The Year of the Near Miss’ was Newsweek magazine’s label for the year 1987. In July it was prophesied that near collisions would increase by 75% over the number reported by the FAA for 1986. The number of near-miss reports did rise, but final NTSB statistics showed that general aviation had the lowest number of accidents reported since the establishment of the NTSB in 1939. This apparent paradox in the complex collision avoidance equation suggests that we should reexamine current medical standards in terms of their usefulness as predictors of piloting performance.
In the selection and screening of pilots it is desirable to use medical tests that can predict pilot capabilities and limitations in the performance of key tasks, such as early target detection, in a manner conducive to safe operation of the aircraft. The standard flight physical is designed to evaluate key biomedical factors that are believed to have an impact on piloting performance. Numerous vision tests are incorporated into the physical exam: visual acuity, color differentiation and glaucoma tests are typical. The pass/ fail criteria for each test were initially established on the basis of professional consensus. Very little field validation data are available to support these standards. Further, experts do not agree on any standard value nor on a standardized procedure to assess a given measure of visual performance.
Eight years ago, the major airlines were still enforcing the age-60 mandatory retirement rule for their air-crews. Former pilots from a major air carrier challenged that rule and recently won a $17.2 million settlement.
When the age-60 rule was lifted it sent a shock wave throughout the biomedical community. It tasked that community to create standards and tests that were appropriate predictors of performance (or performance decrement). The vision-test battery of the physical exam came under careful scrutiny. Was 20/20 acuity necessary for safe pilot performance? Why normal color vision? Is it necessary to check depth perception (or even binocular vision) or is monocular vision acceptable? What about vision tests that have recently emerged that seem to be more highly correlated with pilot performance?
This article will discuss each of the present-day visual standards and how they may relate to performance. Further, future emerging technologies will be discussed along with their applicability to the ultimate vision-test battery.
Visual acuity
Measurements of visual acuity assess the integrity of a very small percentage (5%) of the entire functioning retina. This area is called the fovea. The fovea is specialized to provide sharp daylight and color vision. It is interesting to note that the remaining 95% of the retina is only cursorily tested during the vision screening exam, yet it is the key contributor to our “peripheral awareness” and night vision.
Most people know that their visual Acuity is 2O/20, 20/40 or 2O/lOO, but they typically do not know what “visual acuity” means, except that 20/20 is the “goal.” 20/20 acuity is simply a fraction with the top number (numerator) representing a standard size letter (measured as an angle of 5 minutes of arc at a distance of 20 ft from an observer) and the bottom number (denominator) is an equivalent test distance at which the letter was resolved. For example, 20/40 means that what you see at 20 ft a normal observer (20/20) could see at 40 ft.
Tests for visual acuity can be divided into those involving detection, recognition or resolution. The physical exam only assesses resolution; however, detection may be a more appropriate measure for aviation safety. Further, it is not clear how 20/20 acuity is linked to the successful performance of key piloting tasks. It has been documented that vision degrades from 20/ 20 to 20/30 in air-to-air scenarios, and the “environmentally induced” reduction in vision may be sufficient justification for establishing high acuity standards (ie, 20/20).
However, traditional acuity testing may be an outdated, inadequate basis for assessing visual capability because:
it is irrelevant to many tasks
good acuity does not imply good task performance
measuring visual acuity is typically the first step used by the clinician to discover visual functional problems but it may be irrelevant to performance screening
standards for measuring acuity are inconsistent.
Color vision
For the Class 1 medical certificate the applicant is required to “discriminate” color – that is, to discern small differences between similar hues. Pseudoisochromatic plates are used to measure color discrimination. The plates are typically composed of a randomized matrix of dots with an imbedded number or figure that the applicant is required to identify. Color “identification” is required for the class 2 and 3 certificate. Identification is simply the ability to make gross categorizations of colors and this may be assessed with the Farnsworth Lantern (FALANT). The examinee is simply asked to identify the color of two vertically positioned spots of light that may be green, red or white.
Color interpretation is dependent upon a number of factors which include the level of light adaptation of the eye, luminance of the target, and physiological changes such as yellowing of the lens and internal media of the eye. A normal observer is able to detect as small as one wavelength difference between two spectral colors; however, this color sensitivity is neither necessary nor required for aviation performance, and typically the requirement is only for distinction between colors hundreds of wavelengths apart (eg, red and green).
For appropriate application of a color-vision requirement, the level of color sensitivity required should be defined by the task. The color-vision test selected should evaluate the pertinent sensitivity. An example of this might be the administration of a red/ green (Farnsworth Lantern) test under daytime and nighttime illumination levels. This test might mimic a pilot’s ability to interpret tower signal lamps.
Depth perception
To qualify for a Class 1 or 2 medical certificate “bifoveal” fixation and the maintenance of fusion “under conditions that may reasonably occur in performing airman duties” is required. Simply stated, this means that individuals must use the central vision in both eyes concurrently when they look at objects. It is reasonable to assume that, if vision is 20/20 in both eyes, this would fulfill the requirement for the appreciation of depth perception. The perception of objects in depth is dependent upon binocular and monocular cues, but screening tests are typically limited to binocular tests (ie, stereoscopic vision). Monocular vision is sufficient for a Class 3 certificate; monocular individuals typically use cues like shading, interposition and apparent size difference to interpret depth in their environment. These cues are obviously sufficient for recreational flying, since there are individuals who have useful vision in only one eye but perform aviation tasks without incident. However, the ability to perform well under monocular conditions is heavily dependent upon the recency of onset of visual loss. Further, without binocular vision, there may be a longer period of adaptation to unfamiliar environmental conditions (eg, an unfamiliar airfield), and this may initially result in a disadvantage to the monocular pilot.
