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NIGHT FLIGHT TECHNIQUES AND PROCEDURES

HEADQUARTERS, DEPARTMENT OF THE ARMY

DISTRIBUTION RESTRICTION: Distribution authorized to US government agencies and their contractors to protect information and technical data that address current technology in areas of significant or potentially significant military application. This determination was made on 5 August 1988. Other requests for this document will be referred to Commander, US Army Aviation Center, ATTN: ATZQ-DAP-SS, Fort Rucker, AL 36362-5035. DESTRUCTION NOTICE: Destroy by any method that will prevent disclosure of contents or reconstruction of the document.

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Training Circular * TC 1-204 No. 1-204 HEADQUARTERS

DEPARTMENT OF THE ARMY Washington, DC 27 December 1988

NIGHT FLIGHT TECHNIQUES AND PROCEDURES

TABLE OF CONTENTS

Page PREFACE v CHAPTER 1 NIGHT VISION

1-1. Night Vision Evaluation 1-1 1-2. Eye Anatomy and Physiology 1-1 1-3. Light Levels 1-2 1-4. Vision Types 1-3 1-5. Day Versus Night Vision 1-4 1-6. Visual Problems 1-7 1-7. Dark Adaptation 1-8 1-8. Night Vision Protection 1-8 1-9. Self-Imposed Stress 1-10 1-10. Scanning Techniques 1-12 1-11. Distance Estimation and Depth Perception 1-14 1-12. Visual Illusions 1-19 1-13. Aircraft Design Limitations 1-23 1-14. Nerve Agents and Night Vision (Miosis) 1-23

CHAPTER 2 AVIATION NIGHT VISION AIDS Section I IMAGE-INTENSIFIER SYSTEMS

2-1. Development 2-1 2-2. Operational Theory 2-2 2-3. AN/PVS-5 Series 2-3 2-4. AN/AVS-6 2-7 2-5. Adjustment Techniques 2-9 2-6. Operational Considerations 2-10

Section II THERMAL-IMAGING SYSTEMS

2-7. Operational Principles 2-19 2-8. System Types 2-19

*This publication supersedes FM 1-204, 11 October 1983.

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2-9. Infrared Characteristics 2-20 2-10. Operational Considerations 2-26

CHAPTER 3 HEMISPHERICAL ILLUMINATION AND METEOROLOGICAL CONDITIONS

3-1. Light Sources 3-1 3-2. Meteorological Effects 3-2

CHAPTER 4 TERRAIN INTERPRETATION

4-1. Visual Recognition Cues 4-1 4-2. Interpretation Factors 4-4

CHAPTER 5 NIGHT OPERATIONS Section I PREMISSION PLANNING

5-1. Mission Briefing and Debriefing 5-1 5-2. Crew Duties 5-1 5-3. Common Terminology 5-1

Section II PREFLIGHT GUIDELINES

5-4. Preflight Inspection 5-2 5-5. Aircraft Lighting 5-3 5-6. Aircrew Preparation 5-4

Section III NIGHT FLIGHT TECHNIQUES

5-7. Limitations 5-4 5-8. Hover 5-5 5-9. Takeoff 5-7 5-10. En Route 5-8 5-11. Landing 5-9 5-12. Pathfinder Operations 5-14 5-13. External Load Operations 5-15

Section IV EMERGENCY AND SAFETY PROCEDURES

5-14. Basic Considerations 5-17 5-15. Electrical Failure 5-17 5-16. Airport Traffic Control Light Signals 5-17 5-17. Visual Night Signals 5-18 5-18. Emergency Landing 5-18 5-19. Ground Safety 5-19 5-20. Air Safety 5-19 5-21. Airspace Management 5-20

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CHAPTER 6 NIGHT TERRAIN FLIGHT Section I TERRAIN FLIGHT MODES AND COMMAND CONSIDERATIONS

6-1. Terrain Flight Modes 6-1 6-2. Command Considerations 6-1

Section II PLANNING GUIDELINES

6-3. General Considerations 6-3 6-4. Cockpit Teamwork and Coordination 6-6 6-5. Aircraft Preparation and Equipment 6-7 6-6. Maps and Visual Aids 6-7 6-7. General Route and Air Control Point Planning 6-8 6-8. Aided Night Mission Map Preparation 6-10 6-9. Aided Night Mission Planning and Briefings 6-10 6-10. Route Planning Cards 6-11

CHAPTER 7 MULTIHELICOPTER OPERATIONS Section I CONSIDERATIONS AND RESPONSIBILITIES

7-1. Planning Considerations 7-1 7-2. Supported Ground Unit Commander Responsibilities 7-2 7-3. Air Mission or Flight Commander Responsibilities 7-3

Section II NIGHT FLIGHT FORMATIONS

7-4. Aircraft Separation 7-4 7-5. Night Formations 7-4 7-6. Basic Night Formation Considerations 7-7 7-7. Formation Takeoff 7-8 7-8. Lead Changes 7-8 7-9. Formation Changes 7-9 7-10. Rendezvous and Join-Up Procedures 7-9 7-11. Formation Breakup 7-9 7-12. Formation Landing 7-11 7-13. Vertical Helicopter IFR Recovery Procedures 7-11

Section III TACTICAL FORMATION FLIGHT

7-14. Free-Cruise Technique 7-12 7-15. Movement Techniques 7-14 7-16. Crew Teamwork 7-15 7-17. Mixed Aircraft Formations 7-15

CHAPTER 8 FIXED-WING NIGHT FLYING

8-1. Preparation 8-1 8-2. Taxi, Takeoff, and Departure Climb 8-1 8-3. Orientation and Navigation 8-3 8-4. Approaches and Landings 8-4

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CHAPTER 9 DROP FLARE EMPLOYMENT

9-1. Target Identification 9-1 9-2. Description 9-1 9-3. Fuse Setting 9-2 9-4. Launch Procedures 9-4 9-5. Flight Pattern 9-5 9-6. Wind-Drift Correction 9-7 9-7. Linear Target Illumination 9-7 9-8. Safety Considerations 9-8 9-9. Training Program 9-9

APPENDIX A. ELECTROMAGNETIC SPECTRUM A-1 APPENDIX B. I2 SYSTEM COUNTERWEIGHTS B-1 APPENDIX C. PNVS FLIR C-1 APPENDIX D. TRAINING PROGRAMS D-1 GLOSSARY Glossary - 1 REFERENCES References - 1

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PREFACE

Night flight has assumed an increasingly important role in Army aviation. The Threat trains around the clock. To counter it, aviators must be able to conduct operations at night as well as during the day. Technological advances in night vision devices are enabling Army aviation to extend its operational capability to a 24 hour-a-day schedule. Ongoing improvements to these devices will further enhance aircrew performance during night operations. This publication provides aircrews a comprehensive document on night flight. It is intended to serve as a reference for night vision, unaided and aided night flight, and night vision training. The proponent of this publication is HQ TRADOC. Submit changes for improving this publication on DA Form 2028 (Recommended Changes to Publications and Blank Forms), and forward it through the aviation unit commander to Commander, US Army Aviation Center, ATTN: ATZQ-ATB-O, Fort Rucker, AL 36362-5218. The provisions of this publication are the subject of international agreements: STANAG 2999 (Edition One), Use of Helicopters in Land Operations STANAG 3627 (Edition One) and AIR STD 44/34B, Helicopter Day and Night Tactical Formation Flying AIR STD 44/33B, Helicopter Tactical or Non-Permanent Landing Sites Unless otherwise stated, whenever the masculine gender is used, both men and women are included. This publication has been reviewed for OPSEC considerations.

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CHAPTER 1

NIGHT VISION

Vision is the most important sense used in flight. During day or night, IMC or VMC, vision is the sense that makes crew members aware of the position of their aircraft in space. The eyes can rapidly identify and interpret visual cues during daylight. During darkness, however, visual acuity is decreased proportionally as the level of illumination decreases. Night vision devices improve the capability of the human eye to see at night. These devices include the AN/PVS-5 and AN/AVS-6, which are commonly referred to as I2 systems or I2 devices. Night vision devices also include thermal-imaging systems. This chapter provides a general discussion of night vision and scanning techniques. FM 1-301 contains additional information.

1-1. NIGHT VISION EVALUATION

a. During the initial flight physical examination, an aviator is interviewed to determine if he has difficulty seeing at night. If the inter-view indicates the aviator has adequate night vision, visual testing is not required.

b. The ability to conduct night flight safely is based on how well crew members can see at night and how well trained they are in using their night vision. Although the limits of night vision vary from person to person, most crew members never learn to use their night vision to its fullest capacity. A crew member with an average night vision capability who uses night vision techniques is more effective than a crew member with superior night vision who does not.

