An investigation of strategies for controlling damaged transport aircraft

There have been a number of instances in which an aircraft that suffered in-flight damage to the airframe or control system has been brought safely to the ground using unconventional means of control. To help explore control strategies that might be successful in such situations, a method has been developed that allows a quick and systematic investigation of such cases by determining a control law for a desired maneuver using optimal control theory. The control law is optimized for a chosen aircraft configuration, which could have damage to its control system or airframe, subject to imposed state and control constraints. The formulation of the method results in a tracking problem in which the user-defined state trajectory, which can be nonlinear, is solved by numerically integrating the Riccati equation. As a result, a performance index that contains measures of the minimum control effort, as well as tracking and terminal state errors, is minimized. The usefulness of the method developed is demonstrated with several sample maneuvers on aircraft with and without damage to their primary control systems. In many cases, of course, positive control is not possible, while in others, it is found that unconventional control strategies, sometimes with as little as differential thrust being available, can be employed to get a damaged aircraft safely to the ground.