Emergency Landing Using Thrust Control and Shift of Weight

Dryden Flight Research Center, Edwards, California

Normally, the damage that results in a total loss of the primary flight control of a transport airplane, including all the engines on one side, would be catastrophic. Dryden Flight Research Center has conceived of a method of responding to a total loss of hydraulic pressure and failure of engines on one side: An emergency controller would utilize the engines that are still working on the other side, along with transfers of fuel among tanks to effect lateral shift of the center of gravity (CG), in order to steer the airplane to an emergency landing.

Figure 1: An MD-11 Airplane is shown here landing with thrust control only (that is, with control surfaces not moving).

Many occurrences have made it necessary to use engine thrust to supplement or even replace the normal flight controls of aircraft. Most of these occurrences have resulted in crashes, in which a total of more than 1,200 lives have been lost (see “Throttles Land Disabled Jet” by Michael Dornheim, Aviation Week & Space Technology, September 4, 1995, pages 26 and 27).

Dryden Flight Research Center has developed a propulsion-controlled aircraft (PCA) system, in which computer-controlled thrust provides an emergency control capability without using any of the normal control surfaces. Using a PCA system, an F-15 and an MD-11 airplane have landed without using any moving control surfaces. In all the cases studied, the thrust of the engines on both sides of the airplane was available. Figure 1 shows the first MD-11 PCA landing accomplished by use of thrust modulation with all the engines working and no moving control surfaces.

Consider an airplane on which a bomb blast has disabled the hydraulics (making the control surfaces inoperative) as well as the engine or engines on one side. Could an emergency controller be designed to land the airplane? A preliminary investigation has shown that one wing engine can be operated to obtain limited flight control if the lateral center of gravity (CG-Y) is shifted toward the wing with the working engine. Simulations of the MD-11 with all conventional flight controls inoperative and a wing engine inoperative have shown positive control capability within the available range of CG-Y offset. Simulations of such four-engine airplanes as the B-720 and B-747, have also shown positive control capability within the available ranges of CG-Y offsets.

U.S. Patent 6,126,111 documents this emergency controller. On large commercial airplanes, engines are usually located under both wings, either singly or in pairs, and large amounts of fuel are carried for long journeys. The fuel tanks are distributed throughout the wings and cargo sections of the airplanes. The transfer of fuel among the tanks of an airplane to shift its CG-Y is a well known and frequent practice.

Figure 2. The Asymmetric Forces that arise when the engines on one side and the control surfaces are inoperative are countered by use of an emergency controller, which varies the thrust of the working engines and transfers fuel among tanks to shift the center of gravity towards the working-engine side.

Fuel can be transferred selectably and independently among all fuel tanks. On the MD-11 airplane, each wing tank holds 42,000 lb (≈19,000 kg) of fuel and the maximum CG-Y shift observed in tests was 45 in. (1.14 m) in 15 minutes, corresponding to an average rate of change of CG-Y of about 3 in. (≈7.6 cm) per minute. On the B-747 airplane the wing fuel capacity is 84,000 lb (≈38,000 kg) in each inboard tank and 30,000 lb (≈13,600 kg) in each outboard tank, and the maximum CG-Y shift observed was 70 in. (1.78 m).

The upper part of Figure 2 illustrates the case of an airplane with inoperative control surfaces and only one operating engine. The CG-Y is shifted toward the side with the working engine. The thrust of the operating engine creates yawing and rolling moments to counter the rolling moment resulting from the drag of the non-operating engine. The lower part of Figure 2 depicts the emergency control scheme. The results of simulations of the use of this control scheme on the MD-11 and B-747 have shown that level flight can be maintained. Increasing thrust rolls the airplane away from side of the working engine. Roll rates of 5°/s are typical. This control scheme makes it possible to align the airplane with the center line of a runway and, most likely, to have a survivable landing. Of course, practice landings in a simulator or during a high-altitude flight above a real runway are needed to increase the likelihood of a survivable landing.

This work was done by John J. Burken, Bill Burcham, and Jeanette Le of Dryden Flight Research Center.

This invention has been patented by NASA (U.S. Patent No. 6,126,111). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed the Patent Counsel, Dryden Flight Research Center; (661) 276-3720. Refer to DRC-96-55.