1-2. EYE ANATOMY AND PHYSIOLOGY

The eye is similar to a camera. The cornea, lens, and iris gather and control the amount of light allowed to enter the eye. The image is then focused on the retina. Functionally, the visual receptive apparatus (retina) has two types of cells: the cones and the rods. Vision is possible because of chemical reactions within these cells. Figure 1-1 shows the anatomy of the human eye.

a. Cones. Cone cells are used primarily for day or high-intensity light vision. The concentration of cones in the central retina (fovea centralis) permits high visual acuity in high illumination. The chemical iodopsin is always present in the cone cells. Regardless of the ambient light condition, this chemical is readily available so that the cones can immediately respond to visual stimulation.

b. Rods. The rods are used for night or low-intensity light vision. The peripheral retina is almost exclusively associated with rods. Peripheral vision is less precise than central vision, because the rods perceive only shades of gray and vague form or shape. Rhodopsin, commonly referred to as visual purple, is the photochemical found in rods. As the light level decreases, the amount of rhodopsin in the rods

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builds and the rods become more sensitive. Rods are about one-thousand times more sensitive to light than cones. When illumination decreases to about the level of full moonlight (0.1 footcandle), the rods take over from the cones. The period of highest light sensitivity usually occurs after 30 to 45 minutes in a dark environment. The rod cells may become up to 10,000 times more sensitive than at the start.

Figure 1-1. Anatomy of the eye

1-3. LIGHT LEVELS

Measuring light levels can be complex and confusing. Many different units of light measurement and terms are used for various scientific, engineering, and industrial applications. Terms of measurement are usually familiar only to those who work directly with light measurement problems. Some of the terms important to aircrews are defined in the paragraphs below.

a. Illumination. Illumination is the amount of light that strikes a surface at some distance from a source. The common unit of measurement is the footcandle. A footcandle is the density of light falling on the inner surface of a sphere of 1-foot radius when a point source of light with an intensity of one international candle is placed at the center of the sphere.

b. Luminance. Luminance is the amount of light per unit area reflected from or emitted by a surface. It is an important measurement for visual displays and is usually expressed in millilamberts or footlamberts. Luminance is frequently called brightness. However, brightness is influenced by contrast, adaptation, and such factors as the physical energy in the stimulus. Figure 1-2 shows examples of luminance levels found during commonly experienced conditions.

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Figure 1-2. Commonly experienced light levels

c. Reflectance. Reflectance is the relationship between illumination reaching a surface and the resulting luminance. A perfectly diffusing and reflecting surface is one that absorbs no light and scatters the illumination in the manner of a perfectly flat surface. Such a surface has a reflectance of 100 percent. If illuminated by 1 footcandle, it would have a luminance of 1 footlambert from all viewing angles. In actual practice, the maximum reflectance of a nearly perfectly diffusing surface is about 75 percent.

d. Contrast. Contrast is a measure of the difference in luminance between an object and its background. Contrast can vary from 100 percent (negative) to zero for objects darker than their backgrounds and from zero to infinity (positive) for objects brighter than their backgrounds. Contrast increases when the difference in luminance between an object and its back-ground increases. Contrast is zero when the luminance of an object and its background is the same.

1-4. VISION TYPES

The three types of vision are photopic, mesopic, and scotopic. Each type functions under different sensory stimuli or ambient light conditions. Night vision involves mesopic and scotopic vision. Photopic vision at night is possible only when sufficient levels of artificial illumination exist.

a. Photopic Vision. Photopic vision is experienced during daylight or when a high level of artificial illumination exists. The cones concentrated in the fovea centralis of the eye are primarily responsible for vision in bright light. Because of the high

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light level, rhodopsin is bleached out and rod cells become less effective. Sharp image interpretation and color vision are characteristic of photopic vision.

b. Mesopic Vision. Mesopic vision is experienced at dawn, at dusk, and during full moonlight. Vision is achieved by a combination of cones and rods. Visual acuity steadily decreases as available light decreases. Color perception changes because the cones become less effective. As cone sensi-tivity decreases, crew members should use off-center vision and proper scanning techniques to detect objects during low light levels.

c. Scotopic Vision. Scotopic vision is experienced under low light levels. Cones become ineffective, resulting in poor resolution of detail. Visual acuity decreases to 20/200 or less. This enables a person to see only objects the size of or larger than the big "E" on visual acuity testing charts from 20 feet away. (A person must stand at 20 feet to see what can normally be seen at 200 feet under daylight conditions.) Also, color perception is lost. A night blind spot in the central field of view appears at low light levels. The night blind spot occurs when cone-cell sensitivity is lost.

1-5. DAY VERSUS NIGHT VISION

Differences between day and night vision involve color, detail, and retinal sensitivity. Day vision is superior to night vision in every respect.

a. Color. One major difference between night vision and day vision is that color vision decreases or is lost at night. With decreasing light levels, the eyes shift from photopic vision (cones) to scotopic vision (rods). With this shift, the eyes become less sensitive to the red end of the spectrum and more sensitive to the blue part of the spectrum, as shown in Figure 1-3. Perception of colors is not possible with the rods. Colors of nonilluminated objects cannot be determined at night under very low illumination. Light and dark colors at night can be distinguished only by the intensity of reflected light. If, however, the brightness or intensity of a color is above the threshold for cone vision, the color can be perceived. This is why, for example, signal flares and runway markers can be properly identified at night.

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Figure 1-3. Photopic (cone) and scotopic (rod) sensitivity to various colors

b. Detail. Perception of fine detail is impossible at night. Under low illumination, visual acuity is greatly impaired. At 0.1 footcandle (the level of full moonlight), acuity is one-seventh as good as it is in average daylight. Therefore, objects must be large or nearby to be seen at night. Identification of objects at night is based on perceiving generalized contours and outlines, not on small distinguishing features.

c. Retinal Sensitivity.

(1) Another important distinction between night vision and day vision is the difference in the sensitivity of various parts of the retina. The central part of the retina is not sensitive to starlight illumination levels. During darkness or with low-level illumination, central vision becomes less effective and a night blind spot (5 to 10 wide) develops. This results from the concentration of cones in the fovea centralis and para fovea, the area immediately surrounding the fovea of the retina. The central field of vision for each eye is superimposed for binocular vision. Because the night blind spot for each eye occurs in the central field of view, binocular vision cannot compensate for the night blind spot. Therefore, an object viewed directly may not be detected, as shown in Figure 1-4.

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Figure 1-4. Night blind spot

(2) The night blind spot should not be confused with the physiological blind spot (the so-called day blind spot) caused by the optic disk. The physiological blind spot is present all the time, not only during the day. This blind spot results from the position of the optic disk on the retina. The optic disk has no light-sensitive receptors. The physiological blind spot covers an area of approximately 5.5 by 7.5 and is located about 15 from the fovea. Because of the overlap of binocular vision, this blind spot is normally not noticed unless one eye is not used. The physiological blind spot becomes an important consideration when monocular night vision devices, such as the PNVS, are used.

(3) Because of the night blind spot, larger and larger objects will be missed as distance increases. To see things clearly at night, an individual must use off-center vision and proper scanning techniques. Figure 1-5 shows the effect of distance on the night blind spot.

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Figure 1-5. Effect of distance on the night blind spot

1-6. VISUAL PROBLEMS

Several visual problems or conditions affect night vision. These include presbyopia, night myopia, and astigmatism.

a. Presbyopia. This condition is part of the normal aging process, which causes the lens of the eye to harden. Beginning in the early teen years, individuals gradually lose accommodation; that is, the ability to focus on nearby objects. When individuals are about 40 years old, their eyes are unable to reliably focus at the normal reading distance without reading glasses. As presbyopia worsens, instruments, maps, and checklists become more difficult to read, especially with red illumination. This difficulty can be corrected with certain types of bifocal spectacles that compensate for the inadequate accommodative power of the eye lenses.

b. Night Myopia. Myopic individuals do not see distant objects clearly; only nearby objects are in focus for them. At night, blue wavelengths of light prevail in the visible portion of the spectrum. Because of this, slightly nearsighted (myopic) individuals will experience visual difficulty at night when viewing blue-green light that could cause blurred vision. Also, image sharpness decreases as pupil diameter increases. For individuals with mild refractive errors, vision may become unacceptably blurred unless corrective glasses are worn. Another factor to consider is "dark focus." When luminance levels decrease, the focusing mechanism of the eye may move toward a resting position and make the eye more myopic. These factors are more important when the aircrew looks outside the cockpit during unaided night flight. Special corrective lenses can be prescribed to correct for myopia.

c. Astigmatism. Astigmatism is an irregularity of the shape of the cornea that may cause an out-of-focus condition. If, for example, an astigmatic person focuses on

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power poles (vertical), the wires (horizontal) will be out of focus in most cases. If the astigmatism is 1.00-diopter or greater, the aviator must be individually evaluated before flying with I2 devices that preclude the wearing of eyeglasses. An example is full-faceplate devices used with daylight filters.

