Guidance strategies for microburst escape.

Abstract

This study compares three escape guidance laws for microburst encounters during final landing approach: Altitude-Guidance, Dive-Guidance, and Pitch-Guidance from the point of view of safety. It also introduces Modified Altitude- and Dive-Guidance laws. In this study, we use a full, 6-DOF, nonlinear, rigid-body aircraft model, including the effects of windshear and wind vorticity, and a model of microburst with turbulence. We also model the effect of stall prevention on the escape path. We first construct a new safety metric that quantifies the aircraft upward force capability in a microburst encounter. In the absence of turbulence, the safety factor is analytically proven to be a decreasing function of altitude. This suggests that descending to a low altitude may improve safety in the sense that the aircraft will have more upward force capability to maintain its altitude. In the presence of stochastic turbulence, the safety factor is treated as a random variable and its probability distribution function is analytically approximated as a function of altitude. This approximation reveals that the probability of safety factor being less than a given value has a minimum, i.e. safety increases as the altitude decreases up to a certain altitude, then starts decreasing. In the dissertation, two different approaches are used for comparison. (1) In a sample analysis approach, typical samples of the time histories of various variables are analyzed. Additionally, an animation of an aircraft escaping a microburst is produced and the behavior of the aircraft along with its inertial velocity and airspeed vectors are studied. (2) In a statistical approach, the probability distribution of the minimum altitude is estimated by the Monte Carlo Method when the statistical properties of the microburst parameters are known. Both approaches suggest that, within the modeling assumptions of this dissertation, and in the absence of human factors, altitude and dive guidance with low commanded altitude may provide better safety than pitch guidance. That is, once the escape maneuver is initiated, the aircraft should be directed to a low recovery altitude with full thrust as long as the recovery altitude is higher than an optimal value. However, the drawback of descending to the so-called optimal altitude is that the aircraft may unnecessarily descend to the optimal altitude even when it is possible to safely recover from a microburst with a higher recovery altitude. The analytic approximation of the probability distribution function of the safety factor is used to determine the highest safe altitude at which the aircraft may descend, hence avoiding to descend too low. This highest safe altitude is used as the commanded altitude in Modified Altitude- and Dive-Guidance. Monte Carlo simulations show that these Modified Altitude- and Dive-Guidance strategies decrease the probability of minimum altitude being less than a given value without compromising the probability of crash. That is, an aircraft with Modified Altitude-- or Dive-Guidance generally has a higher recovery altitude without increasing the risk of ground contact or stall.