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7.1 What Are the Requirements for the Selection of Military Pilots in the Portuguese Air Force?

One of the most demanding environments in which humans can work, in what concerns to vision function, is the interior of airplane cabins, especially the ones in military aircrafts.

The environment of the aircraft cabins is characterized by certain conditions which have a huge impact on human physiology and, particularly, on the physiology of vision.

Theoretically, common workplaces can be conceived or adapted in order to respect in the best possible way the physiology of their users. It happens that the interior of military planes, because of their specific requirements, has limited possibility of adaptation.

Several environmental factors to which the aircrews are exposed may influence the physiology of vision and may contribute substantially to the degradation of visual function:

- Hypoxia: the low relative concentration of oxygen in the air of the interior of aircraft cabins may induce hypoxia, which limits vision. Vision is the first of the five senses to be altered by the decrease of tissue oxygenation. Several functional changes in the vision and organic changes in the retina have been identified, which vary from slight changes of night vision in the altitudes of the Physiological Zone (0-3000 meters), decrease of accommodation and decompensation of heterophorias between 3.000 and 5.000 meters (Adaptation Zone), installation of diplopia or of tubular vision in altitudes of 5.000 to 8.000 meters, and even the occurrence of irreversible damage in the retina in what is named the Lethal Altitude Zone (above 8.000 meters). Hypoxia-induced by a flying incident with sudden loss of cabin pressure in a plane flying at 35.000 feet (approximately 10.000 meters), which is the cruising altitude of most commercial flights, elicits rapid loss of cognitive capability. The Time of Useful Consciousness (period of time elapsed from the exposure to an oxygen-poor environment to the loss of capability to take normal corrective or protective action) is only 30 to 60 seconds.

- Low humidity: the relative humidity in the pressurized aircraft cabins is typically very low (about 20%). This can influence visual performance directly by producing dryness of the ocular surface, and indirectly by contributing to the dehydration of the individuals.

- Speed: the human visual system was conceived to function at low displacement speeds. Aircrafts move at high speeds, demanding of the aircrews great visual performance and faster reaction to visual stimuli. As an example, if a fighter pilot is traveling in the interior of the cockpit of his plane at 1.000 kilometers per hour, the amount of time between the moment another plane appears in his visual field, his retina integrates that stimulus, transmits it to the central nervous system and the pilot becomes aware of the stimulus, he may have been displaced 300 meters and he will have traveled about one kilometer until he will be able to take the decision to deviate his plane in order to avoid a collision.

- Acceleration: the pilot inside an airplane, when performing maneuvers eliciting positive acceleration (positive G-force), suffers circulatory changes with a decrease in the venous return with the consequent decrease in the systolic volume and cardiac output with a secondary decrease in arterial pressure. When the perfusion pressure on the interior of the ophthalmic artery is inferior to the intraocular pressure, the blood flow in the interior of the eye lowers ant the pilot may experience a sudden loss of peripheral vision or, if exposed to greater acceleration forces, may experience G-Lock, so much feared by fighter pilots. It is because of the violent effects of acceleration to the circulatory system of pilots that modern high-performance planes are equipped with systems designed to automatically compress the lower limbs and abdomen of pilots (anti–G suits). These will automatically insufflate according to the acceleration exerted on the pilot, allowing better venous return to the pilot’s heart and thus limiting the effects of the acceleration forces.

- Altitude: visual environment at the high altitudes of aircraft flight also interferes with the physiology of vision. Flying above the clouds inverts the normal distribution of light, as those reflect the sunlight turning the inferior half of the visual field brighter, the opposite of what happens in the ground. Also, the absence of visual references in the horizon of crewmembers may induce supplementary accommodation leading to the disturbing phenomenon of Space Myopia.

- Vibration: the vibration felt in the interior of aircrafts, especially helicopters, may impair vision, particularly when the frequency is above 15 Hz.

- Lightning levels: in many situations, pilots perform long night missions and are submitted to the difficulties of working in low light, what can induce changes in their depth of field because of induced mydriasis. Another example of the influence of light onboard aircrafts is Flicker Vertigo. Nausea, vertigo or even seizures, were described in helicopter pilots, elicited by light stimuli of certain frequencies, generally due to the interception of sunlight by the helicopters rotating blades.

- Fatigue: long hours of flight, frequent in operational missions, many times occurring under limit conditions, lead to fatigue that alters vision capabilities and may cause a decrease in stereopsis due to decompensation of previous heterophorias.

About 80% of the sensorial information that allows the aviator to control his aircraft is acquired visually.
The (military) pilot is expected to have good:

- Far vision, in order to be able to correctly identify objects and obstacles outside the plane;

- Near vision, allowing the reading of maps and documents;

- Intermediate vision, for the good reading of the flight instruments;

- Stereopsis, necessary for measuring of distances in landings or when flying in formation;

- Chromatic vision, necessary for the correct and fast identification of warning lights and color displays on the cockpit, as well as for the adequate reading of color maps or the identification of targets in the exterior of the plane.

Efficient night vision capability is also needed, especially because more and more operational military missions take place at night.
The pilot vision is also hampered by the possibility of the interference of optical means with vision, like the glasses of the canopies, helmet visors, laser protective goggles or night vision goggles. In addition, the pilots must perform in every possible weather condition.

One of the main features of aeronautics, civilian and military, is the constant concern with safety. So, it is understandable that a particularly performing visual system is required from the pilots and, especially, from the young pilot military when recruited.

Candidates to military pilot in the Portuguese Air Force pass through an extensive array of medical tests. In addition to the extensive assessment of the medical history, ophthalmologic examination, oculomotor evaluation, color vision testing, perimetry testing, corneal topography, cycloplegic refraction, anterior segment optic coherence tomography (OCT) and contrast sensibility tests are done.
The criteria for selection are published in the legislation from the Ministry of Defence.

Distance Visual Acuity

Under 10/10, uncorrected, in each eye

Previous Refractive Surgery

Cycloplegic Refraction

Myopia (any degree)
Hyperopia greater than +2D
Astigmatism greater than 0.50D

Colour Vision

Any degree of dyschromatopsia

Oculomotor Balance
Esophoria greater than 10∆
Exophoria greater than 5∆
Vertical phoria greater than 1∆

Imperfect Stereopsis

Even though when these criteria of admission for new pilots were published they were comparable to those applied by the majority of other national Air Forces, many have changed their recruitment policies modifying certain demands.

As an example, the US Air Force, presently, may enroll pilot candidates previously submitted to refractive surgery and can perform refractive surgery on eyes of pilots already on duty.


LTC, Flight Surgeon, Ophthalmologist
Portuguese Air Force