1-7. DARK ADAPTATION

Going suddenly from bright light into darkness is a common occurrence. For example, people experience this when they enter a movie theater during the day or leave a brightly lit room at night. At first they see very little, if anything. After several minutes, they can see dim forms and large outlines. As time goes by, more details of the environment become apparent as further dark adaptation occurs.

a. Dark adaptation is the process by which the eyes increase their sensitivity to low levels of illumination. Individuals dark-adapt to varying degrees and at different rates. During the first 30 minutes, the sensitivity of the eye increases roughly ten-thousandfold. Very little increase in sensitivity occurs after that time.

b. The lower the starting level of illumination, the more rapid complete dark adaptation is achieved. For example, less time is required to dark-adapt completely after leaving a darkened theater than after leaving a brightly lit hangar.

c. Dark adaptation for optimum night visual acuity approaches its maximum level in about 30 to 45 minutes under minimal light conditions. If the dark-adapted eye is exposed to a bright light, the sensitivity of that eye is temporarily impaired. The amount of impairment depends on the intensity and duration of the exposure. Brief flashes from a white (xenon) strobe light, commonly found on aircraft, have little effect on night vision because the pulses of energy are so short. On the other hand, exposure to a flare, a searchlight beam, or lightning may seriously impair night vision. In such cases, the recovery of a previous maximum level of dark adaptation can take from 5 to 45 minutes in continued darkness.

d. Night vision devices affect dark adaptation. If a previously dark-adapted crew member wearing an I2 device removes the device in a darkened environment, a 30-minute dark adaptation level can be regained in about two to three minutes. No dark adaptation period is necessary before using the I2 device. Vision with I2 devices is primarily photopic, but the low light levels produced by I2 devices do not fully bleach out rhodopsin. Use of the device does not seriously degrade dark adaptation.

1-8. NIGHT VISION PROTECTION

Night vision should be protected when possible. Some of the steps crew members can take to protect their night vision are described below.

a. Equipment.

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(1) Sunglasses. Repeated exposure to bright sunlight has an increasingly adverse effect on dark adaptation. This effect is intensified by reflective surfaces such as sand and snow. Exposure to intense sunlight for two to five hours decreases scotopic visual sensitivity for as long as five hours. Also, a decrease occurs in the rate of dark adaptation and degree of night visual capacity. These effects are cumulative and may persist for several days. If a night flight is scheduled, crew members should wear military neutral density (N-15) sunglasses or equivalent filter lenses when exposed to bright sunlight. This precaution will increase the rate of dark adaptation at night and improve night visual sensitivity.

(2) Oxygen supply. Unaided night vision depends on optimum function and sensitivity of the rods of the retina. Lack of oxygen to the rods (hypoxia) significantly reduces their sensitivity. This increases the time required for dark adaptation and decreases the ability to see at night. Without supplemental oxygen, an individual's night vision declines measurably at pressure altitudes above 4,000 feet. Because I2 device output is photopic and central vision is the last to be degraded by a lack of oxygen, aided night vision is not significantly affected. At night, aviators should use oxygen, if available, when operating unaided above a pressure altitude of 4,000 feet.

b. Precautions.

(1) Airfield lighting. At a fixed airfield, light sources that may impair the aircrew's dark adaptation should be eliminated. Aircraft scheduled for night flight should be positioned, if possible, on a part of the airfield where the least amount of light exists. Maintenance and service crews should practice light discipline. Hover lanes should be established and marked with minimal appropriate lighting. This will preclude the use of the landing light or searchlight during hover operations. Airfield lighting should be reduced to the lowest intensity. The aviator should select departure routes that avoid highways and residential areas where artificial illumination can impair night vision. Runway and takeoff-pad lights, when practical, should be reduced for departing traffic.

(2) High-intensity lighting. During night missions, aircrews may be exposed to high-intensity lighting such as city lights, flares, searchlights, lightning, and artillery flashes. These may cause a total or a partial loss of night vision. If a flash of high-intensity light is expected from a specific direction, the aviator should turn the aircraft away from the light source. When such a condition occurs unexpectedly and direct view cannot be avoided, a crew member can preserve his dark adaptation by shutting one eye and using the other to observe. Once the light source is no longer visible, the eye that was closed can provide the required night vision. This is possible because dark adaptation occurs independently in each eye.

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However, it should be remembered that problems with depth perception can occur when the aviator views with one dark-adapted eye. This is particularly true when an aviator hovers near terrain obstacles or makes an approach. Techniques to counter expected light conditions are discussed below.

(a) Lights in built-up areas. The aviator should select flight routes that avoid built-up areas which may have heavy concentrations of light. If this condition is inadvertently encountered, the aviator should alter the flight route to avoid overflying the brightly lit area. A decrease in dark adaptation from a single light source, such as a farmhouse or an automobile, can be minimized by looking away from the light.

(b) Flares. When a flare is used to illuminate the viewing area or is inadvertently detonated nearby, the aviator should maneuver to a position along the edge of the illuminated area. This procedure reduces exposure to the light source.

(c) Weapon flashes. To reduce the effect of weapon flashes from aerial weapon systems, the aviator should limit the time during which the ordnance is expended. Rockets can be fired in almost any combination without seriously impairing night vision. When firing automatic weapons, the aviator should use short bursts of fire. Closing an eye or looking away from the firing will also minimize the loss of dark adaptation.

1-9. SELF-IMPOSED STRESS

Night flight is more fatiguing and stressful than day flight. Many self- imposed stressors limit night vision. Crew members can control this type of stress. The factors that cause self-imposed stress are discussed below; crew members can remember them by the acronym DEATH.

a. Drugs. Drugs can seriously degrade visual acuity during the day and especially at night. A crew member who becomes ill should consult a flight surgeon.

b. Exhaustion. If crew members become fatigued during a night flight, they will not be mentally alert. Exhaustion causes crew members to respond more slowly, even in situations requiring immediate reaction. Exhausted crew members tend to concentrate on one aspect of a situation without considering the total requirement. Their performance may become a safety hazard, depending on the degree of fatigue. Rather than use proper scanning techniques, they are prone to stare.

(1) Illness. Increased temperature and a feeling of unpleasantness usually are associated with illness. High body temperatures consume a higher than

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normal rate of oxygen. As a result, hypoxia may be induced and night vision may be degraded. In addition, the unpleasant feeling associated with sickness distracts a crew member's attention and degrades his ability to concentrate on night flying requirements.

(2) Poor physical conditioning. To overcome this limitation, crew members should participate in regular exercise programs. Crew members who are physically fit become less fatigued during flight and have better night scanning efficiency. However, too much exercise in a given day may leave crew members too fatigued for night flying.

(3) Inadequate rest. Adequate rest and sleep are important before flying. Commanders should refer to the crew endurance scheduling guide in AR 95-1 when developing crew work and rest schedules.

c. Alcohol. Alcohol is a sedative. Its use impairs both coordination and judgment. As a result, crew members impaired by alcohol fail to apply the proper techniques of night vision. They are likely to stare at objects and to neglect scanning techniques. The amount of alcohol consumed determines the degree to which night vision is affected. The effects of alcohol are long-lasting; hangovers also impair visual scanning efficiency.

d. Tobacco. Of all the self-imposed stressors, cigarette smoking most decreases visual sensitivity at night. Smoking significantly increases the amount of carbon monoxide carried by the hemoglobin in red blood cells. This reduces the blood's capacity to combine with oxygen so less oxygen is carried in the blood. Hypoxia caused by carbon monoxide poisoning affects peripheral vision and dark adaptation. The results are the same as those for hypoxia caused by high altitude. Smoking 3 cigarettes in rapid succession or 20 to 30 cigarettes within a 24-hour period may saturate from 8 to 10 percent of the capacity of hemoglobin. Smokers lose 20 percent of their night vision capability at sea level. This equals a physiological altitude of 5,000 feet.

e. Hypoglycemia and Nutritional Deficiency.

(1) Hypoglycemia. Missing or postponing meals can cause low blood sugar, which impairs night flight performance. Low blood sugar levels may result in stomach contractions, distraction, a breakdown in habit pattern, a shortened attention span, and other physiological changes.

(2) Vitamin A deficiency. Insufficient consumption of vitamin A may impair night vision. Foods high in vitamin A include eggs, butter, cheese, liver, apricots, peaches, carrots, squash, spinach, peas, and most types of greens. A balanced diet usually provides enough vitamin A. Excessive quantities of vitamin A will not improve night vision and may be harmful.

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1-10. SCANNING TECHNIQUES

Dark adaptation is only the first step toward increasing aircrews' ability to see at night. Applying night vision techniques will enable aircrews to overcome many of the physiological limitations of their eyes. Because the fovea centralis is automatically directed toward an object by a visual fixation reflex, scanning techniques require considerable practice and concerted effort on the part of the viewer.

a. Scanning. Scanning techniques are important in identifying objects at night. To scan effectively, crew members look from right to left or left to right. They should begin scanning at the greatest distance an object can be perceived (top) and move inward toward the position of the aircraft (bottom). This scanning pattern is shown in Figure 1-6. Because the light-sensitive elements of the retina cannot perceive images that are in motion, a stop-turn-stop-turn motion should be used. For each stop, an area approximately 30 wide should be scanned. This viewing angle will include an area approximately 250 meters wide at a distance of 500 meters. The duration of each stop is based on the degree of detail that is required, but no stop should last longer than two to three seconds. When moving from one viewing point to the next, crew members should overlap the previous field of view by 10 . Other scanning techniques, such as the ones illustrated in Figure 1-7, may be used if appropriate to the situation.

