Controlling aircraft with engine thrust only: nonlinear challenges

Aircraft control systems are usually very reliable because of redundancy and multiple control surfaces. However, there are rare occasions when potentially disastrous flight control system failures do occur. At such times, the use of appropriate modulation of engine thrust to stabilize the aircraft may be the only chance of survival for the people aboard. In several cases where complete loss of control systems has occurred in multi-engine aircraft, pilots used propulsion system to regain limited control of the aircraft with various degrees of success. In order to evaluate the feasibility of using only engine thrust modulation for emergency backup flight control, the NASA Dryden Flight Research Center has been conducting a series of analytical studies and flight tests on several different types of aircraft in a propulsion controlled aircraft (PCA) program. Simulation studies have included B-720, B-727, MD-11, C-402, C-17, F-18, and F-15, and flight tests have included B-747, B-777, MD-11, T-39, Lear 24, F-18, F-15, T-38, and PA-30. One objective was to determine the degree of control available with manual manipulation (open-loop) of the engine throttles. Flight tests and simulations soon showed that a closed-loop controller could improve the chances of making a safe runway landing. The major work to date has concentrated on three aircraft (F-15, F-18, and the MD-11). Successful landings using PCA controllers were performed on the F-15 and MD-11 without the use of control surfaces. The feasibility of using only propulsion system for emergence flight control when the conventional flight control system is inoperative has been established by the propulsion controlled aircraft (PCA) program at the NASA Dryden Flight Research Center. This paper reviews some notable nonlinear phenomena observed in flight tests of the PCA, including nonlinear dynamics of the engines, interactions between the propulsion system and airframe, and cross coupling in the aircraft dynamics. These nonlinearities which can be easily accommodated by a conventional flight control system becomes significant challenges to improvement of the performance of the PCA, because the engines are slow and have limited control authority. The flight test data for an F-18 aircraft is presented to illustrate some of these aspects. It is suggested in this paper that a PCA controller properly designed by using a nonlinear method will likely be more effective, given the highly nonlinear environment. A recently