Fault Tolerant Flight Control: A Physical Model Approach

Safety is of paramount importance in all transportation systems, but especially in civil aviation. Therefore, in civil aviation, a lot of developments focus on the improvement of safety levels and reducing the risks that critical failures occur. When one analyses recent aircraft accident statistics, it is clear that a significant portion is attributed to “loss of control in flight”. A recent worldwide civil aviation accident survey for the 1989 to 2003 period, conducted by the Civil Aviation Authority of the Netherlands (CAA-NL) and based on data from the National Aerospace Laboratory NLR, indicates that this category accounts for as much as 17% of all aircraft accident cases. This has led to a common conclusion: from a flight dynamics point of view, with the technology and computing power available on this moment, it might have been possible to recover a part of the aircraft in the accident category described above on the condition that non-conventional control strategies would have been applied. These non-conventional control strategies involve the so-called concept of fault tolerant flight control (FTFC), where the control system is capable to detect and adapt for changes in the aircraft behaviour. One FTFC strategy option is using a model based control routine. This research focuses on a physical modular approach. In this setup, not only a reconfiguring controller is needed, but also a suitable FDI/identification strategy. This research focuses on both components. In this reseach project, a real-time aerodynamic model identification procedure has been combined with a model based adaptive control method. A manual as well as an autopilot version have been developed. The autopilot version has been evaluated on desktop simulations, the manual version has been tested in the Simona Research Simulator involving professional airline pilots. Both tests have demonstrated promising results. The autopilot performance is very good, and the manual controller has demonstrated to increase handling qualities and to reduce pilot workload of the damaged aircraft. These are very promising results that motivate further research in this field.Control and SimulationAerospace Engineerin

Solid Focal Liver Lesions Indeterminate at Contrast-enhanced CT or MR imaging: The Added Diagnostic Value of Contrast-Enhanced Ultrasound.

This paper presents a study on fault tolerant flight control of a high performance aircraft using multivariate splines. The controller is implemented by making use of spline model based adaptive nonlinear dynamic inversion (NDI). This method, indicated as SANDI, combines NDI control with nonlinear control allocation based on an onboard aerodynamic spline model and a real-time identification routine. The controller is tested for an aileron hardover failure and structural damages which change the global aerodynamic properties of the aircraft. It is shown that the controller can quickly tune itself in failure conditions without the need of failure detection and monitoring algorithms. Instead, self-tuning innovation based forgetting is applied to reconfigure the onboard aerodynamicmodel. The controller is able to tune itself each time a model error is detected and does not require any external triggers for re-identification. Multivariate splines have a high local approximation power and are able to accuratelymodel nonlinear aerodynamics over the entire flight envelope of an aircraft. As a result the identification routine gives a robust adaption of the aerodynamic model in case of a failure.Control & OperationsAerospace Engineerin

Pseudo Control Hedging and its Application for Safe Flight Envelope Protection

This paper describes how the previously developed concept of Pseudo Control Hedging (PCH) can be integrated in a Fault Tolerant Flight Controller (FTFC) as a safe flight envelope protection system of the first degree. This PCH algorithm adapts the reference model for the system output in case of unachievable commands due to control input saturation. As an example, this algorithm has been applied in the pitch rate and velocity control loops of a high fidelity Boeing 747 simulation model where its beneficial influence has been illustrated. The nonlinear adaptive control law used for this example is a triple layered nonlinear dynamic inversion algorithm, based upon the concept of time scale separation.Control & OperationsAerospace Engineerin

Effects of structural failure on the safe flight envelope of aircraft

The research presented in this paper focuses on the effects of structural failures on the safe flight envelope of an aircraft, which is the set of all the states in which safe maneuver of the aircraft can be assured. Nonlinear reachability analysis basedonan optimal control formulation is performed to estimate the safe flight envelope using actual aircraft control surface inputs. This approach uses the physical model of an aircraft, where the aerodynamic stability and control derivatives are calculated using Digital Datcom. Symmetrical damages to a Cessna Citation II are considered with 25, 50, 75, and 100% spanwise vertical tail tip losses, leading to gradual shrinkage in the safe flight envelope. Based on the estimated safe flight envelopes, a discussion on the effects of structural damages and different flight conditions on the safe flight envelope is presented. In particular, the interpolatibility of the resulting safe flight envelopes is demonstrated. This property is essential for a novel database-driven flight envelope prediction method, where a database of safe flight envelopes is created offline to be accessed later in real time.Control & SimulationWind Energ

Formation Flight Control System for In-Flight Sweet Spot Estimation

A formation flight control system has been designed that addresses the unique environment encountered by aircraft flying in formation and in the upwash of the leading aircraft. In order to test the control system a simulation environment has been created that adequately represents the aerodynamic coupling effects between aircraft flying in formation and implements an INDI-based formation flight control system. Since the exact position of the sweet spot cannot be known the research presented in this paper uses an advanced extremum seeking algorithm that utilizes an EKF to estimate the gradients. Simulations have shown the algorithm to be robust to errors induced by turbulence and having the ability to consistently find the sweet spo