In commercial aviation, there have been several recent cases of unstabilized approaches that have resulted in crash landings short of the runway. Some of the direst consequences of these incidents may be prevented with the addition of a level of autonomy — a supervisory envelope protection scheme that anticipates loss-of-control accidents and intercedes to prevent them.
A model predictive automatic recovery system determines when it must intervene to prevent an imminent accident resulting from a poor approach. It estimates the altitude loss that would result from a go-around (GA) maneuver at the current flight condition. If the loss is projected to violate a minimum altitude threshold, the maneuver is automatically triggered. The system deactivates to allow landing once several criteria are met.
The system consists of three primary components: the flight and propulsion control override mode, the flight path predictor, and the trigger system. When activated, the override mode exerts full authority over the primary flight and propulsion controls in order to modify the flight trajectory and avoid ground contact. The flight path predictor detects the onset of unsafe operation, and the trigger system utilizes current and projected flight conditions to activate or disable the override mode.
The control mode is functionally similar to the go-around flight mode for modern transport aircraft. For this work, relatively simple control logic was developed for demonstration purposes, but a GA mode could alternatively be used. Pitch and roll control algorithms were utilized to build a control mode that executes a rudimentary GA maneuver.
The flight path predictor is a simplified, two-dimensional rigid-body physics model of the aircraft that estimates the maximum altitude loss that would occur if the vehicle were to execute the full-throttle pull-up maneuver. Lateral dynamics of the aircraft are neglected since the primary application considered for this work is the approach/landing phase. The model accounts for longitudinal forces and moments due to aerodynamics and propulsion.
The purpose of the trigger system is to determine if the override mode must be activated to avoid unintended ground contact prior to the runway threshold by querying the flight path predictor at regular intervals. It also checks several criteria to determine if the override mode should be deactivated for landing.
Piloted simulations demonstrated that the system worked as intended, allowing normal operation in all instances, but intervening consistently when an unrecoverable situation was imminent.
This work was done by Jonathan Litt, Yuan Liu, Albert Owen, and Thomas Sowers of Glenn Research Center.
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