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Guest Editorial

Wiley Post and visual standards for pilots

In “Wiley Post, Around The World with No Stereopsis” (in this issue of Survey) Elshatory and Siatkowski have highlighted the remarkable achievements of one of Oklahoma’s favorite sons, Wiley Post, a pioneer in aviation. His drive and interest in aviation brought him to overcome obstaclesda prison sentence for armed robbery and loss of his left eyedthat would have stopped a lesser individual. He set airspeed records in a flight around the world, initially with a co-pilot, but then solo, utilizing the first radio directional navigation and an autopilot designed by the Sperry Company. As an individual with limited resources, he also pioneered high altitude flight, working with the B.F. Goodrich Company to produce a pressurized suit, the first step in the evolution of our current space suits. The authors also provide some insight on the role of stereopsis, not just in aviation, but also in overall function. Since David Wheatstone11 published his work on stereovision in the 1830s12 and invented the first way of producing images that would induce a stereo effect (later revised and improved by David Brewster4), stereopsis has fascinated not only ophthalmologists, but also the general population. During the 19th century the stereoscope could be found as entertainment in most upper class drawing rooms. It has always seemed remarkable that individuals without stereopsisdnot just those who are monocular, but also those that have amblyopia and never developed stereovisiondcan function as well as they do. The importance of stereopsis in one’s personal space (arm’s length) can be illustrated to anyone who has been binocular by simply occluding one eye and trying to reach for an object. Over time, however, the one-eyed person will learn to use monocular cues. Individuals who have never been binocular utilize parallax and other clues effectively, even for working with near objects. A number of ophthalmologists and other surgeons have successfully functioned without stereopsis, including performing microsurgery. A different question that has never been answered is the role of stereopsis at distances beyond arm’s reach. Aviation, more than any other activity, has provided an arena for the debate about the importance of stereopsis. Because it was easier to attribute crashes to “pilot error” than to design and construction deficiencies (“about 90 percent of the accidents to graduate pilots were due to defects in the pilots themselves”)3 it

is not surprising that a “great effort was made to increase the physical standards for flying training and to determine the attributes which good pilots should be required to possess,”3 It is interesting that the criteria (especially ophthalmic and otolaryngologic) for pilots in the U.S. Army Air Corps were established quite arbitrarily, primarily by William Holland Wilmer, a reserve officer in the U.S. Army at the time the United States entered World War I and later the director of the Wilmer Eye Institute established at Johns Hopkins in 1929. It was recognized early on that “it is possible for a man to fly in spite of one or many handicaps; the object to be obtained, howeverdthe defeat of the enemyddemands that only such fliers be sent against him as are the very best air-fighting materialdnot merely men who are able to fly.”1 Unlike other countries’ air services, where various other criteria were emphasized (including vestibular function, reaction time, tolerance of hypoxia, and measures of equanimity),3 the U.S. army had a surplus of volunteers and could apply any criteria, no matter how arbitrary, and still fill the roster (“At the present time in the United States a high standard is required because this country is in a position to pick and choose.”6) Those American pilots that flew with the Lafayette Escadrille prior to the entrance of the U.S. into World War I had no such criteria, and one of the foremost aces for the Americans, William Thaw, flew with useful vision in only one eye. Nonetheless, in May 1917, the army issued “Form 609,” which outlined in detail the physical requirements for prospective pilots. These criteria specifically included stereopsis (listed as number 2 on the form) tested initially with the stereoscope. “The one-eyed man may succeed; the possession of two eyes, however, would render success more certain.”1 Visual “deficiencies” were the largest cause of disqualification, including uncorrected acuity of less than 20/20, color vision defects, presence of a phoria greater than 2 diopters, and lack of “normal depth perception.”3 In spite of questions about the importance of vestibular criteria written into Form 609dand raised by Isaac Jones,6 who tried to retract the criteria he helped introducedthe form remained essentially unchanged through the end of World War I into the 1920s. Although civilian flying organizations, based largely on similar automobile organizations, began in the first decade of

