# Unaided Night Flight In today's aviation environment, night missions are seldom conducted completely unaided. There are times, however, that aviators may determine that unaided flight is preferable to aided flight. There are definite reasons for doing so, as well as some potential problems that need to be avoided. The basic principle is to choose the sensor that is most appropriate for the mission and flight environment. *** ## **Problems** One of the biggest problems with unaided flight is that dark areas can be difficult or impossible to see into. The level of available detail is inconsistent due to the highly variable light level. Obstacles can be very difficult to see. Additionally, the flight environment (visibility or weather) may change while the aircrew is effectively "heads down" in the lighted area. *** ## **Considerations** The following are considerations of unaided night flight: cockpit lighting, altitude/airspeed changes, glasses, and dark adaptation. ### **Cockpit Lighting** Cockpit illumination should be kept at the lowest level consistent with safe operation of the aircraft (figure 4-1). This does not mean 'as dim as possible.' As ambient level decreases from twilight to darkness, intensity of the cockpit lights should be continually reduced to a low, usable level, reducing any glare or reflection off the windscreen. Cockpit instruments should be instantly and easily readable. The delay required for aviators to adapt their vision to cockpit illumination levels that are too low can result in unacceptable loss of situational awareness, particularly at NOE altitudes. Crewmembers should be discreet in the use of supplemental lights to avoid detection by enemy forces. \[ Insert Picture of AH64 Night Cockpit Lighting] ### Altitude /Airspeed Changes During unaided flight operations, allow time to adjust to flight conditions. This includes readjustment of instrument lights and orientation to outside references. During the adjustment period, the aircrew's night vision continues to improve until optimum night adaptation is achieved. Remember to allow additional time and distance to allow for reduced ability to identify landmarks and obstacles. It is important to recognize the level of one's ability to accurately interpret the terrain. It may become necessary to slow down or increase altitude to allow the reaction time necessary to react to changes. It is much easier to out-fly one's ability to see unaided than when using an NVD. ### Glasses Many people suffer from slight visual defects. The importance of correcting for these becomes greatly magnified when operating in marginal conditions, such as limited visibility or unaided flight. Remember to keep them clean. ### Dark Adaptation Vision is possible due to chemical reactions within the eye. Night vision requires a buildup of one of these chemicals (Rhodopsin) in the rod cells in the retina. The average time required to attain peak sensitivity or adaptation to a dark environment is 30 to 45 minutes. Depending upon the individual's starting level, dark adaptation is about 80 percent complete within 30 minutes, but it may take hours or even days to reach individual maximum dark adaptation. Full dark adaptation never really occurs in the cockpit because of the light intensity provided by instrument lights. The physiological processes involved are fully explained in TC 3-04.93. #### Sunglasses Colored sunglasses ***DO NOT*** protect dark adaptation. For sunglasses to be effective, all visible light has to be attenuated, not just portions of the visible spectrum. Colored or yellow visors and spectacles do not protect dark adaptation. To protect unaided night vision, allow for scanning close to the sun, and provide normal color vision, use dark sunglasses with a neutral gray tint. Tinted sunglasses that are too dark may reduce visual acuity unless the ambient brightness remains excessively high. Lighter sunglasses may not be dark enough to protect retinal sensitivity for night vision. Therefore, it has long been recognized that military flyers should use sunglasses on sunny days. These sunglasses should have a visible luminance transmission of 15 percent or less. Sunglasses identified on the authorized protective eyewear list should provide adequate protection for night vision if they are worn consistently when outdoors in bright sunlight. #### Bright Lights Exposure to bright sunlight also has a cumulative and adverse effect on dark adaptation. Reflective surfaces such as sand, snow, water, and man-made structures intensify this condition. For example, exposure to intense sunlight for 2 to 5 hours decreases visual sensitivity for up to 5 hours and also decreases the rate of dark adaptation and degree of night visual acuity. #### Flash Blindness Dark adaptation recovery time depends on duration and intensity of the exposure. While dark adaptation of the rods develops rather slowly over a period of 30 to 45 minutes, it can be lost in a few seconds of exposure to bright light. If the eyes are exposed to bright light after dark adaptation, their sensitivity is temporarily impaired. The degree of impairment depends