In 1983, Dr James Mitchell summarized the importance of depth perception in the industrial setting in the following manner:
“Although it has been implied that a relationship exists between normal depth perception and safety, some industrial experience has shown that the number of applicants for placement who demonstrate a significant lack of depth perception is so infinitesimal as to make its determination as a routine part of every (screening) examination completely unnecessary.”
We must be extremely careful not to generalize a statement like this to all tasks within the aviation community. Stereopsis, the binocular form of depth perception, is typically used at distances of 20 ft or less and may be extremely important to taxiing, low altitude/minimum separation flight, and certain key cockpit tasks that may require arms-length manipulations.
Present screening tests evaluate stereopsis and fail to consider monocular depth cues and their contribution to the perception of depth. It may be more appropriate to include monocular and binocular tests of depth perception in our vision-screening batteries.
Field of vision
Typically the field of vision is assessed by very coarse means (eg, finger-waving in the periphery) and individuals are rarely referred for a more complete visual-field assessment.
Today, there are wonderful automated visual-field testing devices that plot the integrity of central and peripheral visual fields in a relatively short period of time. It is important to remember in this context that peripheral vision involves approximately 95% of the usable retina; it is employed extensively in daily activities to detect potentially harmful moving and flickering objects that enter our visual field from oblique directions. Further, peripheral vision is a key navigational aid in our locomotion through space. It would, therefore, seem most appropriate to select a screening device that would tell us more about the pilot’s peripheral vision than is attainable with the coarse measures currently in use.
Automated perimetry is being recommended and tested in research protocols involving vision-screening of automobile drivers, industrial workers and the visually impaired. It will inevitably be recommended for aviation screening.
Dark focus
When an individual is placed in an unstructured environment (total darkness, glaring sunshine) there is a tendency for the visual system to develop an artificial near sightedness. This condition was termed “dark focus” by Dr H. Leibowitz and was believed to represent an intermediate resting state for the focusing system (accommodation) of the eyes. In flying, this condition is potentially dangerous, because the pilot may be unaware that his focus shifts from infinity to a few feet.
There are currently no simple screening machines to asses dark focus. Research is currently underway to simplify the testing device and methodology.
Contrast sensitivity
In the 1960s vision scientists took an engineering technique for measuring transmission characteristics of lenses and modified it for assessment of human vision. The technique involved transmitting a series of bar gratings through the lens and assessing the quality of the projected image. The same technique is used in visual assessment. That is, the individual is exposed to a series of bar gratings of varying contrast and his responses give rise to a curved line of data called the “contrast sensitivity function” (CSF). The CSF is a measure of visual sensitivity over a broad range of contrasts. Visual acuity (20/20) represents only one point on the entire curve. It is apparent that the CSF is a much more appropriate measure of visual performance for the aviation environment.
There are screening tests for CSF Currently available but there is no consensus on how the results should be interpreted and applied to aviation problems. Perhaps with continued clinical applications this test may eventually be accepted as an appropriate screening tool. The US Air Force is incorporating it into their medical examination for air crews. Possibly their data will be translated for general aviation applications.
Dynamic visual acuity
It is well documented that, when target velocity increases, the ability to resolve the target decreases. This is an extremely important concept to the aviation community, where closing velocities can exceed 1000 mph. Safe operation in high density airspace is heavily dependent upon our ability to rapidly detect other aircraft. The ability to maintain fixation and identify the other aircraft are equally important. Dynamic visual acuity (DVA) is a measure of our ability to maintain focus on rapidly moving objects.
DVA is not currently used as a clinical or screening tool, but with advances in computer technology it is not unreasonable to assume that it may be effectively used some day to carefully quantify differences among pilots in the decrement in visual performance for a given task.
Vision-screening methodologies at present have not changed significantly from those used in the 1940s. The purpose of this article is to familiarize the pilot with the current tests and to suggest that the standards for medical classification should be reevaluated and perhaps modified to define more appropriately the vision necessary to successfully perform piloting tasks, rather than establishing criteria on the basis of the highest clinical standards.
It may be possible one day to create a battery of vision tests, using existing and emerging technologies, that will more appropriately define the visual standards based on “zones” of useful vision rather than separate and distinct cutoff scores. If the tests are developed in this fashion it will not pose a threat to the pilot population by making the standards more rigorous. Rather, it will indicate which piloting tasks the individual is capable of safely executing.
In recent years, there has been a convergence of interest among legal, clinical and scientific communities in developing new tests of vision performance. Devising a battery of vision screening tests and standards to satisfy our aviation requirements is an extremely difficult task. However, with current electronic and computer advancement, this task can be accomplished in such a way as to resolve some current dilemmas over the issuance of aviation medical credentials.
Dr. William Monaco
bill@flightsight.com
Flight Sight
