paper presents the results from traffic testing on a low-strength subgrade flexible pavement section with a substantially thick unbound granular subbase layer. Data from in situ instrumentation and heavy weight deflectometer (HWD) testing were analyzed at different stages of trafficking to evaluate the pavement structural deterioration imposed by the test gears. At the end of traffic testing, posttraffic forensic investigation studies were conducted to investigate the failure mechanism of pavement structures. The results showed th...

Effect of Dynamic Aircraft Gear Loads on Asphalt Concrete Strain Responses

Abstract Asphalt strain gauges were installed in both the longitudinal and transverse directions at the Federal Aviation Administration's National Airport Pavement Test Facility (NAPTF) to measure the load-induced Asphalt Concrete (AC) tensile strains during full-scale traffic testing with a dual-tridem Boeing 777 gear configuration and a dual-tandem Boeing 747 gear configuration. The results are presented for four flexible test sections; two sections with 127 mm of AC and two with 254 mm of AC. An axisymmetric finite element pavement analysis software was used to model the NAPTF flexible test sections and compute the AC strain responses. The measured AC strain magnitudes were significantly influenced by AC temperature variations. Dramatic increase in peak strain responses were observed as the NAPTF flexible test sections approached structural failure. There was a good agreement between the field-measured and computed strains during the early life of the test sections.

Severity effects of dual-tandem and dual-tridem repeated heavier aircraft gear loading on pavement rutting performance

In this paper, the rutting behaviour of flexible test pavements subjected to multiple-wheel heavy aircraft gear loading at the National Airport Pavement Test Facility (NAPTF) are characterized. Two series of traffic tests were conducted. During the first series, a Boeing 777 (B777) aircraft gear and a Boeing 747 (B747) gear were trafficked on two low-strength subgrade and two medium-strength subgrade flexible test sections until the test sections were deemed failed. The second series of traffic tests involved repeated loading of six-wheel aircraft gear (the same as B777 gear) and four-wheel gear on low-strength subgrade test sections with variable granular subbase thicknesses. The results showed that the mean rut depths (RD) accumulated under B777 loading and B747 loading were similar. For a similar number of load repetitions, low-strength subgrade test sections with reduced subbase thicknesses yielded larger RD. The NAPTF failure criteria did not yield consistent RD.