Figure 1-6. Scanning pattern

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Figure 1-7. Alternate scanning pattern

b. Off-Center Viewing.

(1) Viewing an object using central vision during daylight poses no limitation. If this same technique is used at night, however, the object may not be seen because of the night blind spot that exists during low illumination. To compensate for this limitation, crew members must use off-center vision. This technique requires that an object be viewed by looking 10 above, below, or to either side of the object. In this manner, the peripheral vision can maintain contact with an object. Figure 1-8 illustrates an example of the off-center viewing technique.

(2) The technique of off-center vision applies only to the surveillance of targets that are minimally illuminated or luminous. Under these conditions, cone vision is not stimulated. Central vision is best used when an object or a target is bright enough to stimulate the cones and needs to be seen with considerable detail. When the object or target begins to fade, it should be redetected using off-center vision and retained until central vision recovers sufficiently to permit further observation.

(3) With off-center vision, the images of an object viewed longer than two to three seconds will disappear. This occurs because the rods reach a photochemical equilibrium that prevents any further response until the scene changes. This produces a potentially unsafe operating condition. To overcome this night vision limitation, crew members must be aware of the phenomenon and avoid viewing an object for longer than two or three seconds. The peripheral field of vision will continue to pick up the object when the eyes are shifted from one off-center point to another.

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Figure 1-8. Off-center viewing technique

1-11. DISTANCE ESTIMATION AND DEPTH PERCEPTION

Distance estimation and depth perception cues are easily recognized when crew members use central vision under good illumination. As the light level decreases, the ability to judge distances accurately is degraded and visual illusions become more common. A knowledge of distance estimation and depth perception mechanisms and cues will assist crew members in judging distances at night. These cues may be monocular or binocular. Monocular cues are more important for crew members than binocular cues.

a. Monocular Cues. The monocular cues that aid in distance estimation and depth perception include motion parallax, geometric perspective, retinal image size, and aerial perspective.

(1) Motion parallax. This cue to depth perception is a means of judging distances under reduced illumination. Motion parallax refers to the apparent motion of stationary objects as viewed by an observer moving across the landscape. When the crew member looks outside the aircraft, perpendicular to the direction of travel, near objects appear to move backward, past, or opposite the path of motion. Far objects seem to move in the direction of motion or remain fixed. The rate of apparent movement depends on the distance the observer is from the object. For example, as an aviator flies low level, objects near the aircraft will appear to rush past the aircraft while a mountain range near the horizon will appear stationary. As the aviator flies across a power line that extends to the horizon, that part of the power line near the aircraft will appear to move swiftly, opposite the path of motion. Toward the horizon, the same power line will

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appear fixed. Objects that appear to be fixed or moving slowly are judged to be a greater distance from the aviator than objects that appear to be moving swiftly.

(2) Geometric perspective. An object may appear to have a different shape when viewed at varying distances and from different angles. Geometric perspective cues include linear perspective, apparent foreshortening, and vertical position in the field. They are illustrated in Figure 1-9.

(a) Linear perspective. Parallel lines, such as runway lights, tend to converge as distance from the observer increases. This is illustrated in part A of Figure 1-9.

(b) Apparent foreshortening. The true shape of an object or a terrain feature appears elliptical when viewed from a distance. As the distance to the object or the terrain feature decreases, the apparent perspective changes to its true shape or form. Part B of Figure 1-9 illustrates how the shape of a body of water changes when viewed at different distances at the same altitude.

(c) Vertical position in the field. Objects or terrain features farther away from the observer appear higher on the horizon than those closer to the observer. The higher vehicle in part C of Figure 1-9 appears to be closer to the top and, thus, at the greater distance from the observer. At night, crew members can mistake lights on elevated structures or lights on low-flying aircraft for distant ground structures because of the lights' higher vertical position in the field.

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Figure 1-9. Geometric perspective

(3) Retinal image size. The brain perceives the actual size of an object from the size of an image focused on the retina. Four factors are considered in determining distance using the retinal image. They are known size of objects, increasing or decreasing size of objects, terrestrial associations, and overlapping contours or interposition of objects.

(a) Known size of objects. The nearer an object is to the observer, the larger its retinal image. By experience, the brain learns to estimate the distance of familiar objects by the size of their retinal images. Figure 1-10 shows how this process works. A structure projects a specific angle on the retina based on its distance from the observer. If the angle is small, the observer judges the structure to be at a great distance. A larger angle indicates to the observer that the structure is close. To use this cue, the observer must know the actual size of the object. If the observer is not familiar with the object, its distance would be determined primarily by motion parallax.

Figure 1-10. Known size of objects

(b) Increasing or decreasing size of objects. If the retinal image size of an object increases, the relative distance is decreasing. If the image size decreases, the relative distance is increasing. If the image size is constant, the object is at a fixed relative distance.

(c) Terrestrial associations. Comparing an object, such as an airfield, with an object of known size, such as a helicopter, helps to deter- mine the object's size and apparent distance from the observer. Objects ordinarily associated together are judged to be at

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about the same distance. For example, a helicopter observed near an airport is judged to be in the traffic pattern and, therefore, at about the same distance as the airfield. Figure 1-11 illustrates terrestrial association.

Figure 1-11. Terrestrial association

(d) Overlapping contours or interposition of objects. When objects overlap, the overlapped object is farther away, as illustrated in Figure 1-12. This overlapping is especially important to consider at night during a landing approach. Lights disappearing or flickering in the landing area indicate barriers between the landing area and the aircraft. The flight path should be adjusted accordingly.

Figure 1-12. Overlapping contour

(4) Aerial perspective. The clarity of an object and the shadow cast by it are perceived by the brain and are cues for estimating distance. Several aerial perspective factors are used to determine distance.

(a) Variations in color or shade. Subtle variations in color or shade are clearer the closer the observer is to an object. However, as distance increases, these distinctions blur. For example, the side of

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a hill from a distance will appear to be a uniform shade with no distinguishable shape. As the aircrew flies closer to the hill, the shades produced by individual trees and the spaces in between those trees become noticeable. Thus under high light levels at night, color or shade can provide cues for distance estimation.

(b) Loss of detail or texture. As a person gets farther from an object, discrete details become less apparent. For example, when a cornfield becomes a solid color and the leaves and branches of a tree become a solid mass, the objects are judged to be far away. Because reduced illumination also decreases resolution, these cues will disappear shortly after sunset or be limited to close viewing distances.

(c) Position of light source and direction of shadow. Every object will cast a shadow from a light source. The direction in which the shadow is cast depends on the position of the light source. If the shadow of an object is toward the observer, the object is closer than the light source is to the observer. Figure 1-13 illustrates light and shadows.

Figure 1-13. Light and shadows

b. Binocular Cues. Binocular cues depend on the slightly different view each eye has of an object. Consequently, binocular perception is useful only when the object is close enough to make an obvious difference in the viewing angle of both eyes. In the flight environment, most distances outside the cockpit are so great that binocular cues are of little, if any, value. In addition, binocular cues operate on a more subconscious level than monocular cues and are performed automatically.

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1-12. VISUAL ILLUSIONS

Decreasing visual information increases the probability of spatial disorientation. Reduced visual references also create several illusions that can induce spatial disorientation. Many types of visual illusions can occur in the aviation environment. Included among them are autokinesis, ground light misinterpretation, relative motion, reversible perspective illusion, false horizons, altered reference planes, and height perception illusion. Others include flicker vertigo, fascination (fixation), structural illusions, and size-distance illusion.

a. Autokinesis. When a static light is stared at in the dark, the light appears to move, as shown in Figure 1-14. This phenomenon can be readily demonstrated by staring at a lighted cigarette in a dark room. Apparent movement will begin in about 8 to 10 seconds. Although the cause of autokinesis is not known, it appears to be related to the loss of surrounding references that normally serve to stabilize visual perceptions. This illusion can be eliminated or reduced by visual scanning, by increasing the number of lights, or by varying the light intensity. The most important of the three solutions is visual scanning. A light or lights should not be stared at for more than 10 seconds. This illusion is not limited to light in darkness. It can occur whenever a small, bright, still object is stared at against a dull dark or nondescript background. Similarly, it can occur when a small, dark, still object is viewed against a light, structureless environment. Anytime visual references are not available, aircrews are subject to this illusion.

b. Ground Light Misinterpretation. A common occurrence is to confuse ground lights with stars. When this happens, aviators unknowingly position aircraft in unusual attitudes to keep the ground lights--believed to be stars--above them. For example, some aviators have mistaken the lights along a seashore for the horizon and have maneuvered their aircraft dangerously close to the sea; they believed they were flying straight and level. Aviators have also confused certain geometric patterns of ground lights. For example, aviators have identified moving trains as landing zone lights and have been badly shaken by their near misses. To avoid these problems, aviators should cross-check aircraft instruments. Also, position lights of other aircraft in formation can be mistaken for ground lights and might be lost against the horizon when another aircraft is at or below the altitude of the observer. Figure 1-15 illustrates ground light and skylight illusion.

