title: Optical Tau theory : current and future roles in fixed-wing flight operations
Creators: name: Ridgway, Garnet R
Publisher: name: University of Liverpool Repository
Year: 2013

description: Commercial air travel is widely regarded as one of the safest methods of transportation in terms of fatalities per distance travelled, and the annual number of fatal airliner accidents has been in decline since the end of the Second World War. However, a small but significant number of fatal accidents still occur each year, indicating that there is scope for further improvement in flight safety. A review of airliner safety statistics concluded that the greatest proportion of fatal accidents over the last ten years have occurred in the approach and landing phase of flight. In spite of recent advances in flight deck automation for large transport aircraft, certain piloting tasks are still performed manually by the pilot. The flare manoeuvre (an aft longitudinal stick input in the final moments before touchdown) is an example of such a task, and is often undertaken based solely upon the visual information available through the windscreen. Previous studies have shown the flare to be considered the most difficult piloting task undertaken during typical fixed-wing missions. Additionally, there is no single consensus amongst the existing body of work as to the precise nature of the piloting strategies used to perform the flare manoeuvre. Recent studies at the University of Liverpool (UoL) have sought to apply theories of visual perception to such piloting tasks in order to gain an understanding of how pilots make use of the available visual information. In particular, the optical parameter “time-to-contact”, or “tau” has been shown to provide an appropriate basis for understanding and modelling pilot behaviour for “gap closure” type manoeuvres. Such manoeuvres, of which the flare is an example, involve the pilot controlling the motion of the aircraft between a specified start and end point. The overall aim of the work reported in this Thesis was to build upon these findings to further develop the current and future roles of tau theory in fixed-wing piloting tasks. The first objective of this research was to establish the nature of the strategy used by pilots to initiate the flare manoeuvre. A number of previous studies have investigated this area, often with conflicting results; this study, therefore, sought to identify and address some of the limitations of these previous investigations. A piloted simulation experiment was undertaken using a model of a generic large transport aircraft (GLTA) in the HELIFLIGHT simulator at UoL. The results suggested that pilots use a constant, critical value of time-to-contact with runway, , to initiate the flare manoeuvre. In addition it was demonstrated that commanding flare initiation at a constant value of through use of a Head Up Display (HUD) resulted in more successful manoeuvres (in terms of vertical velocity at touchdown ) than any of the other parameters tested. This further demonstrated the appropriateness of the tau-based flare initiation strategy. The second aspect of the work presented in this Thesis was concerned with the development and evaluation of a tau-based pilot aid for the flare manoeuvre. This was based on both the findings of the flare initiation investigation and of a previous study at UoL. The concept was used to drive a set of HUD symbology which was implemented onto the GLTA simulation model to enable piloted evaluation. The tau-based HUD was evaluated against both a baseline Head Down Display (HDD) and an in-service example HUD in a piloted simulation experiment. The results showed that the tau-based concept provided a performance advantage over the baseline HDD, and performance comparable with the in-service example HUD. Recommendations were made for further refinement of the concept in future design iterations. A previous study at UoL identified two types of tau-based piloting strategy for the flare manoeuvre. Specifically, it had been observed that pilots used either a strategy in which the aircraft performed a continuous vertical deceleration until touchdown (“type 1” ), or a strategy in which the vertical deceleration was completed before touchdown (“type 2”). In the case of the type 2 flare, the deceleration phase was typically followed by a phase of approximately constant vertical velocity. A piloted simulation experiment was undertaken to test the hypothesis that the type 2 flare strategy was adopted to compensate for the paucity of the visual information available, i.e. the fact that the pilots could not directly observe the landing gear. Three groups of novice pilots performed a simplified flare task using varying levels of visual information; the standard windscreen view, a simulated video feed showing the main gear and a HUD representation of the main gear. The results supported the hypothesis, and also showed that an improvement in performance could be derived from enabling the pilot to directly observe the gap closure formed by the landing gear and the runway. The final aspect of this study sought to extend the tau-based approach to fixed-wing flight control to other phases of flight. To this end, two methods of tau-based pilot modelling for fixed-wing aircraft were described and evaluated. The first of these computed a tau-based reference trajectory that was passed through a conventional stability control augmentation system (SCAS) in order to minimise the error between it and the aircraft’s current trajectory. The second method used an approximation of the inverse dynamics of the aircraft to generate the appropriate open-loop control input. The error minimisation model was shown to provide appropriate guidance for a typical range of manoeuvres for a light fixed-wing training aircraft. The perfect control method was shown to provide appropriate guidance for the single manoeuvre tested, and as such was recommended for further investigation. Overall, through the investigation of piloting strategy, this study showed the current role of tau theory to be as an appropriate, succinct method of describing pilot behaviour for a range of fixed-wing flight tasks.