126 Transportation Research Record: Journal of the Transportation Research Board, No. 2214, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 126–135. DOI: 10.3141/2214-16 dependent on the operating conditions of the aircraft and the pavement concerned, and the relationship is a highly complex one. Research studies have shown that wet-pavement skid resistance is affected by all of the following factors: tire structural properties, pavement sur- face properties, aircraft speed, wheel load, tire inflation pressure, and pavement surface water-film thickness (3–7 ). Because of the relatively large number of variables involved and the rather wide range of values possible for each variable, it is impractical to collect the required measurements and rely on statistical regression methods to estimate wet-pavement aircraft braking distance. Similarly, analytical approaches that employ estimated pavement skid resistance values based on limited ground friction measurements would not be able to produce sufficiently accurate braking distance calculations to cover the wide spectrum of wet-weather aircraft operating conditions. To overcome the difficulty and limitations encountered by the cur- rent methods of evaluating wet-pavement aircraft braking distances, in this study an improved procedure for calculating wet-pavement braking distance based on engineering mechanics and fluid dynam- ics theory is presented. The procedure uses a three-dimensional finite element algorithm developed earlier by two of the authors for simu- lating skid resistance of vehicular tires (8). The adaptation of the algorithm to model skid resistance of aircraft tires involved calibra- tion and validation of the model before it was applied to perform skid resistance calculations for aircraft braking distance analysis. EXISTING METHODS OF AIRCRAFT BRAKING DISTANCE ESTIMATION Aircraft braking distance is significantly affected by the available braking force, which depends on the runway surface friction co- efficient. Therefore, a key component of any aircraft braking esti- mation method is the evaluation of the runway friction coefficient, especially under wet-pavement conditions. Researchers have gener- ally employed two approaches to evaluate wet-pavement friction behavior: one uses statistical methods derived from experimental data from aircraft test runs, and the other employs analytical techniques based on the theory of mechanics. In a study conducted by Croll and Bastian (9), braking distance was calculated with data from full braking runs on wet pavement using a Falcon 20 type research aircraft. From the test results, regression mod- els were developed as given in Equation 1 to show the variation of braking coefficient μ B (wet) versus ground speed V G on a wet runway surface: where the ground speed V G is measured in knots. This relationship takes into account friction coefficient variation with speed during μ B G V wet ( ) = - 0 237 0 00103 1 . . () Computation of Aircraft Braking Distances H. R. Pasindu, T. F. Fwa, and Ghim Ping Ong Wet runways are known to be a major contributing factor in overrun accidents. Studies have shown that low skid resistance caused by reduced friction on wet pavement leads to a longer braking distance and increases aircraft overrun risk. The common approaches to compute aircraft brak- ing distance and overrun risk are based on either statistical predictive equations derived from accident data or analytical methods using esti- mated ground friction values. Both approaches are unable to calculate aircraft braking distance accurately because they cannot account for the complex relationship between wet-pavement skid resistance and the highly varied aircraft operating conditions (which are characterized by factors such as aircraft tire type, landing speed, wheel load, tire inflation pressure, and thickness of runway surface water film). To overcome this limitation, an improved procedure for calculating wet-pavement braking distance based on engineering mechanics and fluid dynamics theory is proposed. An analytical simulation model is first developed to evaluate the wet-pavement skid resistance available to an aircraft under a given oper- ating condition, which is defined by aircraft speed, tire structural proper- ties, pavement surface properties, wheel load, tire inflation pressure, and pavement surface water-film thickness. With actual measured data reported in the literature, a numerical example is presented to demon- strate the application of the proposed procedure. The example illustrates the calibration and validation of the tire model, followed by computation of braking distances under different operating conditions of wheel load, tire inflation pressure, landing speed, and water-film thickness. Aircraft landing safety remains one of the major issues in the aviation industry. Twenty-five percent of the fatal aircraft accidents worldwide took place during aircraft landing (1). Landing accidents are mainly caused by runway excursions in which the aircraft overruns the run- way length or veers off the side of the runway. In 2008, 30% of world- wide aircraft accidents were runway excursions, and on the basis of Boeing’s statistical data, overrun accidents have been steadily increas- ing over the last few years (1). Overrun occurs when an aircraft land- ing distance exceeds the runway length available during a landing ground roll. The rate of landing overrun accidents for turbojet aircraft is estimated to be around 0.25 per million landings (2). Aircraft braking distance is the main component in the calculation of total aircraft landing distance. It is significantly affected by pave- ment skid resistance available to the aircraft. During wet weather, pavement skid resistance is reduced substantially from its dry-weather value (3, 4 ). The magnitude of wet-pavement skid resistance is Department of Civil Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260. Corresponding author: T. F. Fwa, ceefwatf@ nus.edu.sg.