Visual Illusions



This really didn't happen, its just a visual illusion...

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             Visual Illusions. Many types of visual illusions can occur in the aviation environment. It is critical that the pilot be familiar with visual illusions that may affect safe flight. Visual illusions cause loss of orientation, which is referred to as Spatial Disorientation (SD). Defined, SD is the inability of the pilot to sense correctly the position, motion or attitude of the aircraft in relation to the surface of the earth and the gravitational vertical. Additionally SD is the inability to determine the position or attitude of the pilot's aircraft relative to other aircraft. Decreasing visual cues and references increases the probability of SD. This lack of cues, coupled with an erroneous interpretation of visual cues can create additional illusions that also induce SD. Illusions generally occur at night in both the unaided and aided (Night Vision Goggle or NVG) modes of flight. In some instances illusions may occur during the day in low contrast environments (i.e. desert, water, snow). Remember, when encountering visual illusions there is no substitute for good crew coordination, proper scanning techniques and cross referencing instruments. Discussed below are the most frequently encountered visual illusions and ways to overcome them.


             A common acronym used to remember these visual illusions is:


          F  F  F    C  R  A  S  H    S  A  R    C


             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 cockpit task. Because outside references for attitude are less obvious and reliable at night, aviators should rely on them less. Utilizing instrument crosschecks can help prevent this situation. Hovering over uneven terrain can lead an aviator to interpret sloped ground as level ground and make control inputs that will cause the aircraft to drift instead of maintaining a stationary position. Crew coordination is paramount to over come the illusion of False Horizons.


             Flicker Vertigo. Much time and research has been devoted to the study of the Flicker Vertigo illusion. A light flickering at a rate between 4 and 20 cycles per second can produce unpleasant and dangerous reactions such as nausea, vomiting, pilot vertigo, and, on rare occasions, convulsions and/or unconsciousness. These reactions tend to be exaggerated by fatigue, frustration or boredom. During the day, especially while flying instruments, this can be caused by sunlight flickering through the rotor system or propellers. At night, it can be caused by an anticollision light reflecting off the clouds when in instrument meteorological conditions (IMC). If this phenomenon causes safe flight to be compromised try to eliminate the source of the flicker.


             Fascination (Fixation). This illusion occurs when aviators ignore orientation cues and fix their attention solely on a goal or object. Target hypnosis is probably the most common type of fascination. This is typified by the attack helicopter or fixed-wing aviator so intent on aiming at and hitting a target during a gunnery run, that he delays the pull-up or break-off point too long and the aircraft strikes the ground before he has a chance to recover. This may also occur during an NVG approach. The aviator is restricted in his field of view and uses the *Infra-Red (IR) searchlight which pin points the landing area. Fascination also takes place during confined area operations where the helicopter could drift into near obstacles. Crew coordination coupled with a proper scanning is the most effective method for defeating the illusion of Fascination (Fixation).


             Confusion of Ground Lights (Ground Light Misinterpretation). It is not uncommon to confuse ground lights with stars. This occurs when an aviator unknowingly positions an aircraft in an unusual attitude to keep the ground lights (believed to be stars) above their line of sight. For example, it is easy to interpret lights along the seashore as lights on the horizon and maneuver the aircraft dangerously close to the water in the attempt to fly straight and level. To overcome this illusion, aviators should crosscheck aircraft instruments, which should include a radar altimeter. It is also common to interpret a pattern of lights on the ground as the "landing Y." For instance, the position lights on the top of an AH-64 or an UH-60 are Y shaped and can be easily mistaken for the landing area when one of these aircraft is sitting on the ground. Crew coordination may prevent the illusion of Confusion of Ground Lights from occurring.


             Relative Motion Illusion. The Relative Motion illusion is one of several types of motion illusions that may be encountered. The most common example is an aircraft hovering while another aircraft hovers into position alongside the first one. When this second aircraft appears in the edge of the peripheral vision, the movement of the second aircraft is misinterpreted to be movement of your aircraft and the aviator attempts to correct for this "movement", which often results in an incorrect control input. The Relative Motion illusion may also be encountered during multihelicopter operations when aircrews interpret movement of other aircraft as movement of their own. Other motion illusions include:


                   Lack of Motion Illusion. During low level flight, the lack of discernible terrain (low contrast) may result in the perception of near zero airspeed causing the pilot to increase airspeed unnecessarily. Additionally, at a hover with a total lack of visual references the pilot may drift without detecting it.


                   Wave Drift Illusion. While hovering over water at night with a lack of discernible references, the motion of waves blowing out forward or away from the aircraft creates an illusion of the aircraft drifting opposite the motion of the waves. The illusion causes the pilot to instinctively adjust the cyclic to drift with the motion of the waves and out of the stationary hover. This sensation may also be encountered while hovering over tall grass, loose ground or loose references (i.e. unanchored chem lights).


