# Operational Considerations Operational considerations such as visual cues should be a factor at all stages of planning. The following considerations should be briefed during mission rehearsals to expand on situational awareness and mission contingencies. *** ## Distance Estimation and Depth perception Distance estimation and depth perception are closely related. Distance estimation relates to determining distance to objects, while depth perception primarily refers to the relationship of objects to each other. The quality of both is affected by ambient light, type and quality of NVG, degree of contrast in the field of view, and viewer's experience utilizing monocular cues. Objects tend to appear further away than they actually are. Reduction in visual acuity negatively influences distance estimation, because we expect objects that are less distinct in detail to be farther than ones that possess sharp detail. ### Cues Both depth perception and distance estimation are visual processes that are usually automatic. The loss or degradation of these cues is not recognized unless they are demonstrated or a conscious effort is made to remain aware of these limitations. We utilize two types of cues: binocular and monocular. ### Binocular The binocular factors of convergence and stereopsis are involved with depth perception. Both of these systems are primarily used at distances less than 10 meters (30 feet). At greater distances, it is usually considered that the information provided by these systems is minimal, and the brain primarily relies on monocular cueing for distance and depth information. Convergence is used to measure of the difference in the angle of the two eyes. The steeper the angle between the two eyes as they look at an object, the closer that object is to the viewer. Stereopsis is caused by the difference in the images on the retina of the two eyes. The amount of overlap in the overall scene provides distance information to the observer. Convergence is the more significant factor of the two in judging the distance of near objects. Binocular depth perception is primarily used when hovering or when flying at NOE altitudes. Depth perception using ANVIS is particularly degraded at short distances (10 meters or less). This results from the eye's fixed focus in the NVG (approximately 6 feet). This does not match the information by the other native short-range visual tools, convergence and stereopsis. Further confusion arises due to the fact that the image in the ANVIS is very blurred (the ANVIS being focused for 50 meters). The aviator's visual system cannot resolve the inconsistency in these inputs and is unable to determine the distance to the object in question. This becomes particularly apparent when hovering near the ground, taking off, and landing. Until the aviator develops other tools to assist in distance estimation and depth perception, the aviator's perception is degraded, particularly in determining hover height. ### Monocular Monocular cues are derived from experience and are subject to interpretation. Monocular cues can assist in identifying possible hazards to include man-made structures, associated terrain, and actual position of the ground in reference to present altitude and position. Although these monocular cues provide depth perception for all distances, they become more dominant as distance between the observer and the object in question increases. Anything that adversely impacts NVG resolution also impacts the perception of these cues. Therefore, as aircrew NVG visual acuity decreases due to lower illumination or lower contrast scenes, the cues are less discernible, resulting in poorer depth perception. The types of monocular cues include geometric perspective, retinal image size, aerial perspective, and motion parallax #### *Geometric Perspective* An object appears to have a different shape when it is viewed at varying distances and from different angles. Geometric perspectives include linear perspective, apparent foreshortening, and vertical position in the field: * Linear perspective (figure 4-37)-parallel lines such as railroad tracks appear to converge as distance from the observer increases.
* Apparent foreshortening (figure 4-38)-the shape of an object or terrain feature appears distorted when viewed from a distance at both higher and lower altitudes. Round objects appear elliptical (oval and narrow), while square objects take the shape of a trapezoid. As the distance to the object or terrain feature decreases, the apparent perspective changes to its true shape or form. When flying at lower altitudes and at greater distances, crewmembers might not see objects clearly. If the mission permits, pilots should gain altitude and decrease distance from the viewing area to compensate for this perspective. Once altitude increases and distance between the aircraft and viewing area decreases, the viewing field widens and enlarges so objects become apparent.
* Vertical position in the field-objects or terrain features at greater distances from the observer appear higher in the field of view than those closer to the observer. When viewing a scene, a higher vehicle appears closer to the top and at a greater distance from the observer. Before flight, crewmembers should already be familiar with the actual sizes, heights, and altitudes of known objects or terrain features within and around the planned flight route. If situation and time permit, crewmembers can reference published information to verify actual sizes and heights of objects and terrain features within their flight path. In addition, crewmembers should cross-reference the aircraft altitude indicator to confirm actual aircraft altitude is adequate to safely negotiate the object or terrain feature without prematurely changing aircraft heading, altitude, or attitude.
