Page:CAB Accident Report, Eastern Air Lines Flight 663.pdf/13

Rh mental bewilderment and confusion. The aircraft pilot is susceptible to many types of illusions which result in spatial disorientation, such as vertigo, ocular gyral illusion, autokinetic illusion, and inadequate stimuli. Each of these types can be reproduced separately in a laboratory but they might be difficult to separate during actual flight. Some of these illusions will be considered separately herein.

Spatial disorientation results from reliance on the physiological sensing elements of the body which can give false or conflicting information to the senses. The primary device to provide orientation with respect to the horizontal, vertical, depth and distance is the eye. Vision, on the other hand, can give miscues to the physiological senses. A frequently experienced example of this miscue is an indication of motion when, in fact, you are standing still. In a stationary train the movement of an adjacent train often gives the impression that your train is in motion. When vision is no longer available, instruments must be relied upon to eliminate disorientation. Rotation through many degrees for an extended period of time, twenty seconds or more, can give a false impression of straight flight due to the actions of the semicircular canals in the inner ear. Movement of the head during the rotation will result in the impression of a violent pitch up or down, dependent on the direction of rotation with relation to the direction of head movement. The literature on this subject is quite complete so that it will not be discussed at length herein. Of particular interest in this accident are ocular gravic illusion results from the forces of gravity and acceleration acting upon the body where the body attempts to orient itself to the resulting vector of these forces. J. R. Harper in the January-February 1965 issues of "Cockpit" reports that these sensations, if relied upon, would tell us only that we are going up or down or from side to side. He states that when a turn is entered with a 30-degree bank our muscle and tendon pressures by themselves and without a visual horizon would tell us we are climbing. Reference to instruments would eliminate the effect of this illusion.

Autokinetic illusion results from the continual observance of an isolated light in the dark where no other visual references are available. The observed light, although in a fixed position, gives the impression that it is moving. The excursions of the light can be quite large and will be in different directions and magnitude for different observers. In order to experience this illusion the observer must fixate on the light for a period of about 20 seconds. Autokinesis can be easily broken by movement of the eye from the light to another object. Commander Walter Goldenrath reported in the June 1965 Newsletter of SAFE (Space and Flight Equipment) as follows "Inadequate stimuli or reduction in the intensity and quality of the visual stimulus will impart false sensations and there by result in disorientation. These illusionary effects are caused by such factor as haze, glare, fog, dusk, and darkness. Even when they are not severe they will reduce the visual stimulus levels to a point where orientation to the earth or other objects becomes faulty. This is particularly hazardous when flying over snow, water, or other areas barren of clearly defined landmarks. It results primarily imin [sic] marked decrease in depth and distance perception."