Extensive redundancy is designed into aircraft flight-control systems to ensure a low probability of failure. During recent years, however, major failures of flight-control systems have occurred in several airplanes, leaving engine thrust as the flight-control mode of last resort. In some of these emergency situations, engine thrust was successfully modulated by the pilots to maintain flightpath or pitch angle, but in other situations, lateral control was also needed. In the majority of such cases, crashes resulted, and over 1,200 people died because of control-system failures.

alt
Figure 1. This Auto-Throttle Servocontrol System responds to velocity and pitch-angle feedback by modulating engine thrust to damp the phugoid oscillation and maintain a commanded pitch angle, making it possible to land the MD-11 airplane by use of thrust control only.

Thus, the challenge has been to create sufficient thrust-modulation control to safely fly and land an airplane. A thrust-modulation control system designed for this purpose was flight-tested in a propulsion-controlled aircraft (PCA) — an MD-11 airplane. The results of the flight test showed that without any operational control surfaces, one can land a "crippled airplane" (U.S. Patent 5,330,131). This flight program also verified that the "weak link" is the phugoid mode, which is also known as the long-period pitch mode. The low phugoid damping and a high pilot workload made manual landings exceedingly difficult near the ground. However, a PCA system makes landings feasible. The installation of the original PCA system entailed modifications not only of the flight-control computer (FCC) of the airplane but also of each engine-control computer. Inasmuch as engine-manufacturer warranties do not apply to modified engines, the challenge was to create a PCA system that does not entail modifications of the engine computers.

The response to the challenge was a method of providing longitudinal (pitch) control through modification of only the program of the FCC, without any changes in the engine-control computers and without changes in cockpit hardware. In the event of a failure in the primary flight-control system, the engines can be used to dampen the phugoid mode, and in the case of a multiple-engine airplane, they can be used to land the airplane safely. This method relieves the pilot of the longitudinal-control task, enabling the pilot to concentrate on using differential thrust to keep the wings level for landing. This may seem difficult at first, but lateral control is made simpler when the phugoid motion is highly damped by the closed-loop system shown in Figure 1.

alt
Figure 2. The Phugoid Oscillation is excited when the engine thrust is increased in manual open-loop control. The plots in the left part of this figure represent the dynamics of an MD-11 airplane in which the thrust of each of the wing engines has been increased by 6,000 lb (≈27 kN). On the other hand, the phugoid oscillation is suppressed when the system of Figure 1 is used to control the wing and tail engines.

This control system, denoted the auto-throttle servocontrol system, strives to maintain a commanded pitch angle in the face of velocity and pitch-angle feedback. The pilot commands this system by use of a thumb wheel. The output of this system drives an auto-throttle servo forward or back, thereby causing the engines to increase or decrease thrust, and thereby, in turn, controlling the pitch of the airplane in such a way as to keep the phugoid mode well damped. In the many multiple-engine airplanes in which auto-throttle systems are already in place, no hardware changes are needed.

Suppose that one were flying a wide-body airplane in which control surfaces had been rendered inoperable by a full hydraulic failure and the airplane was not equipped with the present auto-throttle servocontrol system. In order to increase the pitch angle, one would slowly increase the thrust, as shown in the left part of Figure 2. As the thrust was increased, the phugoid mode would be excited, causing the pitch and velocity to oscillate. On the other hand, if the airplane were equipped with the present auto-throttle control system, then the airplane would behave as shown in the right part of Figure 2. The pitch angle in the case illustrated would increase by 1°, tracking the command very well. The phugoid mode would be well damped. The thrust would be increased and decreased automatically, as needed to control the motion of the airplane.

This work was done by John J. Burken and Bill Burcham of Dryden Flight Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.

This invention has been patented by NASA (U.S. Patent No. 6,041,273). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

Technology Commercialization Office,
Dryden Flight Research Center;
(661) 276-3720.

Refer to DRC-96-07.