# A Study of Emergency Aircraft Control During Landing
## 2 Related Work  
The idea of using collective and asymmetric thrust variations as an aircraft emergency stabilisation and control mechanism was originally introduced more than 30 years ago and investigated mainly on theoretical grounds. In the civil aviation sector to date, only a couple of emergency thrust control systems are known to pass at least extensive simulation testing. One of the main requirements connecting the existing solutions was the low cost of system implementation. This factor obviously influenced the choice of control algorithms and, therefore, the maximum achievable precision of the system. Inspired by a story of the United Airlines Flight 232 disaster from 1989, which occurred due to loss of total hydraulic control, research conducted by the NASA Dryden Flight Research Center between 1991 and 1997 was the first to determine the potential of using engine thrust for aircraft emergency control [4]. The scope of the research included analytical studies, a simulation, and in-flight trials. One of the main project objectives was to investigate the degree of control that could be achieved by manipulating thrust levers among a diverse set of aircrafts, including Boeing 747, McDonnell Douglas MD-11, Learjet 24, and even Piper PA-30. For each of the machines studied, unique difficulties in performing a safe landing were found as a result of the aircraft's phugoid oscillations and Dutch roll, slow engine response, and poor control moments [5]. In response to the identified problem, a control system called Propulsion-Control Aircraft (PCA) was developed to provide emergency control in flight. Computations of the appropriate thrust control commands (instantaneous values of all the EPR coefficients) passed on to the FADEC controller were performed by two control systems in the longitudinal and lateral channel. The task of suppressing phugoid oscillations was accomplished by proportionally combining the signals of airspeed error, pitch rate, and pitch angle with a filtered-out DC component. Characteristic features of the PCA system included up to four control modes in the longitudinal channel, distinguished by the tail engine management method (for three-engine jets) and the level of integration with the aircraft's onboard systems, as well as the option for coupling with the ILS via special final approach algorithm, making the commanded flight path angle dependent on the altitude above the tarmac [6]. The method of combining control outputs from the PCA channels, however, has not been described in the literature. Despite successful in-flight trials, the system did not meet with approval from the FAA, who were of the opinion that the implementation of such a system would not be financially justified considering the extremely low likelihood of total control failure [7]. The concept of thrust-based emergency control had not been revisited until a research programme conducted by Mitsubishi Heavy Industries (MHIs) in 2010. The development of its \"Thrust-only Flight-control\" (TOFC) system was focused on the control of a four-engine Boeing 747-400 [8]. The proposed control law consisted of two main modules: the first one computing required aircraft angular acceleration based on commanded flight path angle, roll angle and feedback signals, and the second one responsible for optimal distribution of engine control commands. This ensured an independent thrust utilisation of each of the aircraft's engines. One of the TOFC system's distinctive features was a novel approach to controller design based on the H-infinity techniques. The programme also explored various methods of cockpit system integration, from information display for supporting pilot actions to full integration with the control column and ILS. The MHI's project was concluded with emergency landing simulations and evaluation by professional pilots. Concurrently with the work of Japanese engineers, research into aircraft emergency control technology was also conducted in Europe by the German Aerospace Center (DLR).