Longitudinal emergency control system using thrust modulation demonstrated on an MD-11 airplane

This report describes how an MD-11 airplane landed using only thrust modulation, with the control surfaces locked. The propulsion-controlled aircraft system would be used if the aircraft suffered a major primary flight control system failure and lost most or all the hydraulics. The longitudinal and lateral–directional controllers were designed and flight tested, but only the longitudinal control of flightpath angle is addressed in this paper. A flight-test program was conducted to evaluate the aircraft’s high-altitude flying characteristics and to demonstrate its capacity to perform safe landings. In addition, over 50 low approaches and three landings without the movement of any aerodynamic control surfaces were performed. The longitudinal control modes include a wing engines only mode for flightpath control and a three-engine operation mode with speed control and dynamic control of the flightpath angle using the tail engine. These modes were flown in either a pilot-commanded mode or an instrument landing system coupled mode. Also included are the results of an analytical study of an autothrottle longitudinal controller designed to improve the phugoid damping. This mode requires the pilot to use differential throttles for lateral control. Nomenclature Alon longitudinal state derivative matrix Blon control input derivative matrix c.g. center of gravity *Aerospace Engineer. †Chief, Propulsion Branch. Associate Fellow AIAA. ‡Flight Control Engineer. Copyright  1996 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. 1 American Institute of Aero Clon state output matrix Dlon control input observation matrix EPR engine pressure ratio (turbine and inlet total pressures) FADEC full-authority digital engine control computers FCC flight control computer FCP flight control panel sink rate, ft/sec ILS instrument landing system flightpath error feed-forward gain, deg pitch integrator error gain, 1/sec pitch rate feedback gain, deg/deg/sec velocity error feedback gain, deg/kn pitch angle feedback gain, deg/deg/sec center engine washout gain, lb MCDU multifunction control and display unit PCA propulsion-controlled aircraft PIO pilot induced oscillation q pitch rate, deg/sec t time, sec uu x axis velocity perturbation, ft/sec Vel velocity or airspeed, kn s Laplace transform ww z axis velocity perturbation, ft/sec xlon longitudinal state vector α angle of attack, deg γ flightpath angle, deg ḣ Kvc Kvi Kq K rs sec Kthad Kvm nautics and Astronautics flightpath angle command, deg velocity error θ pitch attitude, deg pitch attitude rate, deg/sec φ roll attitude, deg Introduction Aircraft flight control systems are designed with extensive redundancy to ensure a low probability of failure. During recent years, however, several aircraft have experienced major flight control system failures, leaving engine thrust as the only control effectors.1,2 In some of these emergency situations, the engines were used to maintain control of the airplane flightpath angle, γ. In the majority of the cases surveyed, crashes resulted, and over 1200 people have died.1 The challenge was to create a sufficient degree of control through thrust modulation to control and safely land an airplane with severely damaged or inoperative flight control surfaces. Meeting this challenge is the objective of the Propulsion-Controlled Aircraft (PCA) Emergency Backup System. The PCA emergency backup flight control system requires that the airplane have at least two engines, preferably two wing engines. In addition, the normal control surfaces can not be locked in a hardover position which could exceed the moments resulting from the thrust of the engines. The National Aeronautics and Space Administration, Dryden Flight Research Center, Edwards, California, has performed nonlinear and linear analytical studies and conducted several flight-test programs investigating the PCA concept. Results of these programs2–6 show that gross control can be obtained by manually moving the throttles. However, making a safe runway landing is exceedingly difficult because of low phugoid and dutch roll damping coupled with the high pilot work load near the ground. To improve the performance and reduce the pilot work load, the PCA program was developed. The goal was to make flying an airplane with the PCA system a viable task with minimal or no previous pilot training with this system. This report describes the longitudinal PCA control systems and flight test results of four modes: • Mode A—using the wing engines only for control of flightpath angle, γ. • Mode B—using the tail engine for speed control in conjunction with mode A. • Mode C—using all the wing and tail engines for dynamic control of γ and speed control. • Mode D—using an existing autothrottle system for γ control. The autothrottle system was developed to provide a simpler implementation that did not require changes to the engine controllers. This system was not flight tested, but simulation results are presented.§ Within control modes A, B, and C, the pilot has the option of selecting the instrument landing system (ILS)coupled with PCA for approach and landing. This option virtually eliminates the pilot work load. Two ILS landings using the wing engines (mode A) were performed, and one is presented in this report. The lateral–directional controller is described in reference 7. Test Vehicle Description The MD-11 airplane is a large, long-range, threeengine, wide-body transport. This airplane is 202 ft long, has a wing span of 170 ft, and a maximum takeoff gross weight of 618,000 lb (fig. 1). Flight Control Systems The MD-11 airplane has a mechanical flight control system with irreversible hydraulically powered actuators. The hydraulic power provided by three independent systems is intended for fail-safe capability. Essential control functions may be maintained by any one of these three systems. Pitch control is provided by dual elevators on each horizontal stabilizer, and pitch trim is provided by a moveable horizontal stabilizer. Inboard and outboard ailerons supplemented by wing spoilers provide roll control. A dual rudder mounted on a single vertical stabilizer provides yaw control. The lateral dynamics is controlled by the yaw damper. The longitudinal stability augmentation system controls the pitch dynamics. The aerodynamic surfaces are controlled by hydraulic actuators. The flight control computers (FCC) were built by Honeywell, Phoenix, Arizona, and operate at 20 samples/sec. The MD-11 airplane is equipped with a flight management system which integrates autopilot, navigation, and autoland functions. The automatic pilot control includes a thumbwheel for commanding flightpath angle, . §NASA has a patent pending for mode d. γ cmd γ err