#### *Retinal Image Size* An image focused on the retina is perceived by the brain to be of a given size. The factors that aid in determining distance using the retinal image are— * Known size of objects (figure 4-39)-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 image. An object projects an image on the retina based on its distance from the observer. If the image is small, the observer judges the object to be a great distance away, while a larger image indicates the object is close. To use this cue, the observer must know the object's actual size and have prior visual experience with it. If no experience exists, the observer determines the distance to an object primarily by motion parallax.
* Increasing or decreasing size of objects-if the retinal image of an object increases in size, the object is moving closer to the observer. If the retinal image decreases, the object is moving further away. If the retinal image is constant, the object is at a fixed distance. * Terrestrial association (figure 4-40)-comparison of one object such as an airfield with another object of known size such as a helicopter helps in determining the relative size and apparent distance of the object from the observer. Objects ordinarily associated together are judged to be at 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.
* Overlapping contours (figure 4-41)-an object partially concealed by another object is behind the object concealing it. Crewmembers must be especially conscious of this cue when making an approach for landing at night. Lights disappearing or flickering in the landing area should be treated as barriers and the flight path adjusted accordingly.
#### *Aerial Perspective* An object's clarity and its shadow are perceived by the brain and cues for estimating distance. Crewmembers must use the following factors to determine distance with aerial perspective: * Fading colors or shades (figure 4-42)-the colors of objects appear to fade with distance. However, an object viewed through haze, fog, or smoke can appear less distinct and at a greater distance than it actually is. Conversely, if atmospheric transmission of light is unrestricted, the object can appear more distinct and closer than it actually is.
* Loss of detail or texture-sharpness and clarity of details or texture is lost or is less apparent with distance. For example, at a distance a cornfield appears to be a solid color, tree leaves and branches appear to be a solid mass, and objects appear to be at a great distance. When an aircraft is operating near the ground, crewmembers can see the grass or gravel immediately below, in front of, and alongside the aircraft. They can use these details to help them determine airspeed and altitude. If they maintain that view as the aircraft slowly ascends, the crewmembers notice the clarity and detail of the surface fades and eventually blends in with the terrain as a whole, making accurate determination of airspeed and altitude difficult or impossible. Again, environmental factors such as fog or dust may degrade apparent texture and increase the apparent distance to an object. * Position of light source and direction of shadow (figure 4-43, page 4-37)-every object casts a shadow in the presence of a light source. The direction in which the shadow is cast depends on the position of the light source. If an object's shadow is cast toward an observer, the object is closer to the observer than the light source.
#### *Motion Parallax* This is often considered the most important cue to depth perception. It is the primary visual cue used in hovering. Motion parallax is the apparent, relative motion of stationary objects as viewed by a moving observer (figure 4-44). When judging airspeed, near objects appear to move past or opposite the landscape. 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. Rapidly moving objects are judged to be near while slow moving objects are judged to be distant. When hovering at night, keeping a tree or similar object near the helicopter stationary in relation to another object greatly aids in keeping the helicopter steady. Remember that this does not detect movement toward or away from the two objects. This awareness must be maintained in at least two directions to detect helicopter movement.