on the intensity and duration of the exposure. Brief flashes from high-intensity, white xenon strobe lights commonly used as aircraft anti-collision lights have little effect on night vision because the energy pulses are of such short duration, lasting only milliseconds. Exposure of 1 second or longer to a flare or searchlight, however, can seriously impair night vision. Depending on brightness (intensity) and exposure duration or after repeated exposures, complete dark adaptation recovery time can range from 5 to 45 minutes or longer. Accordingly, during night operations aircrew members should avoid bright lights or protect one eye. Dark adaptation is an independent process in each eye. Even though a bright light may shine into one eye, the other eye will retain its dark adaptation if it is protected from the light. Aircrew members should avoid looking at flares, flames, or gun flashes to avoid temporary flash blindness. If a supplemental light must be used, it should be as dim as possible while still being readily usable (the lowest easily readable level) and should only be used for the shortest possible period. *** ## Types of Vision Types of vision are related to the light level—photopic, mesopic, scotopic (figure 4-2). They relate to the level of detail available to the aviator under different conditions. These concepts are more fully explained from a medical perspective in TC 3-04.93.
### Photopic Vision Photopic vision is experienced during daylight or under high levels of artificial illumination. Cones concentrated in the fovea centralis are primarily responsible for vision in bright light. Due to the high-level light condition, rod cells are bleached out and become less effective. Sharp image interpretation and color vision are characteristics of photopic vision.
### Mesopic Vision There is a common misconception that the rods are used only at night and the cones only during the day. Actually, both rods and cones function simultaneously over a wide range of light intensity levels. The transition zone between photopic and scotopic vision, where the level of illumination is equivalent to twilight or dusk, is called mesopic vision. It is experienced at dawn, dusk, and under full moonlight. The brightness output from NVG also falls within this range. Visual acuity steadily decreases with declining light. Color vision is reduced or degraded as light levels decrease and the cones become less effective. The central or night blind spot becomes more apparent as cone cell sensitivity is lost. Mesopic vision can be the most dangerous visual conditions for crewmembers. How degraded the ambient light condition is determines what type of scanning or viewing technique crewmembers should use to detect objects and maintain safe and incident-free flight.
### Scotopic Vision Scotopic vision is experienced in low light environments such as partial moonlight and starlight conditions. Cones become ineffective in these conditions (approximately 50 percentage illumination and below), causing poor resolution in detail. Visual acuity decreases to 20/200 or less, and color perception is lost. Scotopic vision degrades color perception to shades of black, gray, and white unless the light source is of adequate intensity to stimulate the cones. The central or night blind spot is fully evident. Scanning, off-center and peripheral vision are used while viewing with scotopic vision.
*** ## Retinal Blind Spots There are two areas in the retina that can lead to impaired night vision. These two blind spots are outlined below and can have a significant impact on flight operations. They are more fully described in TC 3-04.93. The day blind spot is offset from the center of the retina. It is where the optic nerve attaches to the retina. No cones or rods are present at the attachment point. It is slightly smaller than the night blind spot. Viewing with binocular vision compensates for the day blind spot, as each eye overlaps the other when viewing. Note that this compensation no longer exists for aviators using a single occluding monocular, such as the helmet display unit (HDU) used in the AH-64.
The night blind spot occurs due to the total absence of rod cells in the fovea (figure 4-7). It occurs when the cone cells become inactive in low light conditions and involves an area from 5 to 10 degrees wide in the center of the visual field. It is necessary to look about ten degrees to one side or the other of the object in question, allowing the rods to detect the object. At these low light levels, though, the object tends to fade away when stared at for longer than a few seconds. This is because the rhodopsin reaches chemical equilibrium and stops transmitting visual information. The night blind spot causes difficulty when individuals do not move their head or eyes but continue to look directly at or near an object. The size of the objects that can be hidden by the night blind spot increases as the distance between the observer and object increases. The night blind spot is compensated for by using proper scanning techniques and off-center viewing.
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