                   Waterfall Illusion. While hovering over water at night with a lack of discernible references, the downward motion of water particles in the rotor wash creates an illusion that the aircraft is rising. The illusion causes the pilot to instinctively lower the collective out of a fixed hover position.


             Autokinesis. When a static light is stared at in the dark, the light may appear to move. This phenomenon can be readily demonstrated by staring at a lighted cigarette, or a single, small point of light, 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. The illusion can be eliminated or reduced by visual scanning, increasing the number of lights, or by varying the light intensity. The most important of these 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 and 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 lacking, or not available, aircrews are subject to the illusion of Autokinesis.


             Structural Illusion. Heat waves, rain, snow, sleet, or other factors that obscure vision cause this illusion. Straight lines appear curved, a position light may appear double, or to move when viewed through a rain shower. Awareness that the Structural Illusion is occurring together with crew coordination should be sufficient to overcome it.


             Height Perception Illusion. When flying over desert, snow, water, or other areas of poor contrast, aircrew members may experience the illusion of being higher above the terrain than their actual height. This is due to the lack of visual references. This illusion can be diminished by observing any shadows cast by near objects, such as the landing gear at a hover. It is also important to maintain a continuous scan of the flight instruments, which should include a radar altimeter, in addition to the normal scan outside the aircraft. Another technique involves dropping some object, such as a chemical light stick, on the ground prior to landing (see Motion Illusions above). This will help the crew identify the actual surface and to better estimate the actual altitude, while acquiring a more accurate understanding of the conditions. Restrictions to visibility, such as haze, rain, snow, smoke or fog can also produce this illusion. The result of this illusion is inappropriate rates of descent, or other control inputs that could be difficult to counteract. For any of these occurrences, it is critical to employ good crew coordination to combat the effects of the Height Perception Illusion.


             Size-Distance Illusion. At night it is very difficult to judge distance estimation of lights. A bright light may be perceived to be closer than dimly lit lights that may be at the same distance while dimly lit lights may appear to be further than a bright light at the same distance. During NVG flight it is not uncommon for crewmembers, especially with low experience, to perceive lights to be closer than they actually are. This is due to the amplification inherent with NVGs. Additionally, viewing a stationary source of light that is increasing or decreasing in luminance (brightness) may cause the crewmember to interpret the light as approaching or retreating. An attempt to overcome this illusion can be accomplished by associating the lights viewed with features on a map. While using NVGs, it may be necessary to look under periodically to obtain a better perception with unaided vision. Crew coordination will also help in ascertaining the distance of lights, thereby reducing the effects of the Size-Distance Illusion.


             Altered Planes of Reference. When approaching a line of mountains or clouds, aviators will perceive the need to climb to ensure clearance, even when the aircraft altitude is sufficient. This illusion can be diminished by cross-checking the aircraft altitude with the terrain altitude and by observing the terrain beyond the mountains. If you are unable to see any terrain beyond the mountains, then it is likely that the aircraft altitude must be increased. Additionally, this illusion may occur when flying parallel to a line of clouds and the aviator tends to tilt the aircraft away from the clouds. Cross-checking the instruments will overcome the illusion of Altered Planes of Reference.


             Reversible Perspective Illusion. At night, an aircraft may appear to be going away when, in fact, it is approaching. This illusion often occurs when an aircraft is flying parallel to the course of another aircraft. To determine the actual course and direction of another aircraft, the aircrew should observe aircraft lights (for color and configuration) and their relative position to the horizon. If the intensity of the lights increases, the aircraft is approaching and vice versa. To counter the Reversible Perspective Illusion, remember the 3 R's when identifying the direction of travel of other aircraft. If the Red aircraft position lights are on the Right side, the aircraft is Returning (coming toward the observer). While using NVGs, the colors of the position lights are not seen. Due to *monochromatic adaptation, when looking under or around the NVGs, the colors can be distorted so that white lights may appear pinkish. Due to the NVG's amplification, using the intensity of aircraft lights for distance estimation may be misleading. Other aircraft using NVGs could be using the IR searchlight, could be using dim position lights, could be masked, or using no lights at all. Anytime an aircrew member cannot determine if an aircraft is definitely moving away, and the aircraft is not moving across the windscreen, use good crew coordination and maneuver your aircraft to avoid this potentially deadly collision course.


             Crater Illusion. When operating at night, using NVGs and the IR searchlight, as the aircrew descends into a confined area the illusion of dropping into a crater or hole in the ground may occur. The best defense against the Crater Illusion is to use good crew coordination techniqes.




             * Infra-Red (IR) Light: Light below the lower end of the visible light spectrum for the human eye. It is not visible to the naked, or unaided eye. Through the use of Night Vision Devices (NVD), this light may be seen.


             * Monochromatic Adaptation: NVDs allow the aviator to see at night very well. Unfortunately, they produce only green colored light. Human eyes adapt to this color. When looking away from the devices, all objects will be effected by the color shift until the eyes re-adapt to the natural world. As a result, all light viewed will be shifted towards the green end of the visible spectrum until vision returns to normal.



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