*** ## Other Cues Secondary cues should be utilized to facilitate primary scans. The effects of external sensory equipment failure could be extremely detrimental if secondary scans are not utilized by aircrews. ### Optical Flow Optical flow is referred to as the angular rate and direction of movement of objects because of aircraft movement. This provides the information necessary to interpret speed and direction of motion. If there is no relative motion, there is no optical flow. Normally, peripheral vision is used to detect optical flow; central vision is used to assess its speed. Since the NVG field of view is severely restricted, the optical flow cues will be severely degraded when compared to day flight and central vision tracking becomes the primary means of detection. This leads to one of the most insidious dangers when transitioning to terrain flight from higher altitudes. Because of the reduction in peripheral vision motion, the ensuing "speed rush" that would indicate close proximity to the ground is not available, resulting in reduced ability to judge airspeed and rate of climb or descent. This is why it is very important that aviators maintain an aggressive scan during NVG flight. Duringhigh cockpit workloads or periods of fatigue, scanning is one of the first tasks to be impacted. A dedicated effort must be made to avoid fixation and to maintain the scan necessary to provide this essential cue. ### Unaided Peripheral Cueing How much unaided (normal night vision) peripheral cueing is available and whether or not it is helpful in the NVD environment depends on many variables: illumination level, terrain type, and artificial light sources. Although aircrews are not completely dark adapted, they are partially dark-adapted and able to discern some features outside or around the NVG intensified image. For example, while flying over terrain where cultural lighting is generously scattered, the motion of these lights as they speed by can be detected in the periphery while looking into the NVD image. This adds to overall orientation (situational awareness) by feeding familiar information to the aircrew. When flying in canyons during periods of good illumination, features and motion may be detected in the periphery outside the NVG FOV. When peripheral cueing is added to both the NVG and FLIR image, a good marriage of sensor and real world imagery results in significantly enhanced spatial orientation. ### Spatial Orientation/Disorientation The greatest challenge for NVG operations is the impact of NVG on the aircrew's ability to correctly interpret the image presented. Visual cues provide the strongest input to overall spatial orientation and situational awareness. The visual system is functionally divided into two distinct systems: the central (focal) and peripheral (ambient) systems. Each of these visual systems is impacted by NVG use and is degraded as compared to daytime (photopic) performance. Daytime visual performance is the standard by which we compare NVD performance By virtue of NVD design limitations (for example, FOV, lack of color discrimination or visual acuity), operationally significant misperceptions and visual illusions can occur during NVD aided operations. The challenge for aircrew remains to develop the knowledge base and training necessary to understand and overcome NVD limitations and effectively use NVDs in flight. Night unaided/aided flight increases the likelihood of experiencing visual illusions and spatial disorientation in comparison to the day environment. Degraded visual acuity, fatigue, high task loading, limited field of view, and inexperience are all contributing factors that must be anticipated during night unaided/aided flight. Other factors that can aggravate the tendency for spatial disorientation include extreme aircraft maneuvering and three axis head movements. Peripheral vision also helps reduce or eliminate the effect of middle ear disturbances (vestibular illusions). This is one reason spatial disorientation is more common at night. Spatial disorientation can be induced or aggravated by the following: * Aircraft bank greater than 30 degrees. * Significant or abrupt aircraft maneuvers. * Three axis head movements. * Unfamiliar perception related to a lack of NVG experience. * Degraded visual acuity. * Fatigue. * High task loading. This is due in part to the fact that the peripheral visual system is heavily used as a backstop or sanity check for the vestibular and proprioceptive systems. As the amount and quality of information deteriorates, disturbances in these systems are more likely to produce disorientation. The other major factor lies in the fact that the aviator's primary means of orientation is the horizon. The severely restricted field of view available to NVDs can limit or eliminate the visible horizon, particularly under degraded conditions. Increased reliance on aircraft instrumentation can be necessary to preventing disorientation. Use of HUD helps substantially. It may become necessary to change the flight profile, revise the mission timeline, or revise or abort the mission entirely as conditions deteriorate. *** ## **Airspeed and Ground Speed Limitations:** * Aviators using NVDs should understand the relationship between visual range, forward lighting, and airspeed, especially during terrain flight. * Different light levels affect object identification and limit ground speed; reduce speed for obstacle avoidance. * Factors affecting NVG visibility include type, age, and condition of NVGs, cleanliness of aircraft windows, weather conditions, and individual proficiency. ## **Hazards to Night Flight:** * Hazards include lasers causing flash blindness, retinal burns, and impaired night vision; nerve agents affecting pupil constriction and dark adaptation. * Aircrew coordination is crucial, as assumptions about obstacle visibility can lead to accidents. * Preflight inspections are critical, especially in low light conditions; detect leaks and ensure windscreen cleanliness. * Cockpit lighting should be dimmed gradually to avoid glare; anti-collision lights may distract formations. * Landing lights must be used judiciously; IR searchlights can aid NVG visibility depending on terrain reflectivity. * Position lights should be dimmed to reduce detection risk; supplemental cockpit lighting must be NVG-compatible. * Fatigue poses significant risks, including reduced vigilance, computational skills, communication, and decision-making abilities. * Understanding sleep cycles is essential; shifting to a night routine can disrupt sleep patterns over time. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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