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the 20th century, licensing or regulation of who could fly did not exist until the founding of the Bureau of Air Commerce of the Department of Commerce in 1926. The first license was issued in April 1927, one month before Lindbergh’s New York to Paris flight. Visual criteria, established largely by T. C. Lyster and W. H. Wilmer in Form 609, passed to a surprisingly complete degree into the requirements for licensing, not unexpectedly since the Bureau of Air Commerce was run by former Col. Louis H. Bauer, who had succeeded Wilmer. Depth perception was now measured by the use of the HowardDolman apparatus that replaced the original military requirements of viewing through a stereoscope, but was still designed to limit monocular clues. Prior to World War II, instead of becoming more realistic about the importance of stereopsis, Ames2 and the Dartmouth Eye Institute emphasized the negative potential of aniseikonia and its effect on stereoscopic vision and argued for this exclusion for civilian as well as military pilots.3 As Elshatory and Siatkowski point out, a waiver was obtained for Post, whose employer insisted on following the new regulations. As they also mention, although the requirements are less stringent for private pilots today, the military still requires stereopsis, arguing that this is needed for formation flying and refueling. In a paper summarizing the literature dealing with the role of binocular clues in landings, Entziger noted that “many papers just cite or even re-cite values [of stereopsis] without even taking their determination method into account, thus creating the false impression that a certain value is commonly accepted.”5 He goes on to observe that “the practical range of stereoscopy as a depth cue is much smaller than the theoretical range” and thus “stereopsis would be of practical use [only] up to 20w65 meters.”5 Testing with the military-adopted Howard-Dolman apparatus led him to think that “stereopsis is not as important an aid to the quantitative discrimination of real depth, as is usually supposed.”10 Although “common knowledge” seems to suggest that stereopsis is important for night landings, as far as I am aware, this has never been tested. Even those initially binocular “quickly adapted to monocular flying.”5 Entzinger concludes that “it is very likely that monocular cues [not binocular stereoacuity] do indeed dominate in practical situations.”5 Practical verification of these sentiments could be seen in testing that demonstrated landing of jet trainers by qualified pilots was unaffected by acute monocular occlusion,7 and follow-up of monocular pilots failed to reveal an increased accident rate.9 As early as 1935, Jongbloed8 was unable to demonstrate any degradation in landing performance following blindfolding one eye. Interestingly, in spite of his own findings (and perhaps as a warning to how we interpret

data, particularly when it doesn’t fit our preconceived notions) he nevertheless concluded that one-eyed persons should not fly. We are often prisoners of knowledge without foundation. This failure to appreciate the implications of studies is certainly not the first instance of not allowing facts to revise established dogma. Perhaps it is not so remarkable that Wiley Post and other monocular aviators have been able to perform so well. Whatever the actual visual requirements necessary for flying airplanes, we as ophthalmologists should respect the determination and drive of this remarkable one-eyed aviator.

references

1. Air Service Medical. Washington, Government Printing Office; 1919 2. Ames A. Aniseikoniada factor in the functioning of vision. Am J Ophthalmol. 1935;18:1014e20 3. Armstrong HG. Principles and Practice of Aviation Medicine London, Baillie`re, Tindall & Cox; 2nd Ed., 1943 4. Brewster D. Account of a new stereoscope. Report Brit Assoc Trans Sec. 1849;6e7 5. Entzinger JO. The role of binocular cues in human pilot landing control. AIAC-13 Thirteenth Australian International Aerospace Congress. Australia 2009;1e15. 6. Jones IH. Equilibrium and Vertigo. Philadelphia, PA, JB Lippincott Co; 1918, p 33 7. Lewis CE Jr, Krier GE. Flight research program: XIV. Landing performance in jet aircraft after the loss of binocular vision. Aerospace Med. 1969;40:957e63 8. Jongbloed J. Landing carried out by experienced aviators with the use of one eye only. Acta Brevia Neerland. 1935;5:123e5 9. Mayer HB, Lane JC. Monocular pilots: a followup study. Aerospace Med. 1973;44:1070e4 10. Teichner WH, Kobrick JL. Effects of viewing distance with the Howard-Dolman apparatus. J Optical Soc Am. 1956;46:837e40 11. Wade NJ. Brewster and Wheatstone on Vision. London, Academic Press; 1983 12. Wheatstone C. Contributions to the physiology of visiondPart the first. On some remarkable, and hitherto unobserved, phenomena of binocular vision. Phil Trans R Soc. 1838;128:371e94

Steven A. Newman, MD University of Virginia, Charlottesville, Virginia Corresponding author: Steven A. Newman, MD University of Virginia, Box 800715 Charlottesville, VA 22908-0715. E-mail: [email protected]

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