Method and apparatus for automatically trimming an airplane stabilizer

Abstract

A method and apparatus for managing the trim of an airplane stabilizer whose elevator is controlled by a highly augmented elevator control law is disclosed. The augmented portion of the elevator command ##EQU1## is evaluated(13) to determine if it is above or below a variable threshold whose magnitude is dependent on airspeed. Portions above or below the threshold are integrated (15) to determine if the augmented portion of the elevator command is rapidly or slowly diverging. A stabilizer trim command is rapidly produced if the augmented portion of the elevator command is rapidly diverging and produced more slowly if the augmented portion is slowly diverging. The stabilizer trim command causes the production of a trim up or trim down command (25/27) depending upon whether the stabilizer needs to be moved up or down. Trim up commands (25) are inhibited under certain conditions, such as when the tail of the airplane is near the ground, when the airplane angle of attack is above the point where further nose up trim would be ineffective, or when the stabilizer trim limit has been exceeded. Trim down commands (27) are inhibited when the stabilizer trim down limit has been exceeded. Trim up and trim down commands are also inhibited when the command would drive the stabilizer out of fair with the elevator. Finally, provision is made for direct stabilizer trim control by the pilot of the airplane in the event that the elevator is limited and, thus, the stabilizer is required to produce additional pitch response.