Identification of Runway Friction Coefficient Using Extended Kalman Filter

<div>Aircraft operations during landing or takeoff depend strongly on runway surface conditions. Safe runway operations depend on the tire-to-runway frictional force and the drag offered by the aircraft. In the present research article, a methodology is developed to estimate the braking friction coefficient for varied runway conditions accurately in real-time. To this end, the extended Kalman filtering technique (EKF) is applied to sensor-measured data using the on-ground mathematical model of aircraft and wheel dynamics. The aircraft velocity and wheel angular velocity are formulated as system states, and the friction coefficient is estimated as an augmented state. The relation between the friction coefficient and wheel slip ratio is established using both simulated and actual ground roll data. Also, the technique is evaluated with the simulated data as well as real aircraft taxi data. The accuracy of friction estimation, with and without the measurement of normal reaction force on the landing gear, is analyzed using the simulated data. The friction coefficient vs slip ratio curve, derived from the empirical “Magic formula”, compares well with the estimated maximum tire-to-ground braking friction, and a shift in optimal slip is observed in actuality compared to the predictions. The brake disc friction coefficient is also estimated during the process since the brake torque measurements are not available in the actual data. The estimated friction coefficient, which represents the real characteristics of the runway, can be used to tune the control algorithms of the aircraft’s anti-skid brake management system for various runway conditions. While improvements in anti-skid efficiency alone may not directly prevent all runway excursions, accurate real-time friction estimation enhances the predictability and reliability of braking action, supporting safer operations under degraded or uncertain runway conditions. Moreover, the real-time estimation of tire-to-ground friction coefficient vs slip ratio curves can be used to develop adaptive control algorithms for the brake management system.</div>