Technical report | Distance Discrimination Thresholds During Flight Simulation in a Maritime Environment

Abstract

 The Aeronautical Design Standard 33 (ADS-33) prescribes a set of manoeuvres and limits for assessing the handling qualities of military rotorcraft. In developing an ADS-33 maritime addendum, the Aircraft Maintenance and Flight Trial Unit (AMAFTU) and Defence Science and Technology Organisation (DSTO) have collaboratively worked towards the definition for a maritime hover manoeuvre. Initial testing to validate this proposed manoeuvre in the DSTO Air Operations Simulation Centre (AOSC) flight simulator has proved problematic. Although pilots could successfully complete a land-based hover within performance limits in the flight simulator, they experienced difficulty in maritime conditions. Pilots were unable to perceive small distance changes in the AOSC simulated maritime environment in some conditions.

Executive Summary

The Aeronautical Design Standard 33 (ADS-33) prescribes a set of manoeuvres and limits for assessing the handling qualities of military rotorcraft. In developing an ADS-33 maritime addendum, the Aircraft Maintenance and Flight Trial Unit (AMAFTU) and Defence Science and Technology Organisation (DSTO) have collaboratively worked towards the definition for a maritime hover manoeuvre.Initial testing to validate this proposed maritime hover manoeuvre in the DSTO Air Operations Simulation Centre (AOSC) flight simulator has proved problematic. Although pilots could perform a land-based hover in the flight simulator, they experienced difficulty in maritime conditions, far exceeding the usual degradation in performance observed when shifting from actual flight trial to simulated flight trial. In accordance with the proposed Maritime Hover performance limits (Appendix A), pilots were expected to maintain plan position within 3 ft of the hover's origin, whilst also maintaining heading and altitude. This was to be done with reference to a buoy 50 m away. In the DSTO AOSC Maritime Hover trial, plan position could not be maintained under any conditions.

It was suggested that perceptual limitations were the reason for poor performance in the DSTO AOSC Maritime Hover trial. A short experiment was required to test whether or not a change of 3 ft between the aircraft and the buoy, at distance of 50 m and at a constant altitude, was large enough to be detected. The experiment described in this paper was conducted for this purpose. Its aim was to determine the minimum distance the aircraft must move from its original position for a change in position to be perceived. This minimum distance was defined as the distance discrimination threshold.

For both high and low sea states, the thresholds were found to be larger than the 3 ft 'desirable' performance limit. If a pilot is required to maintain plan position within a limit that is too small to be detected at a perceptual level, it will only be by chance than the hover is successfully completed. This is not adequate assurance for testing a potential manoeuvre in the simulator.

To rectify the problems pilots experience when performing the hover in the simulator, 3 solutions may be employed. These are as follows:

•                         Improve simulator technology to include more of the fine visual cues a pilot expects. 
•                        Specify how far from the buoy the pilot should hover. A 3 ft movement backwards or forwards will change the image projected onto the screen by a larger proportion at closer distances, decreasing the threshold for discrimination.
•                        Review the maritime hover performance limits (for plan position in particular) and widen them to account for the degraded performance observed in the simulator.

A combination of these options may prove to be the most effective solution.

The results of this experiment suggest that the thresholds for distance discrimination are situated very close to or beyond the 3 ft 'desirable' limit of the maritime hover task. This is true in the absence of any task other than identifying distance. It is therefore unreasonable to expect pilots to maintain the current 'desirable' and sometimes 'adequate' distances within the AOSC simulator. While simulator improvement continues, a review of these limits, or alternatively a prescription for the precise distance to hover from the buoy, should be considered. This will ensure that limits for the maritime hover are a true reflection of the aircraft's ability to perform the manoeuvre and that such an ability is not impeded by a pilot's inability to perceive small distance changes in the AOSC simulated maritime environment