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Figure 1-14. Autokinetic illusion

Figure 1-15. Ground light and skylight illusion

c. Relative Motion. The illusion of relative motion can be illustrated by an example. An aviator hovers an aircraft and waits for hover taxi instructions. Another aircraft hovers alongside. As the other aircraft is picked up in the first aviator's peripheral vision, the aviator senses movement in the opposite direction. This illusion may be encountered during multihelicopter operations. Aircrews may mistake the motion of another aircraft for that of their own. The only way to correct for this illusion is to have sufficient experience to understand that such illusions do occur and to not react to them on the controls. The use of proper scanning techniques can help prevent this illusion.

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d. Reversible Perspective Illusion. At night, an aircraft may appear to be going away when it is, in fact, approaching a second aircraft. This illusion often occurs when an aircraft is flying parallel to another's course. To determine the direction of flight, aircrews should observe aircraft lights and their relative position to the horizon. If the intensity of the lights increases, the aircraft is approaching. If the lights dim, the aircraft is moving away. Also, remembering the "3 Rs" will help identify the direction of travel when other aircraft are encountered. If the red aircraft position lights are on the right, the aircraft is returning (coming toward the observer).

e. False Horizons. Cloud formations may be confused with the horizon or the ground. Momentary confusion may result when the aviator looks up after having given prolonged attention to a task in the cockpit. Because outside references for attitude are less obvious and reliable at night, aviators should rely less on them during night flight. Using instrument cross-checks can help prevent this situation. While hovering over terrain that is not perfectly level, aviators might mistake the sloped ground in front of the aircraft for the horizon and cause the aircraft to drift while trying to maintain a stationary position. Figure 1-16 illustrates false horizon illusion.

f. Altered Reference Planes. When approaching a line of mountains or clouds, aviators may feel that they need to climb even though their altitude is adequate. Also, when flying parallel to a line of clouds, aviators may tend to tilt the aircraft away from the clouds.

g. Height Perception Illusion. When flying over desert, snow, water, or other areas of poor contrast, crew members may experience the illusion of being higher above the terrain than they actually are. This is due to the lack of visual references. This illusion may be overcome by dropping an object, such as a chemical light stick or flare, on the ground before landing. Another technique to overcome this illusion is to monitor the shadows cast by near objects, such as the landing gear, or skid shadows at a hover. Flight in an area where visibility is restricted by haze, smoke, or fog produces the same illusion.

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Figure 1-16. False horizon illusion

h. Flicker Vertigo. Much time and research have been devoted to the study of flicker vertigo. A light flickering at a rate between 4 and 20 cycles per second can produce unpleasant and dangerous reactions. Such conditions as nausea, vomiting, and vertigo may occur. On rare occasions, convulsions and unconsciousness may also occur. Fatigue, frustration, and boredom tend to intensify these reactions. During the day, the problem can be caused by sunlight flickering through rotor blades or propellers. At night, it can also be caused by an anticollision light reflecting against an overcast sky, haze, or the rotor system. This can be corrected by turning the anticollision light off.

i. Fascination (Fixation). This illusion occurs when aviators ignore orientation cues and fix their attention on a goal or an object. This is dangerous because aircraft ground-closure rates are difficult to determine at night; normal daylight peripheral movement is reduced or absent. Target hypnosis is a common type of fascination. For example, an aviator intent on hitting a target during a gunnery run may delay pull-up so long that the aircraft contacts the ground. Preventing this illusion requires increased scanning by the aviator.

j. Structural Illusions. Structural illusions are caused by heat waves, rain, snow, sleet, or other factors that obscure vision. For example, a straight line may appear to be curved when seen through a desert heat wave or a wing-tip light may appear to double or move when viewed during a rain shower.

k. Size-Distance Illusion. This illusion results from viewing a source of light that is increasing or decreasing in luminance (brightness). The aviator may interpret the light as approaching or retreating. For example, when an aviator, hovering near a second aircraft, changes the position lights from DIM to BRIGHT, the other aircraft may appear to jump toward him.

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1-13. AIRCRAFT DESIGN LIMITATIONS

The design of Army aircraft may degrade a crew member's ability to see outside the aircraft. To minimize the loss of night vision because of aircraft design shortcomings, an aircrew must properly prepare the aircraft for night flight. Consideration should be given to the aircraft limitations discussed below.

a. Windscreens reduce the ability to see outside the aircraft. Dirt, grease, and bugs must be removed from the windscreen before each night flight.

b. Aircraft instruments are easier to read under high levels of instrument illumination. However, the level of illumination needed for optimum reading interferes with maximum dark adaptation for viewing dim objects outside the aircraft.

c. Interior lights also interfere with dark adaptation. They reflect off the windscreen and reduce outside visibility. Also, interior lights may be detected by the Threat. To reduce the adverse effects of cockpit lights, the aviator should turn off nonessential lights and keep the intensity of essential lights at the lowest usable level.

d. Exterior lights are used to identify the aircraft. During aided terrain flight, the illumination from these lights may degrade the operation of the I2 system. To reduce the adverse effect of exterior lights, the aviator should turn off all lights not required by regulations. The remaining lights should be operated in the DIM mode or properly taped or painted.

1-14. NERVE AGENTS AND NIGHT VISION (MIOSIS)

Night vision is adversely affected when eyes are exposed to minute amounts of nerve agents. When direct contact occurs, the pupils constrict (miosis) and do not dilate in low ambient light. The available automatic chemical alarms are not sensitive enough to detect the low concentrations of nerve agent vapor that can cause miosis.

a. Exposure Time. The exposure time required to cause miosis depends on the concentration of the agent. Miosis may occur gradually as eyes are exposed to low concentrations over a long period of time. On the other hand, exposure to a high concentration can cause miosis during the few seconds it takes to put on a protective mask. Repeated exposures over a period of days are cumulative.

b. Symptoms.

(1) The symptoms of miosis range from minimal to severe, depending on the dosage to the eye. Severe miosis, with the resulting reduced ability to see in low ambient light, persists for about 48 hours after onset. The pupil

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gradually returns to normal over several days. Full recovery may take up to 20 days. Repeated exposures within the affected time are cumulative.

(2) The onset of miosis is insidious because it is not always immediately painful. Miotic persons may not realize their condition even when they carry out tasks that require vision in low ambient light. If the unit is attacked by nerve agents, especially the more persistent types, commanders should assume that personnel otherwise fit for duty will experience some loss of night vision. No effective drug is available to remedy the effects of miosis without causing other visual problems that may be just as severe.

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CHAPTER 2

AVIATION NIGHT VISION DEVICES

The two types of aviation night vision devices are image-intensifier systems and thermal-imaging systems. I2 systems amplify both visible and near infrared light energy. They greatly improve night vision, but they require some degree of light to function. I2 systems also do not work well under very low ambient light and adverse weather conditions. The most recent advance in night vision devices is thermal imaging, which detects infrared energy. Thermal-imaging systems detect heat radiated by objects and do not need light to function. They are less affected by weather conditions than I2 systems. Thus far, only the AH-64 has a thermal-imaging pilotage system, the PNVS. Appendix A discusses the portions of the electromagnetic spectrum that are sensed by image-intensifier systems and thermal-imaging systems.

Section I IMAGE-INTENSIFIER SYSTEMS

2-1. DEVELOPMENT HISTORY

a. A type of goggles for aviators to use during night helicopter operations was first demonstrated in 1969. Because Army tactical doctrine at that time did not require low-level or NOE night flight, I2 system development stopped. In 1971, however, the Army reevaluated tactical helicopter employment and determined that NOE operations were necessary at night as well as during the day. As a result, the AN/PVS-5 was adopted as an interim pilot's night vision system because it was a significant improvement over unaided night vision.

b. The AN/PVS-5 series contains second-generation image-intensifier tubes. As use of the AN/PVS-5 expanded, the major limitation of the full faceplate--lack of peripheral vision--became apparent. Subsequently, the faceplate was modified (cut away) to provide peripheral vision. The AN/AVS-6 was developed to overcome or reduce the limitations of the AN/PVS-5 series. The AN/AVS-6 uses third-generation image-intensifier tubes to increase light amplification approximately two times more than the AN/PVS-5 series. The AN/AVS-6 also increases peripheral vision and has a flip-up feature. Both the AN/PVS-5 series and the AN/AVS-6 are being used. Figure 2-1 shows a comparison of peripheral vision for the various models.

