Investigation of optic flow, time-to-intercept, and pilot workload during aggressive approach to hover maneuvers

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Bachelder, E.
Godfroy-Cooper, M.
Aponso, B.
Blanken, C.
Berger, T.
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This work proposes a novel relationship between pilot workload and optic flow during visual approach-to-land maneuvers. A simulation experiment was conducted at NASA Ames Vertical Motion Simulator (VMS) to evaluate the workload associated with operating two candidate Army Future Vertical Lift (FVL) vehicles: a compound (coaxial-rotor and push-prop) vehicle, and a tilt-rotor vehicle. The UH-60 was included in the evaluation as a baseline reference. Sixteen experienced military pilots flew aggressive visual approaches terminating in a hover while providing Bedford workload ratings in real time. No approach or hover guidance was displayed to the pilot. The out-the-window (OTW) environment (front and chin monitors) was digitally recorded and the optical flow of each video frame computed. Prior work identified a mathematical relationship between pilot workload and the combination of display error rate and stick rate during compensatory tracking tasks. The current work extends this relationship to visual landing approaches, where the pilot is hypothesized to track key optical variables that are available from the OTW scene. Via correlation analysis a set of candidate tracking variables which appears to drive pilot workload is identified: the rate of change of optical flow, and the angle formed between the cockpit glareshield and the intended landing spot. Combined with stick rate these variables are used to generate a Bedford estimate. Actual and modeled Bedford ratings are compared for the compound aircraft (video for the other aircraft will be processed and presented in a future paper. Innovative contributions of this research include: 1) Optical flow from high resolution, high frame rate flight video is computed and analyzed for workload analysis; 2) A modelling technique is developed that produces workload estimates that closely matches actual pilot ratings; 3) A technique based on visual perceptual requirements allows optical flow to be employed in a very simplistic, tractable, yet effective manner; 4) While tau motion theory (i.e. rate of instantaneous time-to-arrive is approximately constant) was roughly observed during the approaches, it appears that tau motion was a result of the pilot adhering to a strategy of minimizing deviation in optic flow rather than being the source of pilot behavior. This preliminary, significant conclusion proceeds from the observation that workload correlated well and was causal with minimizing change in optic flow, but correlated poorly and was often non-causal with changes in tau motion; 5) Using a novel method, Bedford workload ratings were collected in real time without impinging on the flight task, enabling in-situ workload analysis. Potential applications include: a) If a pilot has transferred control to automation during an approach in an Optionally Piloted Vehicle (OPV), pilot trust may be higher if he/she observes system behavior that resembles what a skilled operator would produce, i.e., optic flow control; b) Control of optic flow may be an effective, robust method for autonomously executing power-off (autorotative) flight to the ground