Visual Standards in Aviation: Implications for Safety, Performance, and Training Assessment

Abstract

Objectives: The goal of this thesis was to provide evidence to either support the current aviation visual medical standards or to begin to define new ones. Two aspects of vision (visual acuity and contrast sensitivity) were intentionally degraded to assess their effects on flight performance. Four experiments were conducted that examined this topic. Experiment one assessed the flight performance of novice pilots during two simulated flight scenarios: approach to landing or a short flight circuit and landing during clear and calm weather conditions with vision degraded with scattering lenses. Experiment two built on this by having novice and intermediate pilots completed a simulated flight circuit and landing during three adverse weather conditions (high wind, heavy rain or high wind and heavy rain) with their vision degraded by scattering lenses or by defocusing lenses. Experiment three examined whether vision could be used as a probe to evaluate flight instructors. This study had pilot participants complete a simulated flight circuit and landing scenario with their vision degraded using both scattering lenses and defocusing lenses while two flight instructors subjectively assessed their flight performance. The fourth and final experiment expanded on these studies by assessing both the flight performance and stress responses of pilot participants during a simulated flight circuit and landing while their vision was degraded either by scattering lenses or defocusing lenses. Methods: Twenty participants were recruited for study 1. Pilot participants completed either an approach to landing flight simulation or a short flight circuit and landing simulation in clear, calm weather conditions with their vision degraded with scattering lenses to either 6 or 8 levels of degradation respectively. Twenty-six pilot participants were recruited for study 2 where they completed a short flight circuit and landing simulated flight in three weather conditions (wind, rain, wind and rain) with 5 levels of vision degradation. Vision was degraded by either scattering lenses or defocusing lenses. Study three examined using vision as a probe to assess flight instructor agreement and repeatability. Twenty pilot participants completed a simulated short flight circuit with their vision degraded using both scattering and defocusing lenses to 8 different visual acuity levels. There was a total of five flight instructors recruited for study three in which two instructors were present to assess the performance of a single pilot participant. This was repeated on a second day with the same pilot participant and the same flight instructors. Study four examined the effect of vision degradation (either scatter or defocus) on both the flight performance and on pilot stress levels (which were monitored via eye-tracking and heart rate sensors) in thirty-seven pilot participants. These participants were tasked with completing a short flight circuit and landing and some of the participants had randomized minor (increased oil pressure) or major (engine failure) emergency scenarios introduced. Results: Results from all four studies showed that flight performance (vertical speed, airspeed, altitude, pitch, roll, landing hardness, landing accuracy) was not significantly impacted by mild and moderate visual degradation. Only severe degradation had an impact on performance. These studies also show that pilot participant contrast sensitivity may be a better indicator of performance than visual acuity, as declines in performance with milder vision degradation when using defocusing lenses compared to scattering lenses were seen. Studies two through four highlighted the importance of using both objective and subjective grading when assessing pilot performance as it was identified that there was a difference in when objective flight performance metrics (significant at 6/18 with scattering lenses and 6/60 with defocusing lenses) were affected and when flight instructors perceived a change in performance (significant at 6/18 with scattering lenses and 6/120 with defocusing lenses). The final study showed that while at several of the vision degradation levels flight performance was unaffected, pilot participant heart rate and eye-tracking metrics were affected. With heart rate variability and eye tracking metrics (saccade amplitude and saccade velocity) being affected at a degradation level of 6/18. Conclusions: These findings challenge the strict reliance on the use of just visual acuity in the current aviation medical standards. Across four experiments, results showed that flight performance remained largely unaffected when defocus lenses were used but was affected at a much lower visual acuity degradation when scattering lenses were used, suggesting contrast sensitivity may be a more relevant predictor of pilot performance. The studies suggest that current visual medical standards may be unnecessarily strict, thereby restricting the pool of eligible pilots. This work also highlighted the need for a more standardized and evidence-based approach to pilot evaluation. The final experiment further underscored the role of stress in flight safety, as physiological stress responses were observed even when flight performance remained stable. Taken together, these findings suggest that a more comprehensive approach – incorporating contrast sensitivity, objective performance measures, and stress responses – may lead to a more effective and fair pilot vision standard.