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Figure 2-1. Peripheral vision comparison of various I2 models

2-2. OPERATIONAL THEORY

An image intensifier is an electronic device that amplifies light energy. The light enters into the I2 device and is focused by the objective lens onto a photocathode that is receptive to both visible and near infrared radiation. Figure 2-2 illustrates the operation of an I2 device. The photons of light striking the photocathode cause a release of electrons proportionate in number to the amount of light projected through the lens. In turn, the released electrons are accelerated away from the photocathode surface by an electrical field that is produced by the device's power source. The amount of light produced by the I2 tube is proportional to both the number and the velocity of electrons that strike the phosphor screen. The number of electrons striking the phosphor screen is increased by means of the microchannel plate, which is a thin wafer of tiny glass tubes. The glass tubes are tilted in the microchannel plate approximately 8 . Electrons enter these tubes and strike the walls of the tubes. As each electron strikes a wall, more electrons are emitted. Each of these emitted electrons strikes the wall again, producing even more electrons. The accelerated electrons are directed through the microchannel plate and against a phosphor screen placed on a flat plate opposite and parallel to the photocathode surface. The phosphor screen emits an amount of light proportional to the number and velocity of the electrons that strike it. Voltage is applied between the photocathode and the phosphor screen. This accelerates the electrons, "brightening" the projected scene.

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Thus the picture delivered to the user is converted from a small amount of light to accelerated electrons and back to an amplified amount of light.

Figure 2-2. Image intensifier

2-3. AN/PVS-5 SERIES

a. Description.

(1) The AN/PVS-5 series is a self-contained, binocular image-intensifier viewing system with no magnification. The device is passive in normal operation. It measures about 6.5-inches square, weighs 30 ounces, and has a 40 FOV. Power is supplied by a 2.7-volt DC mercury battery or a 3.0-volt DC lithium battery or two 1.5-volt AA alkaline batteries. The device operates satisfactorily between 2.5 volts and 3.4 volts. The full-faceplate version of the AN/PVS-5 completely surrounds the wearer's eyes. Only through-the-tube viewing is possible, and eyeglasses cannot be worn. Under ideal conditions, visual acuity of 20/200 or less unaided can be improved to 20/50 with the AN/PVS-5 series. Figure 2-3 shows the AN/PVS-5 with full faceplate.

NOTE: Flying with full faceplate I2 systems is permitted only during the daytime with daylight filters and with the second crew member unaided.

(2) The weight of I2 devices shifts the CG of the head and helmet system forward. This out-of-balance condition can be relieved by attaching a counterweight to the back of the helmet. Appendix B discusses I2 system counterweights.

(3) The AN/PVS-5 series operates by intensifying ambient light 750 to 1,500 times. This is sufficient to provide good imagery under full moonlight conditions down to marginal imagery during quarter moonlight conditions. Below quarter moonlight conditions, artificial illumination (usually infrared) may be required to light the helicopter's flight path.

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Figure 2-3. AN/PVS-5 series

b. Improvements and Modifications.

(1) AN/PVS-5A. This model has the same image intensifier as the AN/PVS-5 but is constructed differently. The tubes are not interchangeable. The ON-OFF switch on the AN/PVS-5A has a lift requirement to turn on the infrared illuminator. All other features and parts are the same.

(2) Modified faceplate. The faceplate of the AN/PVS-5A was modified for aviation use by the US Army Aeromedical Research Laboratory. The MFP was an interim quick-fix for the AN/PVS-5A to make it safer for crew members to use. Figure 2-4 shows the AN/PVS-5A with the MFP.

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Figure 2-4. Modified faceplate, AN/PVS-5A

(a) The lower portion of the faceplate was removed and its electrical components were relocated to the top. The MFP enables the user to view around the goggles. This permits an aviator to view the cockpit, read maps, and discern the color of aircraft and ground lights. Eyeglasses may be worn with the MFP.

(b) The MFP is mounted between the front of the helmet and the visor cover. It is held in place by the standard vee and side straps. A side strap of surgical tubing can also be used.

(3) GX-5 flip-up modification. The GX-5 uses a pivoting hinge that locks the device in a stowed position or lowers it for use. The hinge is hard-mounted to a section of the visor cover and attached to the helmet visor cover by Velcro. A new frame holds the I2 tubes and attaches to the lower portion of the hinge by a bolt-and-nut assembly. This frame increases peripheral unaided vision. The GX-5 flip-up modification requires the dual-battery pack for power and ON-OFF switching. Figure 2-5 illustrates the GX-5 flip-up modification.

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Figure 2-5. GX-5 flip-up modification, AN/PVS-5A

(4) AN/PVS-5B/C. The AN/PVS-5B/C uses the second-generation intensifier tube of the AN/PVS-5A but with improved objective lenses that gather more light. This results in a substantial improvement in low light flight capability. The AN/PVS-5B/C with the full faceplate is not employed in Army aviation.

WARNING

The AN/PVS-5B/C is designed for ground use only when the tube assemblies are in the original full faceplate. A feature in the faceplate automatically cuts off power if the tubes are exposed to continuous high light levels for more than one minute. Aviation personnel are not permitted to modify the AN/PVS-5B/C faceplate.

c. Flight Helmet Attachment.

(1) A properly fitted flight helmet is essential for comfort and to lessen fatigue. For the same reason, the flight helmet must be correctly modified for attachment of the full-faceplate AN/PVS-5A, MFP, or GX-5 modification. Properly adjusted helmet headbands and nape straps will help prevent the helmet from rotating forward and down during I2 device use.

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(2) Helmet straps are supplied with each device. The aviation helmet kit (vee straps and side straps) provides four attaching points (two male snaps and two 2-inch Velcro strips) that must be attached to the helmet. Instructions for preparing the helmet are described in TM 11-5855-238-10. The SPH-4 helmet is not configured for external attachment of the AN/PVS-5 series without modification.

2-4. AN/AVS-6

a. The AN/AVS-6 is a helmet-mounted, light-intensification device. Figure 2-6 shows the AN/AVS-6. The AN/AVS-6 allows aircrews to conduct operations at terrain flight altitudes during low ambient light levels, to include overcast conditions. It has the same 40 FOV as the AN/PVS-5 series. Under ideal conditions, visual acuity of 20/200 or less can be improved to 20/40 with the AN/AVS-6. Figure 2-7 illustrates the superiority of the AN/AVS-6 over the AN/PVS-5 series because of its improved sensitivity in the red and near infrared region of the spectrum. Light in this portion of the spectrum predominates at night, as shown in Figure 2-8. Not only does the AN/AVS-6 provide greater light amplification but it also amplifies light in that portion of the spectrum that is most predominant at night.

NOTE: The small peak in the blue-green portion of the spectrum in Figure 2-8 is caused by moon illumination. The size of this peak will vary based on the amount of moon illumination.

b. The AN/AVS-6 operates by intensifying ambient light 2,000 to 3,500 times. It can provide sufficient imagery from overcast starlight to moonlight conditions. However, below quarter moonlight conditions, artificial illumination (usually infrared) may be required to light the helicopter's flight path.

Figure 2-6. AN/AVS-6

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Figure 2-7. Relative sensitivity of AN/PVS-5 and AN/AVS-6 systems

Figure 2-8. Spectral distribution of starlight

c. The AN/AVS-6 comes in two versions: AN/AVS-6(V)1 and AN/AVS-6(V)2. The AN/AVS-6(V)1 mounts directly to the standard SPH-4 flight helmet. A special mount and offset binocular, the AN/AVS-6(V)2, is available for SPH-4 helmets modified with a helmet-mounted sight. The AN/AVS-6 is powered by batteries or aircraft interface and has a 30-minute, low-voltage warning indicator. The warning indicator is a dim red light emitted between the binoculars above the FOV when 30 minutes or less of battery life remains.

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d. The AN/AVS-6 is stowable on the helmet in a flipped-up position, which automatically cuts off power to the tubes. Figure 2-9 shows the AN/AVS-6 in the stowed position. The AN/AVS-6 is significantly lighter than the AN/PVS-5 series and has a breakaway feature designed to separate the binocular from the helmet mount under crash loads. The AN/AVS-6 also has an improved unaided peripheral view. It incorporates a minus-blue filter that makes the system insensitive to blue-green cockpit lights and their reflections in the cockpit.

Figure 2-9. AN/AVS-6 in stowed position

2-5. ADJUSTMENT TECHNIQUES

a. Interpupillary Distance. If I2 device eyepieces are not properly aligned with the eyes, less than optimum resolution with the device will be obtained. Proper alignment of the eyepieces is achieved when the distance between the tubes matches the distance between the user's pupils. When the interpupillary distance of the I2 device is properly adjusted, the edges of the images in both tubes will be clear. When the edges are clear, the resultant binocular view through the tubes may appear as a single circle or as two circles. The circle or circles will be overlapped and slightly displaced laterally. Interpupillary distance is adjusted while the tubes are focused at infinity under dark-light conditions with all lens caps removed. The procedure for adjusting interpupillary distance is described below.

(1) Move the tubes away from the eyes as far as possible. This makes edge clarity easier to judge.

(2) With both eyes open, move the tubes closer together and farther apart. Observe the clarity of the edges of the circle in each eye. If the outside edges are blurred, the tubes are too close together. If the inside edges are

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blurred, the tubes are too far apart. If the upper or lower edges are blurred, tilt the tubes.

(3) Move the tubes closer to the eyes as desired without the eyelashes touching the eyepiece lenses. Recheck the tube tilt.

b. Binocular Focus. Each I2 device has a method for dioptric adjustment. This adjustment is used to correct visual refractive errors such as myopia (nearsightedness) and hyperopia (farsightedness). For the AN/PVS-5 series, this is accomplished with the diopter adjust ring. For the AN/AVS-6, it is accomplished with the eyepiece focus ring. When setting the dioptric adjustment, the user may achieve a clear image in each eye (monocular) and yet have a blurred image or accommodative eyestrain when viewing with both eyes (binocular). This occurs when the dioptric adjustment is set for one eye while the other eye is closed or covered. In this situation, the eyes tend to accommodate to a nearer distance than infinity, typically 1 to 3 feet. Over-accommodation or focus imbalance or both between the eyes can cause eyestrain and periodic blurred vision. To achieve a clear and relaxed binocular focus, the user should follow the procedure described below after focusing the tubes for each eye and adjusting interpupillary distance.

(1) Focus at infinity and view a distant object.

(2) Slightly blur the image in one tube, left or right, with the focus knob (AN/AVS-5 series) or objective focus ring (AN/AVS-6). The amount of blur should allow recognition of general object shapes but not fine details in the blurred tube.

(3) With both eyes open, adjust the diopter adjust ring or eyepiece focus ring for the clearest image in the nonblurred tube.

(4) Return the blurred tube to infinity focus, blur the other tube, and repeat the process.

2-6. OPERATIONAL CONSIDERATIONS

a. Magnification. I2 systems do not amplify an image. An object viewed through an I2 system will be the same size as if it were seen with the unaided eye.

b. Lights.

(1) With an I2 device, individuals can detect light sources that may not be visible to the unaided eye. Examples include lights from other aircraft, flashlights, burning cigarettes, and chemical light sticks. As the ambient light level decreases, aircrews can more easily detect these light sources but are less able to estimate distance correctly.

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(2) Performance of I2 systems is directly related to the ambient light. During periods of high ambient light, resolution is improved and objects can be identified at greater distances, although not to the degree possible during daylight. To light the flight path of a helicopter in low ambient light, the aviator may have to use an additional light source. Night scenes viewed with I2 devices are shown in Figure 2-10.

(3) I2 devices are adversely affected by bright lights and periods of high ambient light. When exposed to a bright light source, both the AN/AVS-6 and the AN/PVS-5 series are susceptible to whiteout. Saturation of the I2 system appears on the tube as a bright halo effect around the image of the light source. The halo effect also degrades the contrast of adjacent portions of the intensified image. This degradation of performance becomes worse when several bright lights appear in the field of view. Additionally, internal circuitry automatically adjusts output brightness to a preset level to restrict peak display luminance. When an area with bright lights is viewed, the display luminance will decrease ("shut down"). In addition to the halo effect around a bright light source, the overall display luminance of the rest of the viewed scene will dim. The brighter the light source, the dimmer the rest of the viewed scene. The crew member may also experience the dimming effect when viewing in the direction of a full moon at low angles above the horizon.

(4) Tunnel vision limits an individual's ability to see outside an area lit by bright artificial lights such as flares, landing lights, and lights with infrared filters. The ability to see objects within the lighted area depends on the intensity of the light and the distance of the object from the viewer. A crew member should not look directly at a bright light source because it may temporarily degrade the efficiency of the I2 system. When flying with the landing light or searchlight with the pink light filter or infrared band-pass filter on, the aircrew should avoid concentrating on the area illuminated by the light. The aircrew should also scan the area not illuminated by the light for hazards and obstacles.

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Figure 2-10. Night scenes viewed with I2 devices

c. Depth Perception and Distance Estimation. Depth perception and distance estimation are difficult with I2 systems. The quality of an individual's depth perception in a given situation depends on several factors. They include the available light, type and quality of the I2 system used, degree of contrast in the field of view, and viewer's experience. The aircrew must rely on the monocular cues discussed in Chapter 1 for accurate depth perception and distance estimation.

d. Color Discrimination. Color discrimination is absent when scenes are viewed through I2 systems. The picture seen with I2 systems is monochromatic (single color). It has a green hue because of the type of phosphor used on the phosphor screen of the I2 tube. The green hue in I2 systems may cause crew members to experience a pink, brown, or purple afterimage when they remove the device. This is called chromatic adaption and is a normal physiological phenomenon. The length of time the afterimage remains varies with the individual.

e. Scanning Techniques. Although the basic principles of scanning are the same for unaided and aided flight, crew members must consider a few specific items when conducting operations with I2 devices. Flight techniques and visual cues for unaided night flight also apply to aided night flight. Use of the I2 device improves ground reference but significantly reduces the field of view.

(1) The FOV of I2 devices significantly reduces peripheral vision as compared with unaided flight. Thus the crew member must use a continual

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scanning pattern to compensate for the loss. Moving the eyes will not change the viewing perspective; the head must be turned. However, rapid head movement can induce spatial disorientation. To view an area while using an I2 device, the crew member must rotate his head and eyes slowly and continuously. When scanning to the right, he should move his eyes slowly from the left limit of vision inside the device to the right limit while moving his head to the right. In this manner, the crew member will cover a 70 to 80 viewing field with only 30 or 40 of head movement. This technique minimizes head rotation. However, maximum visual acuity can only be attained when the crew member views through the center of the tube. Acuity drops to 20/70 or worse in the periphery of the I2 device FOV. The crew member should scan back to the left in reverse order and avoid rapid head movements because they can induce vertigo. The crew member must develop scanning techniques that involve a mix of unaided and aided vision.

(2) The devices provide the primary source for detailed visual information. When viewed through the devices, illumination sources, such as aircraft position lights and ground lights, may not be accurately interpreted according to intensity, distance, or color. Unaided vision can provide this additional information. With the newly modified I2 device-compatible aircraft cockpits, a slight downward deflection of the eyes will provide all required visual information inside the cockpit.

(3) Practice and experience are necessary to obtain maximum visual information from both unaided and aided vision. Initially, unaided peripheral vision may be somewhat distracting until the crew member develops adequate experience combining through-the-tube viewing with around-the-device scanning.

NOTE: Continuous flight with one lens focused inside and one focused outside the aircraft is prohibited. This can cause spatial disorientation, headaches, eyestrain, and reduced visual acuity.

f. Obstruction Detection. Obstructions that have poor reflective surfaces, such as wires and small tree limbs, are difficult to detect. The best way to locate wires is to look for the support structures. Hazardous wires in high-use areas should be marked with reflective devices.

g. Spatial Disorientation. Maneuvers requiring large bank angles or rapid attitude changes tend to induce spatial disorientation. Therefore, the aviator should avoid making drastic changes in attitude and bank angle and use proper scanning and viewing techniques.

h. Airspeed and Ground Speed Limitations. Aviators using I2 devices tend to overfly their capability to see. To avoid obstacles, they must understand the

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relationship between the device's visual range and forward lighting capability and airspeed.

WARNING

The visual range of the I2 devices may not allow aviators enough time to avoid obstacles. Therefore, aviators must exercise extreme care when using the devices during terrain flight modes. Aviators should reduce ground speed so that they can detect and avoid obstacles when ambient light levels are low or visibility is poor because of weather conditions.

(1) Different light levels affect the distance at which crew members can identify an object. This, in turn, limits the ground speed at which aviators can safely fly at terrain flight altitudes. Ground speed limitations are not quantified because of continuously changing variables affecting the limitations. Variables include the type of aircraft, type and quality of I2 device, supplemental lighting, vision obscurations, and ambient light conditions.

(2) Object acquisition and identification are related to ambient light levels, visibility, and contrast between the object and its background. For safety reasons, light levels required for training may differ considerably from operational requirements. Variables that affect the ability to see with I2 devices include--

o Type of I2 device. o Condition of aircraft windscreen. o Age and condition of tubes and lenses. o Moisture content in the air (humidity). o Individual's proficiency and capabilities. o Proper care and maintenance of the I2 device. o Capabilities of infrared band-pass filter used. o Visibility (haze, fog, rain, low clouds, dust, smoke).

i. Aircraft Lighting. Various sources of lighting (especially red) that are not compatible with I2 systems may degrade the aviator's ability to see with the system. The adverse effects of aircraft lighting on the I2 device are greatest during low ambient light conditions.

(1) Cockpit lights. The initial I2 cockpit lighting used with the full-faceplate AN/PVS-5 series was an "infrared cockpit illuminator." It was a modified map light that flooded the blacked-out instrument panel with a dim infrared light. The instrument panel was readable when an I2 tube was focused inside. Indicator, caution, and warning lights were dimmed to an acceptable I2 level by covering them with tape. Night-Fix, Phase I, employed blue-green lighting that had a negligible effect on I2 devices.

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Blue-green filters pass only blue-green light and block all other light, especially red and infrared. All red lighting was extinguished by an ON-OFF NVG light switch that activated the blue-green lights flooding the instrument panel. Indicator, caution, and warning lights were dimmed by blue-green filters. This system was designed for compatibility with the AN/AVS-6 and is therefore only partially compatible with the AN/PVS-5 series. Night-fix, Phase II, employed the same blue-green filters, but each instrument, radio, panel, and indicator light was illuminated or dimmed individually. All red lighting was removed. Suggested ways to improve I2 cockpit lighting are discussed on the next page.

(a) AN/PVS-5 series. The blue-green lighting should be dimmed to the lowest readable level based on the ambient light level and the aviator's dark adaptation level. This is required because the AN/PVS-5 series is sensitive to all colors of light. Dimming them reduces reflections on the windscreen, which limit I2 performance. The AN/PVS-5 series does not have the minus-blue filter like that used on the AN/AVS-6.

(b) AN/AVS-6. The AN/AVS-6 is designed to be operated with blue-green cockpit lights. The combination of improved performance in the red and near infrared portion of the spectrum and the minus-blue filter makes red cockpit lights noncompatible and blue-green cockpit lighting ideal. The use of red cockpit lighting should be avoided or strictly limited. While the use of blue-green cockpit lights will not degrade system performance, these lights should be dimmed to the lowest readable level.

(c) Both systems. Light reflected on the windscreen of the aircraft may degrade the aircrew's ability to see outside the cockpit. Aircraft lights that cannot be controlled should be covered with a filter compatible with I2 devices or with a light-reducing material such as tape. Flight instruments should be illuminated by interior lighting compatible with I2 devices. The instruments can be read by looking beneath the I2 device.

(2) Supplemental external lights. During Night-Fix, Phase I, a pink light filter was mounted on the standard landing light. It provides supple-mental infrared lighting, useful for 100 to 200 feet, which can be directed forward in the helicopter's flight path to detect obstacles. Figure 2-11 shows the pink light. The speed of the helicopter is reduced based on the range of the light and visibility restrictions. Further aircraft modification includes an improved infrared band-pass filter mounted on the searchlight or landing light. Different bulbs can vary the distance and beam spread of the infrared light path. Figure 2-12 shows the infrared band-pass filter in place.

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Figure 2-11. Pink light

Figure 2-12. Infrared band-pass filter

(3) External lights. I2 flights are degraded by aircraft external lights unless they are properly modified. The lights should be adjusted to the lowest level that will still allow detection by other aircraft or the control tower. The top half of the lower navigation lights and bottom half of the top navigation lights should be painted or taped to allow aviators to see through an unlit area surrounding the cockpit.

(a) Red navigation lights on the left side of the helicopter produce more usable light with I2 systems than green lights. In unaided

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flight, the opposite is true. Aviators switching seats should anticipate this, especially during hovering. Other lights--fuselage, formation, anticollision, electroluminescent panels (slime lights), and infrared position lights--should be turned off or subdued. Otherwise, their use should be based on training, tactical, or airspace requirements. Figure 2-13 shows the front of a UH-1 helicopter viewed through an I2 device. The red light on the left side of the helicopter appears much brighter than the green light on the right side.

(b) External aircraft lights aid the aircrew in interpreting terrain close to the aircraft. To minimize the adverse effects of external lights on night vision, the aviator should operate navigation lights in the DIM position when they are required. The anticollision light can be turned off to enhance training. The pink light filter or infrared band-pass filter should be used for training during periods of low ambient light. Exterior lights of other aircraft will not degrade the vision of an aircrew using I2 systems if the lights are taped and operated properly.

Figure 2-13. Effect of position lights on I2 devices

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j. Weather. When using I2 systems, aviators may fail to detect entry into IMC. This is because the I2 systems may enable aviators to see through obscurations such as fog, rain, haze, dust, and certain types of smoke.

(1) As the density of the visibility restriction increases, aircrews will detect a gradual reduction in light and visual acuity. When they recognize that their visibility is restricted, they should try to determine the severity of the condition and take appropriate action. This may include reducing airspeed, increasing altitude, seeking areas of contrast, or landing. If visual flight cannot be maintained, aviators should execute the appropriate IMC recovery procedures.

(2) Certain visual cues will be evident when visibility restrictions are encountered. A halo may form around sources of illumination when devices are used and atmospheric obscurations are present. The size of this halo effect around lights in the area of operations should be noted. If the halo becomes noticeably larger, a restriction could be developing. Also, an increase in "image noise" may result when atmospheric obscurations are present and the ambient light level is low. This is similar in appearance to the "snow" seen on a television with poor reception.

k. Weapons.

(1) Tube-launched, optically tracked, wire-guided missiles. TOW missile engagements at night should be made with unaided vision and with the appropriate artificial illumination for target acquisition. During TOW missile engagements, aviators may encounter conditions that adversely affect the use of the I2 device. The most critical of these conditions are listed below.

Initially, the target will not be visible because of the light intensity produced by the missile's flight motor.

The aviator's ability to see the target will be impaired by the missile's infrared source as the missile continues downrange.

Damage may occur to the lens of the telescopic sighting unit.

(2) Rockets, cannons, and machine guns. When firing the 2.75-inch folding-fin aerial rocket, aviators will lose sight of the target momentarily. After the rocket leaves the launcher, they will immediately regain sight of the target. Firing the 20-millimeter cannon can cause some visual impairment. Firing the 7.62-millimeter machine gun will cause loss of sight with the target during the entire firing burst.

NOTE: Recovery from bright flash illumination is more rapid with I2 devices than with the unaided eye.

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Section II

THERMAL-IMAGING SYSTEMS

2-7. OPERATIONAL PRINCIPLES

a. Operation of thermal-imaging systems differs from that of I2 systems. Thermal systems operate passively and without regard to levels of visible light. These systems do not transmit energy. Rather, they sense and display the energy radiated from objects. Thermal-imaging systems provide aviators with an image of an infrared scene. This enables aviators to operate in environments that could restrict or prohibit unaided operations.

b. The effectiveness of a thermal-imaging system depends on the difference in detected infrared radiation between the object to be detected and its background. Effectiveness also depends on atmospheric considerations--the degree of obscuration present between the system and the object. Thermal systems are most effective when a great difference in infrared radiation exists between an object and its background and when obscuration is minimal.

2-8. SYSTEM TYPES

Four types of thermal devices are currently used on Army aircraft. The PNVS, thus far, is the only type of pilotage thermal system used. The other de-vices are target acquisition systems. Aviators should consult the appropriate aircraft operator's manual for specific operating instructions.

a. Pilotage System. The PNVS, which is mounted on the AH-64, is a type of pilotage FLIR. Figure 2-14 illustrates the pilotage system. Appendix C provides a detailed discussion of FLIR components.

b. Target Acquisition Systems. A brief description of target acquisition systems is given below. An example of a target acquisition system, the TADS, is shown in Figure 2-15.

(1) C-NITE sight. C-NITE is a thermal sight for firing TOW missiles from the AH-1.

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Figure 2-14. Pilotage system

Figure 2-15. Target acquisition system

(2) Target acquisition and designation sight. The TADS is a day and night sight and laser designator and range finder for the AH-64. The TADS FLIR can be used as a backup to the PNVS.

(3) Mast-mounted sight. The MMS is a day and night sight and laser designator and range finder for the OH-58D.

2-9. INFRARED CHARACTERISTICS

Infrared is measurable electromagnetic energy and is part of the electromagnetic spectrum. The infrared region occurs beyond the visible light range. Infrared is, therefore, invisible to the human eye.

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a. Infrared Radiation.

(1) A definition of reflectance, transmittance, absorptance, and emissivity is given below.

(a) Reflectance--the ratio of radiant energy reflected by a body to the radiant energy incident upon it.

(b) Transmittance--the ratio of radiant energy that, having entered a body, reaches its farther boundary.

(c) Absorptance--the ratio of radiant energy absorbed by a body to the radiant energy incident upon it.

(d) Emissivity--the relative power of a surface to emit heat by radiation. It is the ratio of radiant energy emitted by a body--as a consequence of its temperature only--to that emitted by a reference body (blackbody) at the same temperature. This additional characteristic has considerable significance regarding object infrared radiation. A blackbody is a theoretical standard used for the purpose of laboratory comparison. It is an ideal body or surface that completely absorbs all radiant energy falling upon it with no reflection. A blackbody absorbs 100 percent of infrared energy incident upon it and emits 100 percent of its infrared energy. Therefore, a blackbody is both a perfect absorber and a perfect emitter.

(2) Reflectance, transmittance, absorptance, and emissivity determine the amount of infrared energy that an object will radiate when exposed to "x" level of thermal energy for "x" amount of time. Incident infrared radiation on a body may be reflected, transmitted through, and/or absorbed by the body. Absorbed energy may be emitted over a period of time according to the emissivity of the body. The total amount of infrared energy that an object will radiate is the sum of reflected, transmitted, and emitted energy. Figure 2-16 illustrates incident, reflected, absorbed, transmitted, and emitted radiation.

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Figure 2-16. Infrared radiation

b. Minimum Resolvable Temperature. The FLIR can discriminate objects from their respective backgrounds in the broad range of environmental conditions in which the FLIR operates. FLIR performance is measured by determining the MRT. A thermal-imaging system discriminates an object from its background by measuring the difference between the total infrared radiation of the object and the total infrared radiation of its background. MRT is de-fined as the lowest equivalent thermal difference between an object and its background that can